Typical 12-volt solar panels deliver power to the solar controller at 16-18 volts, or thereabouts, depending on conditions. The purpose of the solar controller is to translate this power into a form usable to charge 12-volt batteries. Basically, the controller is a high-quality battery charger. To complicate things even further, there are now controllers like the Outback Flex, some of the Blue Sky controllers, the Xantrex XW controller, and the Morningstar Tristar MPPT line that can take any voltage in (up to 150 volts), and output 12-volts. This gives you great flexibility in solar panel choice. Only on the smallest systems can you use a solar panel connected directly to the battery bank without risking damage to the batteries. (Note: 12-volts is the nominal battery voltage in this discussion. The actual voltage is always higher than that in various charge stages. But you still refer to the system as a 12-volt system.)

There are only two types of controller technology worth considering; PWM and MPPT. If you see cheaper “shunt” type controllers ignore them (these days you don't see these much).

PWM (pulse width modulation) controllers are the most common. These send very rapid “pulses” of power to the battery bank – the “width” (duration of the pulse) varies depending on the state of charge of the battery bank. They are typically 3 stage chargers, sometimes with a separate equalization stage (called a 4th stage by some marketing people).

Using the Voltage Calculators

The link above will guide you to an interactive voltage calculator. Many people mis-use this and come up with "funky" numbers. Here is some guidance on the inputs.

Make sure you use the Vmp of your panel as the input to calculate the voltage drop/wire size for the run between the combiner and the controller. Or, if you are doing series/parallel on 12-volt panels, calculate the voltage. If you can not enter the specific Vmp for the voltage, then enter then next lowest available voltage. Do not use 12 volts.

Make sure you do not select conduit runs - select no conduit if you have a choice.

75 degrees C, or 167F for temperature.

Single set of conductors, copper, DC if a choice.

If you enter the wire size, start with 4AWG and vary that to get a voltage drop under 3%.

You can vary the amperage based on the total Imp of your panels, but usually use the max for the controller. That way you can add panels and still have a proper voltage drop. If you know you are not adding panels then you can factor that in.

Separately calculate the drop from the controller to the battery bank. Use 13.4 as the voltage - this is the lowest you will see on that link.

To use grid-tie panels you need a solar controller that can convert that higher voltage input to 12-volt (nominal) output. There are a number of controllers that can do this, and they are all MPPT technology. They tend to be higher-priced controllers with advanced features, so you have an overall better package. I particularly like the the Morningstar Tristar 45 and 60 MPPT controllers. In reality, all these controllers can take any voltage input up to 150V (or up to their rated voltage) and down-convert it to their output rating (for our discussion, always 12V, since we are using a 12-volt battery bank). So that opens up a wide variety of panels for potential use.

When using higher voltage panels and expecting the charge controller to reduce the voltage for a 12-volt battery system you need to consider the total system voltage. The charge controllers lose efficiency if required to step the voltage down too much. Morningstar and Outback publish their efficiency curves - taking a look at them is educational (look in the very back of the manuals). The bottom line is that you will want to target a system voltage in the 30-45 volt range if you can (for 12 volt batteries). Of course, you have to balance this with other considerations, such as voltage drop to the controller. (Just another design consideration.....I never said it was easy...)

Obviously, you need to decide if you want to go with higher-voltage panels during the design stage. If you go this route, look for higher voltage panels typically used in grid-tie applications. They will make installation a little easier. You can easily use pairs of 12-volt panels in series, but the wiring will be a little more complicated. The price per watt is always the determining factor for me. If the price was close I’d go with the larger higher voltage panels, assuming they will lay out on the roof properly. Remember - NO shading.

If you are going to use a series/parallel wiring layout the panels should all be the same rating. You do not want to mix panels with different Vmp ratings, since they will not perform to their potential. Lower voltage panels tend to "pull down" the higher voltage ones. Ratings should be within .2-.3 volts of each other (notice the decimal point). Also, the pairs need to be co-located. It is not a good practice to separate them with long wire runs between them.

One consideration of using these higher voltage panels is that they are physically bigger, since they have additional cells. Make sure that the panels will fit on your roof without being shaded.

Solar Controller Summary

Size: make sure you buy bigger than you need. You want to be able to expand your system.
Type: MPPT controllers are the best, and are worth the extra expense if you are building a larger (bigger than 200 watts) system. Their ability to handle higher-voltage systems make that feature alone generally worth the extra cost.
Wiring: Make sure you have big enough wires. Use the calculators - do not guess. It is unlikely that an RV manufacturer supplied heavy enough wires for a proper solar installation. Make sure you check. I never use less than #4 welding cable.
Voltage: consider the benefits of using panels rated at higher voltage, or series combining to get higher voltage. You need to buy a controller that can handle voltage conversion if you go this route.
Features: A remote display is nice, but not required if you have a good battery monitor and a smaller array. Solar controllers operate fine without any attention. Temperature compensation is a feature worth paying for. By all means get this. Equalization can be handled by most inverters more efficiently so is not worth paying extra for.

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The Inverter/Charger

Checklist For Selection

What About My Converter?

I will assume that the inverters that you are interested in are all hardwire inverter/chargers capable of powering most of the appliances in your RV. I don’t recommend a separate battery charger and inverter. The combination inverter/chargers are easier to install, provide better battery charging, and result in much simpler control circuitry. (For advanced solar systems some might argue with my statements above. There are always exceptions to every generalization - but in RV use it is generally best to stick to an inverter/charger in my opinion.)

This puts your choices in the category of higher-cost inverter/chargers. Typically these range in power from 1800 watts to 3000 watts. The “typical” RV is well served with a 2000 watt sine wave inverter/charger, which usually has a 100 amp battery charger included with it. This will run almost all microwave ovens, which is the heaviest load most inverters in an RV see. If you have special needs that require more power, then inverters up to 3000 watts are readily available. Expect to pay between $900-$2500 for a quality inverter/charger, depending on wattage and waveform. Pure sine wave inverters are more costly, and often not required. With inverter/chargers you usually get what you pay for – so beware the inexpensive high wattage inverters.

Microwaves are usually the highest load device you will run on the inverter. Microwaves are sensitive to peak voltage. The higher the peak voltage, the better they run. Both modified sine wave and pure sine wave inverters are dependent on battery voltage levels and load levels for peak power. Thus output voltage will drop a little if the battery is not at peak voltage, or if the load is heavy. That is why most microwaves do not cook as well on inverters as on shore/generator power. Some microwaves will not run on lower-cost modified sine wave inverters, but this is very rare. Unfortunately, there is no way for me to tell you if your microwave will perform acceptably on modified sine wave. The best I can do is to tell you that in dozens of cases I have personally been involved in I have only seen one microwave not perform well with modified sine wave - and that microwave was mid-90's vintage.

A standard strategy with microwave cooking on inverters is to increase the cooking time. Because of the high draw, microwaves are typically used only for reheating when running on inverter power. We have found it best to run microwaves only on high power when running on the inverter.

Some computer monitors show slight waves in them when on inverter power. The modified sine wave inverter that we had in our Kountry Star and also our Royals International ran a Dell LCD monitor and a Viewsonic LCD monitor with no problems.

Most clocks will not run properly on modified sine wave – this is because the inverter output waveform will vary some, based on load. Use battery-powered clocks if you need to keep accurate time.

Battery chargers for small devices may work properly on MSW, or may fail. Check the charger for overheating. If the charger is excessively hot, do not use it on the inverter.

There are many inverters on the market that would work well in your RV. To some extent it is a personal choice. My personal preference is in the Equipment Recommendations section - but I greatly prefer the Magnum products for a whole-house inverter. The sections that follow will help you narrow your inverter selection. I also include my opinion on a variety of inverters in the other sections. Here is my checklist for inverter selection:

Sine wave or modified sine wave? Some people feel they have to have pure sine wave. If you have special needs that you know can’t be handled by a modified sine wave inverter, then go ahead and pay the extra for the pure sine wave. Usually, this is not necessary. Oxygen generators, CPAP machines, residential refrigerators and laser printers often require pure sine wave. Almost all other devices do not. However, with the price of pure sine wave inverters much less than they were in past years it is probably best if you seriously consider them first. At this point in time I would not put a MSW inverter in my personal RV - but you may be able to use one.

You have to figure the size, in watts. Don’t plan on using everything in the RV at once like you do on shore power. Based on that, you should be able to figure your max draw. Usually, 2000 watts is sufficient. Don’t worry about battery charging capability; all the inverter/chargers have sufficient charger output.

What is the transfer switch rating? Also, how are you going to interface to your load center (split box, sub panel, wired inline)? This will narrow your selection further. While there used to be inverters with 50 amp rated transfer switches that could handle two power legs, these are no longer available. So you are really in the situation of requiring a subpanel if you have a 50-amp RV.

What monitoring system is designed to interface to the inverter? Does it have a cumulative ampere-hour capability, or do you have to use a different meter for that? You have to balance monitor system costs – it may be cheaper to buy a monitor that is not part of the manufacturer’s package and use it in conjunction with a cheaper remote control offered by the manufacturer. Does the inverter have the features you need/want? Does the monitor package have the features you need or want (like generator management, if you need it).

Does the instrumentation for the inverter allow you to control all functions of the inverter individually? I like the battery charger to be separately controlled. I choose to use solar for charging almost all the time, even when on shore power. I only use the battery charger if we have many rainy days. Some inverters automatically charge the batteries if shore power is present; this function can not be disabled.
Does the inverter have an equalization mode? You will want to equalize your batteries from time-to-time. Either the inverter needs this function, or your solar controller needs to have it. It is a little easier if the inverter has it, but either will work fine.
Does the inverter have battery temperature sensing? You definitely want this for most efficient battery charging. Most of the inverters in the price range you will look at have this as either a standard feature, or an option. If optional be sure to get it.
Can you change the charger voltage "set points"? This is useful for getting the batteries fully charged. In many cased flooded cell charging algorithms in the chargers do not bring the bulk-charge voltage high enough. Being able to tailor this allows you to adjust the bulk voltage so your batteries get fully charged. In particular, MANY inverters have the bulk charge set point at 14.4 volts (or around there). For most flooded cells you want a set point of 14.8 volts.

What About My Converter?

The problem with older converters is that they do not have a very good battery charger section, and some of them do not even put out regulated (clean) 12-volt power for the DC house systems. If you put in a full solar inverter/charger system then you can potentially eliminate the charge and supply functions of the converter and use it strictly as a "standby" power source. This is discussed below.

If you are taking a phased approach to upgrading your RV electrical components and do not want to buy a large hardwired inverter with a superior battery charging capability, then you may want to upgrade your converter section. First you have to determine if you have the older style converter. A very popular converter, used in many RV's, is the Magnetek 63xx series of converters. They were inexpensive and many manufacturers installed them. Unfortunately, you get what you pay for - they have very low power output, unfiltered power, and single charging set points. They are basically incapable of recharging a battery fully without overheating it. If you have one of these, this is why you have to add water to your batteries often, and why your batteries do not last as long as they should.

If you decide not to upgrade to an inverter/charger, then you might consider replacing the older Magnetek with a modern Intellipower 91xx series with a Charge Wizard. The Charge Wizard converts the 9100 series from a 2-stage to a 3-stage charger. It only costs about $25, and is the best money you will ever spend. It will enable your batteries to fully charge when hooked to shore power, without overheating and boiling off excessive amounts of water. The 9100 series comes in various sizes from 30 amps to 80 amps. Choose the one that meets your needs, but I would not go lower than 60 amps. The advantage of using the 9100 series is that the charger section can use all of the rated power, minus what the RV DC systems are currently using. This is unlike the 6300 series that had a fixed charger section of around 5 (effective) amps. Thus, the 9100 series is effective when used with a generator to recharge your batteries.

If you have an older converter integrated with the 12-volt loadcenter, trying to replace it with upgraded electronics can be difficult. You can always add a newer separate converter and just disable the old converter. Usually, all that is required is pulling a fuse/flipping a breaker and re-routing the DC wires.

Some very good replacement articles have been written that will step you through replacing your converter. I'm not going to try to re-do them here. Do a search on the web and you will find a lot of info. Or look at Converter Upgrades for a good collection of articles. BestConverter also has a good selection of products for upgrading your converter.

Bottom line: replacing an older converter is an upgrade you should consider if you are not going to put in a large inverter/charger. You need an effective battery charger to use with a generator if you intend to boondock for any length of time, and this is the most cost effective way to do it, plus it benefits you when you are on shore power as well. However, if you intend to go to an inverter/charger you might consider skipping to that step now, and investing the $300 from the new converter into the inverter/charger.

When adding a high-powered battery charger many people throw the old converter out. Let me suggest that you keep it, and wire it into the system by connecting its output to the distribution hubs you have added. However, you will leave it unplugged. If the converter was previously hardwired, install a pigtail on it so you can plug it into a standard outlet.

The purpose of doing this is twofold. First, if you have an inverter failure of some sort, you can fall back on the converter while the inverter is being repaired. Secondly, and most important, if you are in a situation where the shore power is of low quality, or of low amperage (say a 15 amp outlet at a friends house, or a rally), then you can not plug into the main shore power to supply your coach - it is simply not enough power to be convenient. Instead, run your coach on the inverter. Then, you can plug in the converter to the 15 amp external circuit and simultaneously charge your battery bank. (Remember, you can’t use the inverter function and the battery charge function of your inverter/charger at the same time). This allows you to have a clean source of power, and recharge the bank at a reasonable rate.

You can’t plug the converter in anywhere except to an external source of power if running on the inverter (as described above). If you plugged it into your RV outlets then when you inverted power you would set up a feedback loop and drain your battery bank, and potentially damage equipment.

Note: The above applies to a separate converter that plugs into a receptacle - which is fairly typical in larger rigs. If you have an integrated converter/charger/12-volt loadcenter then you will have to figure out the wiring on it to accomplish the above. They are all a little different. If you choose not to modify this type of converter, you can just turn off the breaker that feeds power to it. Everything should then work as if it was never there.

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Xantrex Inverters

The purpose of this section is not to push Xantrex products - although they make some decent stuff. It is to help you differentiate their many models of inverters. There are many other brands of inverters available that are as good, or better, than the Xantrex inverters. Take a look at the Magnum and Outback inverters - both of which have an excellent reputation.

Xantrex sells Trace, Heart, and Statpower inverters and associated components - as well as their own lines. They bought all these companies years ago. It can be very confusing trying to figure out the differences in the Xantrex inverters. There are so many that initially it can be overwhelming.

Many of the Xantrex products do not feature monitors with cumulative amp hours, which I (and most experts) consider a requirement. You will have to augment the instrumentation for this function if you use one of these Xantrex inverter lines. All of the inverters discussed here have equalization. Here is my opinion on their various products.

The Prosine 2.0 Inverter/charger is pure sine wave, has separate battery charger controls, has equalization, battery temperature sensing, and comes with a remote control panel. The transfer switch is 30 amp only, so it will require a sub panel if installed in a 50 amp RV. They retail for $2000, but are easy to find cheaper. I would combine it with a Trimetric 2020 meter. If I was considering this inverter, I would look carefully at the RS series as a better alternative.

The RV series (RV2012) is a modified sine wave inverter with a split-phase 50-amp transfer switch. You can use it with a 50 amp RV. It does have equalization and a temperature compensation capability. However, it does not have the ability to turn the battery charger off. As long as AC power is available the charger is in operation. Personally, I don’t like this “feature”; I prefer to have control over charging. Use it with the RC6 remote, which provides basic monitoring and “On/Off” control. Add a Trimetric for advanced monitoring. Retail is $1700.

The DR series of inverters are modified sine wave with a 30-amp transfer switch. The 2412 has a 2400-watt inverter with a 120 amp charger. Like the RV series, the battery charger is always “On”. Use it with the basic RC8 remote, and augment with a Trimetric. Retail runs around $1100. If you are considering this inverter, look at the Freedom 458 2.0 as a better alternative (cheaper, and with a Link 1000 monitor does not require the Trimetric).

The SW series is 24-volt and 48-volt only. Not suitable for most RV applications.

The Freedom 458 series (Heart) is a modified sine wave inverter with a 30-amp transfer switch. It has temperature compensation, equalization and a battery charger that is controllable (on/off). Use it with the Link 1000 monitor (or the newer LinkLITE or LinkPRO) for full system control and monitoring, including cumulative amp hours. This is a nice combination if it’s specifications work for you. Retail is $1225, but they are commonly available for under $900; the Link 1000 runs around $250. This combination is the best value of all the Xantrex inverters in this size range, and works well. I've used it in two of my personal RV's and installed it in others. If you want pure sine wave this won't work for you, however.

The RS series is a pure sine wave inverter available with a 30-amp transfer switch (RS2000) or a split-phase 50 amp transfer switch (RS3000). It has selectable charger control, equalization, temperature compensation and uses the SCP monitor (System Control Panel). The SCP allows control of multiple Xantrex devices from a single control panel. The most likely device you would add would be a generator remote start, or the XW MPPT solar controller. These can all co-exist on a single control network, and be monitored/controlled with a single SCP. Personally, I like this series the most. Although it does not have cumulative amp hours (you still have to add the Trimetric) it has great features, and is moderately priced for what you get. If retrofitting a 50-amp RV I would use the RS3000 since it can handle two legs of 50-amp, thus avoiding adding a sub-panel if you choose.. The RS2000 list price is $1600; the RS3000 is $2000, but you can find the RS2000 for $1125 and the RS3000 for $1395 if you look hard. The SCP sells for around $205. Note: as of 2010 the RS Series is no longer available.

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The Battery Bank

Battery Types
Charging and Charging Stages
Choosing the Battery Type
Battery Bank Sizing and Installation
Wiring Techniques
Maintenance
Specific Gravity Test

Volumes can be written on batteries. If you want to understand exactly how they work and are constructed there are plenty of resources available on the web. One of the best sources for general battery info is Battery FAQ's This section looks at batteries from a practical view, as used in an RV. The best explanation of how to properly wire multiple batteries is http://www.smartgauge.co.uk/batt_con.html (I exposed this link since it may change; you might find it easier if you can see the text). One thing about batteries and RVers - everyone has an opinion. It is a favorite topic of "techies". I've tried not to go into too much technical detail - use the references for a more complete battery "experience".

Deep cycle batteries used in RV systems are all lead-acid type. Ni-Cad and NiFe (Nickel-Iron) batteries are sometimes used in industrial applications but are not well suited for RV use. There are basically four types of lead-acid battery construction; flooded (wet) cell, sealed flooded (maintenance free), gel, and AGM (absorbed glass mat). Flooded batteries are your normal batteries used in cars, and in most RV’s. They can be deep cycle, or “starting” type, and can be “sealed” or have caps on them so that water can be replaced. All gelled and AGM batteries are sealed.

All the batteries used in RV “house” applications should be deep cycle batteries. NOT marine or starting batteries. SLI, or starting batteries, are designed to supply high starting current for brief periods of time. With many relatively thin plates, they are designed for many shallow cycles, and to maximize the current available for a brief starting cycle. In a starting application they are discharged less than 10%, and can last for thousands of cycles. If deeply cycled, they will last as little a 30-50 cycles.

“Marine” batteries have slightly thicker plates and perform a little better than starting batteries, but they are usually not considered deep cycle batteries. Both SLI batteries and Marine batteries are usually rated in CCA (cold cranking amps).

There is no quantitative measurement to define what a "deep cycle" or "true deep cycle" battery is. Thus it is very difficult to choose between batteries of similar type. In general, "deep cycle" batteries are designed to be cycled at 50% or more for many repetitions. They are differentiated from the starting and marine batteries by the thickness and construction of their plates. This is what enables the many deep cycles. Deep cycle batteries have solid lead plates and are quite heavy. “Golf Cart” batteries are not generally deep cycle batteries like L-16 or industrial batteries, but are somewhere in between a starting battery and the industrial traction batteries. A real "golf cart" battery is designed specifically to power a golf cart and is optimized for that discharge/charge cycle profile. But, from a practical perspective they are considered deep cycle. It is hard to make a judgment about what battery is "better" in an alternative energy application when you are restricting yourself to golf-cart-size batteries, or batteries slightly bigger. You can look at warrantee and you can look at the weight of the battery. Both can give you some indications, but neither are definitive. I generally balance price with "anecdotal" use experience. We know that a Trojan T-105 "golf cart" battery performs well, and lasts generally 5 years or so. We also know that most of the Sams Club "golf cart" batteries last about as long, and cost less. SO does that make them as good? Only you can decide.

There is sometimes debate in the RV community on whether to use a bank of 12-volt batteries for the house system. Most experts recommend against this. The argument used in favor of 12-volt batteries is that if one fails you only need to replace the single battery. With 6-volt batteries they have to be used in pairs (combined in series to make 12-volts) and if one battery in the pair fails you need to replace both of the batteries. That is because a new battery, paired in series with an older battery, will be rapidly brought to the same charge condition as the older battery. In other words, if you leave the old battery in the pair, you get 2 old batteries. This mainly applies to batteries older than 3-9 months. The main argument for the use of 6-volt batteries is that they perform much better. While this is an often stated "fact", it is not absolutely true. It is difficult (but not impossible) to find deep-cycle 12-volt batteries, so the lifetime and performance of the bank will generally suffer with 12-volt batteries. If you do decide to go the 12-volt route, look into using 8D truck batteries (if they will fit in the space available). These are available in a deep-cycle version (about 200 Ah), and will perform better than most other 12-volt batteries. There are 12-volt deep cycle batteries, but they are harder to find, and are expensive. I prefer to stick to readily available 6-volt golf cart batteries, but there is nothing wrong with deep-cycle 12-volt batteries, despite what the "RV grapevine" might tell you.

Golf cart batteries last 2-5 years in RV use, gelled cell batteries last 2-5, and AGM’s last 4-7 years. This is typical use; it can be less or more, depending on treatment and how often they are cycled. Industrial deep cycle batteries can last far longer, but are difficult to use in RV’s, because of space and weight considerations. For example, an L-16 battery easily lasts 4-8 years, and large forklift batteries can last even longer. Large single cells used by the telephone companies for backup power can last for over 20 years in that application (because they are usually not cycled often or deeply). Many (but not all) manufactures publish tables of the number of cycles you can expect at different DOD (depth of discharge) for their batteries. We know people who have gotten over 5 years from Sam’s Club golf cart batteries. They treat their batteries well (no deep discharges, proper watering, and regular equalization), and they are lucky. We currently have over five years on our bank of four Sam’s Club 6-volt golf cart batteries and they are still performing within specifications. But we know we are on the downside of their lifespan. I know people that have gotten 10+ years from T-105's. But they are well treated.

When comparing batteries in places like Wal-Mart or Sam’s Club ONE of the factors in determining the "better" battery is its weight. Assuming that they are the same “group” then the heavier battery will often be better (in use), because it has heavier plates. Compare a Trojan T-105, T-125 and a T-145. The T-105 and T-125 are identical in "footprint" size (they fit in standard Group 27 battery boxes). The T-105 weighs 62 pounds and is rated at 225 amp hours, compared to 66 pounds and 235 amp hours for the T-125. The T-145 has the same footprint but is 5/16 higher. It weighs 72 pounds and is rated at 244 amp hours. Of course, there is a price difference among them. The best deal I have ever seen on new T-105’s is $55, in 2003. In January 2005, you could buy the same “class” golf cart batteries at Sam’s Club for $48 each (220 amp hour rating, generally made by Excide or Trojan). In 2007 the Sam's Club golf cart batteries were around $62. If you have the space and can carry the weight, the Trojan L-16H weighs 121 pounds, is rated at 395 amp hours, and measures 11 5/8 x 7 x 16 11/16 high. Don’t ask me to help you put these in, though - I'll watch and give "moral support".

Sometimes you can find “deals” on flooded–cell batteries that have never been put into service (e.g. they are “dry”). Be careful with this. Even though batteries are shipped dry, and stored dry, they still have a little moisture in them. They do deteriorate in this state, so check the date of manufacture on the battery. Don’t pay full price for a flooded-cell battery that is over 18-20 months old, even if it is “dry”. You will not get full life out of such a battery, but it may be worth the price charged. We once bought T-105’s that had been dry stored in a battery warehouse for 2 years. They tried to sell them at full price, until we pointed out their manufacture date. We got four of them for $115 with no tax (total price, not each), so it was worth the risk. They did charge to specifications and load tested to spec. They lasted 3 years, but that is a reduced lifespan.

Here is the bottom line on batteries, from my perspective. There is no compelling evidence that 6-volt is better than 12-volt in this application. A heavy battery is going to be better than a lighter one, but other than that look at price and antidotal use. How you charge and use the bank makes more difference than which set of batteries you have, assuming you buy decent ones to start with. I personally like the Sam's Club 6-volt batteries because of cost/lifetime tradeoffs. But there are good 12-volt batteries out there as well.

Charging

Your battery bank is going to be charged by multiple charging sources, depending on your setup. Your tow vehicle or motorhome engine will charge your batteries when you are moving. As discussed elsewhere, this is not going to provide a very good charge unless you set up a sophisticated engine charging system, but it will top them off if the bank is not at a high state of discharge. If you install solar panels then the solar controller provides the primary charging source for your battery bank. This provides the best charge, since it is multi-stage and slow. Most solar controllers also have an inbuilt mild equalization effect by design. Your inverter/charger has a high power battery charger built into it which is great for a quick charge of a depleted battery bank. This is the fastest way to get your bank restored when boondocking. Lastly, if you leave your converter wired into the system, then under certain circumstances you can also use your converter as a charging source.

Batteries can only “take in” so much amperage during charging without being damaged. Flooded cell batteries are usually charged at no greater than C/3, where C is the 20-hour rating for the bank. Thus a 440 Ah bank can be charged at greater than 100 amps (usually the inverter max). Gel batteries must be charged much slower, at C/5. AGM batteries can accept much higher currents during charging, so are fully charged much faster (an advantage when boondocking and using a genset or solar to charge). Lifeline AGMs can be charged at almost any rate you can push at them (up to Cx4). Optima batteries are typically charged at Cx2. This is a significant advantage to AGM’s as compared to flooded cell batteries.

Both the inverter/charger and the solar controller contain battery chargers that should be using a Pulse Width Modulation (PWM) charging algorithm. This breaks the charging cycle into three phases (some manufacturers say 4 or 5, but these are just variants of the 3 stage PWM cycle). Do not buy a charger that does not use some variant of PWM. Your inverter and solar controller manuals will cover these in detail. I’ll summarize them here.

The three charging stages are Bulk, Absorption, and Float. There are some inverter/chargers that allow you to turn off the float charge (an example is the Xantrex RS, but that is an option).

Stage one of the charging cycle is bulk charging. During bulk charging the batteries are charged at a constant current. The current is determined by the maximum charge rate set by (in) the inverter and is based on the size of the battery bank, and type of battery. You specify this when you "program" the inverter during installation. Voltage rises during this phase until it reaches the bulk charge voltage set for the battery type. For flooded cell batteries this is typically set at 14.4 volts by default, for gel cell types it is 14.1 volts. If temperature compensation is being used, this will vary based on the battery temperature detected. The batteries will start to gas when the bulk voltage is reached. Bulk charging restores about 75% of capacity.

Once bulk voltage is reached the charger enters the Absorption Stage. During absorption the current is gradually decreased at whatever rate maintains the bulk voltage setting (just below the gassing voltage). If voltage starts dropping then current is increased again until voltage remains constant. The absorption stage ends when the current required to hold the voltage at the bulk setting declines to the setting programmed into the inverter. This is often C/40, where C is the total Ah rating of the bank (so for a 440 Ah bank it is about 11 amps), but it can vary based on the manufacturer, and the source of charge (solar controller or inverter). If the current never declines to this point, then a timer will terminate the absorption stage, usually after 12 hours. Most battery experts will tell you that a level of 1 – 1.5 A per 100 Ah of battery rating indicates a full charge. Notice this is different than some inverter manufacturers. If your inverter allows selection of acceptance voltage algorithms, go with one that is consistent with your battery manufacturer’s recommendation. If you don’t have that information available, then use the inverter default for the battery bank size and type. Absorption charging restores the remaining 25% of the batteries capacity. Once you have your monitoring system in place, you can watch this process happen. It won’t take you long to get used to how your batteries are operating.

Note: almost all in-built battery charge algorithms for flooded cell batteries specify 14.4 volts as the bulk set point. For most flooded cell batteries that is not enough to fully charge them. I always set the bulk charge for 14.8 volts, as recommended by Trojan, and others. Always use your battery manufacturer recommendation, but if you do not have one, I would use 14.8 volts.

The Float Stage starts at the termination of the absorption stage. Typically, the batteries are fully charged at this point. The purpose of the float stage is to maintain this full state of charge without causing battery gassing. The voltage is held at a constant 13.5 volts for flooded cell batteries, and 13.4 volts for gel types. Current is held at a low level, to maintain the voltage required. Higher current is available on demand to supply DC loads, but voltage is held at the float set point which is dependant on battery type. Some experts do not believe in a float stage, but most chargers force you into this stage. Some chargers allow you to set up a 2 stage charging algorithm that does not include a float stage. Other chargers, like the Outback, use multiple float stages, separated by a “quiet” time where the charger is not operating. We avoid an application of a constant float stage by only using our solar controller for charging (normally, our battery charger is always "off" - we only turn it on when solar is not recharging our bank fully). This way, float only lasts during the daytime, and only when there is sufficient sunlight to support it.

Most inverter/chargers have an equalization mode (as do many solar controllers). The purpose of equalization is to remove sulfates from the battery plates, and to break up stratification of the acid and water in the battery case, both of which occur over time in normal use. If successful, equalization results in all the cells specific gravity equalizing to a single value, thus the name. Usually, this is manually started on an inverter. On a solar controller you can set up an automatic equalization cycle, or there may be a built-in mild equalization charge placed on the batteries at the start (in the morning) of each charge cycle. Only flooded cell batteries should be equalized. Placing an equalization charge on gel or AGM batteries can harm them. Inverters and solar controllers have settings to prevent this from occurring. During equalization, up to 17 volts is placed on the batteries. This can damage sensitive electronics in the RV (such as refrigerator boards) so the batteries should be disconnected from the RV loads. For equalization to be effective at least 3 amps of current per 100 Ah of battery bank capacity must be available.

Choosing the Battery Type

From a practical perspective you have a choice of Wet-cell, gel, or AGM batteries. Wet-cell (golf cart and other types with removable caps) have a price advantage over gels and AGM batteries. They have the disadvantage of requiring regular maintenance, and proper venting of the fumes released by the gassing that occurs during charging. They can not be placed on their sides, since they are not sealed, and can not be placed in living spaces. Gel and AGM batteries are much more flexible in this regard – they can be placed on their sides, and anywhere in the RV, since they do not gas. They can also be placed in the same enclosure with the inverter. Battery gas is explosive, so wet cell batteries should never be placed in the same enclosure with the inverter (which can “spark” under the right circumstances). There is one really good reason not to use gels and AGM’s – price. They cost 3-4 times as much as flooded cell batteries.

Since AGM batteries have become available, use of gel batteries is probably not a good idea and they are no longer commonly seen in RVs. Gel batteries have to be charged at a much slower rate (C/20) to prevent gassing, they also need a lower voltage during charge. They can also loose water due to evaporation in hot climates (or enclosures). This shortens their life, since there is no provision to replace this water.

AGM batteries have a glass mat that absorbs the acid/water mixture placed between their plates. Even if the case is punctured, they will not leak. They do not loose water, since it is automatically recombined with the acid during the charging cycle. Because of the construction, they are very resistant to vibration and impact (the glass mats cushion the plates), they can accept a full charge, just like wet-cell batteries. In fact, the internal resistance of AGM batteries is so low that they charge faster than other battery types.

AGM batteries are ideally suited for RV use. They are far superior to flooded cell batteries, like golf cart batteries; they take charge at a much higher rate (ideal for solar and genset recharging), they survive shock and vibration better, no maintenance is required, they are sealed so you can put them anywhere (even on their sides, or inside your RV). But, and this is a big but, they are expensive.

If you have never maintained a large house bank before, I highly recommend using the relatively inexpensive Sam’s Club batteries. They perform well for the money, and if you mess up you can replace them easily. Once you are used to battery maintenance and your energy needs are better known you can invest in higher performance batteries if you want. I’ve found that the Sam’s batteries last as long as the Trojans and perform almost as well. In use I see no significant difference.

Concorde is the manufacturer of AGM batteries you see the most (they also make the Lifeline batteries).

Battery Bank Sizing and Installation

One you have estimated your electrical demand you can determine the size of the battery bank required to support your loads. For most RVer’s, a bank of four flooded cell batteries (like Trojan T-105s) will suffice. They will give you approximately 200 Ah of power if drawn down 50%. As discussed, you should try to only draw down 25% of your bank’s usable power. That would mean approximately 100 Ah would be available under normal conditions. If you need to draw them down further, then you can go to 50% DOD without concern.

If your electrical demands are consistently 175 Ah or more, then you should consider expanding your bank size to six batteries. This will allow you the power you need without taxing your battery bank, and will prolong the life of your batteries.

Another sizing consideration you need to consider during the design stage is balancing the battery capacity with the solar charging capacity. If your goal is to be able to recharge your bank from solar then you have to balance the solar and battery sizes. A general rule-of-thumb for recharging is around one watt of solar for one amphour of battery capacity. So four T-105 class batteries (400 Ah, rounded) are optimally charged by a minimum of 400 watts of solar panels. Don't get obsessed with this - it is strictly a rule-of-thumb. In any case, you need to have a battery bank sized properly for your electrical demands. You can always add solar panels later, or use a generator to make up charge.

The biggest issue with batteries is usually finding room for the size bank you need. Sometimes, it is not possible to install the ideal bank size because of space constraints. This means more generator run time, or reducing your electrical demand through conservation. Most 5^th wheels have space for at least two batteries in the space the manufacturer supplied. Sometimes there is space for four, but rarely for six. Motorhomes sometimes have additional space available in the battery compartment, but can be more difficult than 5^th wheels to find sufficient space.

Batteries need to be located together, and in a vented space separate from the inverter and the solar regulator, both of which can generate sparks and arcs that could ignite battery gas. Sometimes you have to use your imagination to find a suitable location. In a 5th wheel you can move the batteries to the front storage compartment and put the inverter and solar regulator in side compartments, or in the main compartment. The inverter needs to be within 10-12 feet of the battery bank - max. You will also need space for fuses, shunts and other components involved in installation. Plan your location carefully, making sure that you can hook up all the required components.

If you have to move the battery location you need to build a new battery box that has proper venting. I have used plastic storage containers for this with great success. Look in K-Mart, Wal-Mart and home centers to find a properly sized container. I use flexible vacuum cleaner hose for the vent line. Take the vent from the side of the box near the top; place a hole the same size as the vent on the opposite side of the box, near the bottom. Convection will assist with venting. Make sure that the box is secured, as well as the batteries within the box. Vibration and movement will kill your batteries very quickly by damaging the plates.

Battery Wiring

Battery Connections - ParallelSerial.jpg (69795 bytes)

Wiring the battery bank depends on the battery configuration. In a typical RV installation you need 12 volts to supply the house loads (we are ignoring busses for this discussion). That means you have to combine two 6-volt batteries in series to produce 12 volts. Pairs of 6-volt batteries are then combined in parallel to sum the amperage available, while maintaining 12 volts. If using 12-volt batteries you simply combine them in parallel.

The size of wire used to interconnect the batteries depends on the maximum load to be drawn from them. The inverter will place the heaviest load on your battery bank. Your inverter manufacturer will tell you what size wire is required for the inverter, based on its distance from the bank. They usually specify that the battery interconnect wiring is to be the same size. If you "overbuild" the inverter feed wiring (by using, say, 4/0 wire when 2/0 would suffice) you can use the next size smaller to interconnect the battery bank. The battery interconnects have to be able to support the maximum load the inverter is capable of. Personally, I would never use less than 2/0 for interconnecting batteries when an inverter is involved. The reason is that even if you have a small inverter now, if you go to a larger inverter you don't want to have to rewire the battery bank. Information on cable building is in the Truck Electrical Center section. You will be much better off building your own cables.

Battery Connections - Remote Pair.jpg (79381 bytes)

Sometimes, in order to build the size bank required, you are forced to locate parts of the bank in different areas. While this is not desirable, if there is no alternative it can be done as long as you keep the distance between the batteries reasonable. For example, in our Royals International 5th wheel the battery box will support four T-105 class batteries. So that is what I have. However, if I needed to add two additional batteries I would place them just inside the nose storage compartment next to the battery box (which is on a slide out tray with an outside door). My inverter is also inside this nose compartment, on the opposite side of the RV. Normally, you never put an inverter in the same compartment with a battery bank because of the possibility of explosion. In this example it is safe to do so because of the separation and because this compartment also contains the genset, so it has excessive ventilation. I would still put the extra two batteries in their own box, and vent that box to the main battery box.

Maintenance

Flooded-cell batteries require routine maintenance. This needs to be performed at least once a month. One of the advantages of gel and AGM batteries is that they are sealed units and do not require maintenance.

Following is the maintenance information provided by Trojan.

Specific Gravity Test

(Flooded batteries only)

1. Do not add water at this time.
2. Fill and drain the hydrometer 2 to 4 times before pulling out a sample.
3. There should be enough sample electrolyte in the hydrometer to completely support the float.
4. Take a reading, record it, and return the electrolyte back to the cell.
5. To check another cell, repeat the 3 steps above.
6. Check all cells in the battery.
7. Replace the vent caps and wipe off any electrolyte that might have been spilled.
8. Correct the readings to 80^o F:

Add .004 to readings for every 10^o above 80^o F
Subtract .004 for every 10^o below 80^o F.

9. Compare the readings.
10. Check the state of charge using Table 1.

The readings should be at or above the factory specification of 1.277 ± .007. If any specific gravity readings register low, then follow the steps below.

1. Check and record voltage level's.
2. Put battery's on a complete charge.
3. Take specific gravity readings again.

If any specific gravity readings still register low then follow the steps below.

1. Check voltage level's.
2. Perform equalization charge. Refer to the Equalizing section for the proper procedure.
3. Take specific gravity readings again.

If any specific gravity reading still registers lower than the factory specification of 1.277 ± .007 then one or more of the following conditions may exist:

1. The battery is old and approaching the end of its life.
2. The battery was left in a state of discharge too long.
3. Electrolyte was lost due to spillage or overflow.
4. A weak or bad cell is developing.
5. Battery was watered excessively previous to testing.

Batteries in conditions 1 - 4 should be taken to a specialist for further evaluation or retired from service.

Back to Page Contents

Wiring

Rooftop and Solar Controller Wiring
Solar Array Wiring Considerations
MC Connectors
Battery to Inverter Wiring
Interfacing to Your Loadcenter
AC Wire Types
Grounding
Neutral Bonding
Installing a Sub Panel
Powering the Entire Loadcenter
"Splitting" a 50-amphere Loadcenter
Monitoring and Control
Recommendations

At the end of The Truck Electrical Center section is information on cable building and wiring methods. Sources for parts and tools are there as well.

There are five main areas of wiring:

DC wiring from the panels to the rooftop wiring hub, or between the panels if not using a combiner.
DC wiring from the combiner on the roof, to the battery bank, which goes through the solar controller.
DC control wires that connect your instrumentation to their sensors.
DC cables that interconnect the battery bank, and connect the battery bank to the inverter.
AC wiring between the inverter and the existing load center (or sub panel, if using one).

If the inverter does not have terminal blocks for the AC input/output connections, use twist-on wire nuts, like in a residential electrical connection. If you have to use twist-on connectors, make sure you tape them to the wires securely to prevent loosening from vibration.

It is important to completely design your system before you start implementing it. If you want to phase in the implementation, that is OK, but you need to design the entire thing first. You also need to consider where you are going to mount components, and their layout. On the left is a picture of the "first generation" electrical center in my old Kountry Star. It is placed on a piece of 3/4" plywood, which makes it convenient to mount components.

Rooftop and Solar Controller Wiring

For now, I will assume you are connecting your rooftop panels in parallel, and that you are using 12-volt panels (nominal rating). We will discuss higher voltage panels and serial wiring a little later.

Just like batteries, solar panels come in 12, 24 and 48 volts (nominal). Why would you use higher voltage panels? Two reasons: first, larger panels typically come in higher voltage, so if you want 175+ watt panels you will be getting higher voltage. Second, higher voltage panels mean less voltage drop on the way to the controller. A technically superior design would be to use higher voltage panels and a solar controller that can convert the output voltage to 12-volts. This allows you to use smaller wiring, or have longer wire runs from the roof to the controller.

With 12-volt panels all wiring is parallel. You simply interconnect all the + and all – lines. You can do this two ways; daisy chain them from panel to panel, with the last panel having the wire that comes down to the solar controller; or with a distribution hub on the roof. These are typically called "combiner boxes". Use of the combiner helps in several ways. First, each panel’s wire runs directly to the combiner, so wiring is easier. Second, adding a panel later is easier, since you don’t have to modify or lift any of the existing panels. Third, you can use smaller wire to interconnect the panels, and run a larger wire from the roof down to the controller. If you have panels grouped together, you can daisy-chain between panels within a group, and then run a single line to the hub. This makes wiring a little easier.

I prefer to use the combiner, but it does increase the cost slightly, and complicates the initial installation a little. Locate the combiner centrally, near the panels, but try to minimize the wire run from the combiner to the solar controller. You can then run one larger wire from the combiner down to the controller. Look at the voltage drop tables in the Solar Regulator section to calculate the wire size required and then use one size heavier. I prefer to use a minimum of #4 wire for the run to the solar controller, which is sufficient in almost every case (but not all cases - you MUST use the voltage drop tables to ensure your wire size is correct). Even if #4 is not required, it gives you some room for growth and does not cost that much more.

Double Distribution Block - 4:1

You can find various types of small distribution hubs several places, including at many Wal-Mart’s in the automotive audio section. Or order them from the internet sources in this article. (Try www.solarseller.com look in section 57, Cat# PDB-175-SIX, about $29). To build a "home made" combiner I use a plastic outdoor junction box, which has a removable lid, with a gasket. It is easy to drill the appropriate sized holes in the sides for routing wires. Use weather-tight wire clamps. I like to use hubs that have at least four outputs (which you will use as inputs for the wires from the panels). You can usually double the wires up, if required. You need one hub for + and one for -, or you can buy a dual hub, which is the one shown. Position it in the box so you can tighten the set screws. Epoxy it into the box, when you are satisfied with the layout.

The alternative to building your own is to buy a simple ready-made combiner box from AM Solar. Check the product section for combiner boxes. I prefer using the CB Combiner Box. The new AM Solar combiner box is the 4/2 box. It will take much heavier wire than the previous boxes available from them. The old boxes would only take #6 wire - these boxes will take #2 easily. You can also find combiners from Outback and Midnite Solar that contain DC-rated breakers in them. These are weather-tight enclosures, but are intended for vertical mount. They can be mounted at up to a 45 degree angle on the roof without a problem (with the back facing the front of the RV, so water is never forced into the lid). The advantage of using the combiner with the breakers is that you can easily test each panel, or string of panels. Plus, each panel is protected from a short or catastrophic failure of another panel. I think the $100 or so that you will spend on the combiner with breakers is well worth it.

An alternative to putting the combiner on the roof when you are using a high voltage system is to place the combiner in a "close" storage compartment and run the #10 wire from each of the panels (or strings) down to it. This is especially nice with the breaker boxes, since it negates any water entry to them. This works well ONLY IF the voltage is high enough so that there is not more than 2% voltage drop to the combiner. Use the voltage calculators.

For the home-built and AM Solar combiners, I try to bring the wires in from the sides of the box that will face the sides of the RV, or from the rear. That way if the entry holes are not perfectly sealed there is less chance of wind forcing water into the box when driving. I bring the single entrance wire (going to the controller) out the back. To hold the box to the roof, use adhesive caulk compatible with your roof material. If you have a fiberglass or aluminum roof you can use adhesive silicone caulk. If you have an EPDM rubber roof, or a vinyl roof use the Dicor adhesive caulk designed for that application – do not use silicone on an EPDM roof, because it will not stick. With Outback or Midnite Solar combiners the wire routing is the same as a standard electric subpanel - from the bottom.

Note: the combiner to the far left is a Midnite Solar 6-position combiner. It can take six parallel-wired panels, or 6 strings of series-wired panels. There is a water-resistant cover that is not shown. The next two boxes are from AM Solar. The middle one is the small CB combiner box that can take four panels in (or double up on the terminal strips for more). With more than four panels I do not recommend the CB - it is too tight to wire. The far right box is the new (larger) combiner box from AM Solar - it is their best box and I highly recommend it if you are not doing breakers. There is plenty of space for lots of wires, and it will easily handle any size up to 1/0.

To hold single wires on the roof I use “puddles” of the appropriate adhesive caulk – embed the wires in the puddle. I put the puddles about every 3-5 feet along the wire run. Once the caulk sets, add a little to the top. I’ve been doing this for years, and have never had a wire come loose. For the “bundle” of wires you might have if you combine the individual wires of multiple panels on the way to the distribution hub (tie wrap them together), use a combination of caulk and one tie-wrap with a screw slot to secure them. Cover the single screw with caulk. Use caulk alone to secure the rest of the bundle to the roof.

All rooftop wire should be UV resistant “tray cable”. Or, you can run conduit if you want, but this is really overkill, and much harder to install. I have very rarely seen conduit on the roof in an RV installation. I use 10 gauge wire between the panels, or from the panels to the hub, and usually #4 welding wire to run down to the controller. Unless you have an unusual distance from the combiner to the controller this is usually more than sufficient. If you have a high-amperage set of panels (lots of panels in parallel) then the #4 may not be sufficient, and you may have to wire the panels so you increase the voltage, and decrease the amperage. More on that technique is discussed below. In any case, use the interactive voltage drop calculators, or the voltage drop tables on this website to determine the size required.

Some RV’s have “solar prep” packages. These typically have 10 gauge wire installed by the manufacturer from the roof to the solar controller location. This is marginal in most circumstances plus it may not terminate in the location that you want it. You will have to decide if you want to use this wire, or run another wire that better maintains voltage. Consult the voltage drop charts and estimate the length of the wire run. In some cases it is easy to add a second wire – in which case you could run a second 10 gauge wire in parallel to the “solar prep” wire supplied by the manufacturer. Or you could just run the 4 gauge wire and abandon the manufacturer’s wire. Wire size and connector quality are particularly important when using an MPPT controller. Heat and bad wire connections will cause an MPPT controller to operate far below its rating, negating any advantage to using it instead of a non-MPPT controller. The voltage-drop chart will tell you what you need to use. Without consulting a voltage-drop chart you are simply guessing. Wire for a 2% or less voltage drop - you should strive for 1%. That way, even if you do not use a MPPT controller initially, you can swap for one later and be within the recommended voltage drop for that controller type. If the input terminals on your solar controller will not accept the wire size you use, simply clip some of the fine wire strands off until it fits. This won’t affect anything.

From the solar controller to the battery bank I usually use the same #4 AWG welding wire, depending on the length of the run. It is critical to minimize voltage drop from the controller to the battery bank. Make sure that your wire is heavy enough. Aim for a 1% drop. There is a fuse installed in this line. On small systems (one or two panels), I use automotive "Maxi" fuses instead of the glass fuses usually supplied in solar installation kits or with some controllers. They are easier to install, and easier to insert fuses. It is also easier to "pull" them if you want to service the lines. This should only be done on smaller systems. With larger systems I use an air conditioner disconnect box with two legs of service on it (shown on the left). One leg (and fuse) handles the input side of the controller - the wire from the roof. The other leg (and fuse) handles the output side of the controller - the wire to the battery. So the wire goes from the roof into the disconnect, then to the controller, then back through the disconnect and on to the battery. This allows you to isolate the controller from all power by simply pulling the disconnect handle out. You can find these boxes at any Home Depot or Lowes in both 40 amp and 60 amp ratings. The SquareD boxes are DC rated. Make sure you use the appropriate fusing and that it is DC-rated; it is likely a larger fuse on the output side.

A neater solution to the requirement to be able to isolate the controller is to use a DC-rated breaker box and fuses. These days I almost always use a Midnite Solar Baby Box for isolation of the solar controller. The small extra expense is well worth it in my opinion. Wiring of these devices is covered in detail in this article on "Wiring the Solar Controller Disconnect", which is a downloadable PDF.

With MPPT controllers, and higher output (larger) systems, the #4 cable you used from the roof to the controller may not be enough for the run to the battery bank. On a 60 amp controller you should use #2 or larger to ensure that there is no voltage drop. Even if the current implementation does not require the larger wire, you may want to use it so that you don't have to rewire if you add panels - in other words, wire for the max output of the controller you are using. Consult the voltage drop tables, and ampacity charts. You need to ensure that you meet BOTH ampacity standards and voltage drop goals. Ampacity charts do NOT account for voltage drop! Remember that an MPPT controller can boost the amperage quite a bit under ideal conditions. Make sure you understand how to figure this and take it into account. It helps if your solar controller is located close to the battery bank. An MPPT controller can make up for voltage loss from the roof fairly effectively, and properly output the correct voltage for the battery charge stage. But if it is too far from the battery bank you will have voltage drop and there is no "machinery" like the controller to compensate for this.

Solar Array Wiring Considerations

In a small solar system that is typical of an RV it is pretty simple to design the array configuration. Most systems on RVs are 1-4 panels, and typically 12-volt panels. These usually have an output of about 135watts each, with an Isc of about 8.4A. Isc is the "short circuit" current (I) of the panel in ideal test conditions. It is the very max a panel can output, and is used for calculating wire sizes and controller sizing. On RVs, these panels are often wired in parallel - all plusses are joined together on the roof (and the negatives) at a combiner box and a single pair of larger wires is used to bring the power down to the solar controller. When wired in parallel in this fashion the voltage stays the same (lets say Vmp of 17.7), and the amperage is cumulative. In this example say 8.4A*4 panels=33.6 amps. You have to send this amount of current (power) down to the controller. At this low a voltage (12-volt nominal) you have to be careful of the wire size so that the voltage is not reduced too far over the length of the run - especially with an MPPT controller that "likes" high voltage.

Lets take an example of 20' of wire run to the controller from the combiner box on the roof - which would not be atypical on an RV. Look in the voltage drop table above for 35A and 21' and you will find you need #2 wire. Not good, since that is a pretty big wire size and expensive. But now lets look at reality: your panels are flat on the roof, and not well ventilated. They are not going to output the Isc, or likely even the Imp (current at "max power"), but lets use Imp at 7.63A * 4 = 30.52A. You are now pretty close to the #4 cable that I recommend that you always run. And in reality you will probably never see 30A off the array - you might out of the solar controller after boost though. But that should be a short run from the controller to the battery.

Since we are marginal in the configuration above, lets look at the effects of running the panels in series with a MPPT controller. Remember, all of the larger controllers are rated to handle a max of 150 volts. In reality, if you follow NEC codes you are constrained to less - say 145 volts.

With the same four panels in series (by the way, these are Kyocera KD135GX-LPU panels in the examples) the voltage combines, but the amperage stays the same. Exactly like with batteries. So in this case: Vmp*4= 17.7V * 4 = 70.8V, and the current stays at Imp 7.63. So you are sending 7.63A at 70.8 volts to the charge controller. And that fancy MPPT controller can take that high voltage, perform some magic on it, and output it at nominal battery voltage (lets say 14. 8 Volts on bulk charge) and whatever amperage is appropriate - generally the max it can push out in bulk mode on a small array like this. In this case lets say it is outputting the 30A from above and boosted it 10% to 33 amps. It is easy to see that with a wire run of 20' and a current of only 8 amps (I rounded up) you have no issue at all with #4 cable to the solar controller from the roof. So there is a lot of benefit to the higher voltage. And you can only do that with an MPPT controller.

But there is an additional complication. The MPPT solar controllers are most efficient when the voltage coming in is about 2 times the nominal battery voltage, more or less. They lose efficiency in down converting really high voltages on input to really low voltages on output. So for the most efficient array configuration we might want to reduce that voltage some and get it closer to 35V. We can do that with two panels in series instead of four (this is referred to as a "string") which will output Vmp * 2 = 17.7 * 2 = 35.4V. Much better. Now we have two strings of serial panels on the roof, and we will parallel them together at the combiner box: 35V @ 15A. How did I get that? Each string is 7.63A, since the two panels are in series. And when you parallel the strings the voltage stays the same, but the Amps adds: 7.63A*2=15.26A. Consulting the table (double it for 24-volt nominal) we have no issue with the distance or the cable size using #4.

One other consideration. You should only put similar (exact) panels in series. Voltage specs (Vmp) for all panels in the string (and all strings in the array) should be within .2-.3 volts of each other. Otherwise you introduce inefficiencies and reduce output.

You can see that it is not real simple to properly design a higher-powered system. But if you take your time and understand the concepts it is not rocket-science.

MC Connectors

MC connectors are a push-in type connector found on almost all higher-output solar panels today. They are now typically MC4 locking connectors, although there may be some MC3 connectors still on the market. MC connectors are integrated with the panels themselves - they are on the ends of the wire pigtails coming out of the panels. There are no longer junction boxes on panels, except in rare exceptions. The pigtails vary in length, but all are long enough to allow two adjacent panels to be interconnected without extending the pigtails.

MC connectors are a mixed blessing, IMO. They greatly simplify wiring, but they also add cost. Personally, I'd rather have a J-box; that is just me...Regardless, we are stuck with MC connectors. They do make for a fail-safe connection and are far less prone to installer error.

There is a positive and negative MC connector on each panel. You use MC extension cables to extend the wire run back to the combiner box. Buy an extension cable twice the needed length and cut it in half - that gives you a separate male and female MC connector, with a bare wire end to connect at the combiner box. If you are running strings of panels then for the serial connections between adjacent panels you can just plug the panels in directly to each other (as appropriate), and then run the opposite end(s) +/- back to the combiner. The extensions come with 12 gauge and 10 gauge UV resistant wire - make sure you get the 10 gauge. The pigtails from the panels are almost always #10.

If you parallel all the panels (or use two strings) then you might want to reduce the wire runs to the combiner. You can do that with MC "Y" connectors. They come in male and female.

This permits a 2-1 reduction.

With the MC4 latching connectors you need an unlatch tool, or you will struggle getting the connectors apart. This can be bought with the connectors. The MC4 connectors are waterproof, but it is still a good practice to wrap them with tape when finished.

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Cables and Battery Connections

I always build my own cables. First, it is cheaper and you generally get a better product. Second, it is difficult or impossible to get the wire lengths and orientation of the lugs correct if they are built commercially.

It is not difficult to build your own high-amperage cables, but you do need the correct tools and parts. For tools, you need a cable cutter that is capable of cutting at least 2/0 cable. Klein makes a compact cable cutter that will work - available at electric supply houses and Home Depot/Lowes for about $25. This will cut 4/0 with a little grunting. Believe me, it is worth buying. If you decide not to use a cable cutter you can cut the wire with a reciprocating saw or hacksaw - clamp it in a vice first. If you go this route you will have ragged ends - use a grinder to smooth the edges out. If you don't you will never get them into the lug - the lugs are pretty much the EXACT size of the wire. You can also use a dremel tool with a small cutoff wheel or even high-quality pruning shears work OK.

You also need a large crimper for the magna-lugs you will use. The picture on the left is an example of a hammer crimper that works acceptably well. You put the lug into the anvil and whack it with a maul. The alternative to crimping is to solder the lugs (more on soldering below). If you decide to solder I recommend Fusion lugs. These have solder and flux in the barrel of the lug. You stick the lug in a vice, heat it with a torch and when the solder melts insert the wire. The problem with this is that it is often difficult to get the wire in, and you have to fool with it. Difficult when you have a hot lug and limited time to get the wire in. I prefer to crimp.

Note that these lugs have closed fronts, and are tin-coated for corrosion resistance. They are pure copper underneath. The lugs, crimper and battery extension posts (see below) are available at Solar Seller, The Solar Biz, or at Wrangler Northwest Power. You will find it useful to call Wrangler Power (800-962-2616) and order their catalog. They have high quality parts, regulators, high output alternators, isolators, lugs, 12-volt fuse centers, etc. described in the catalog. Their website is very difficult to use. Solarseller has better prices than Wrangler, if they have the part. If you can't find wire locally you can get it from Welding Supply. They have colored wire for a reasonable price.

You also need an antioxidant, which is used on the wires before crimping. This helps prevent corrosion and decreases electrical resistance. Apply to the wires and rub in. Squirt a little into the lug before crimping. You should put antioxidant on all wires - no matter the size - before crimping. One brand name available at Home Depot/Lowes and Ace Hardware is Ox-gard.

After crimping you apply an adhesive heat shrink tube (color coded, of course) over the lug. Once melted, the adhesive totally seals the barrel of the lug and greatly minimizes future corrosion. You will probably have to mail order the adhesive heat shrink tubing .

For battery interconnections and the run to the inverter from the battery bank you can buy regular battery cable, or the highly flexible battery cable. For runs from the rooftop solar combiner to the solar controller the industry norm is to use welding cable - since it is highly flexible and far easier to handle. This is easily obtained at any welding supply house. Buy it uncut and cut it yourself when building the cables. You can also use welding cable for battery interconnects and the run to the inverter.

Hooking up the inverter cables is not difficult but there is only one correct way to do it. The positive feed originates from one side of the battery bank, and the negative feed from the opposite end (battery 1 and battery 4, in a 4-battery system). Diagonally loading the bank ensures that all batteries are drawn down equally. If you hook both leads to one battery - no matter which one - that battery will be supplying more of the load than the others, and will get more charge than the others. Rub a little Oxguard on the lug before bolting it down. You may have to drill the lug to a larger size, depending on the lug and the battery. You might want to measure the battery terminal bolt size before ordering lugs.

On the positive battery terminal feeding the inverter you need to insert a fuse of an appropriate size - 25% larger amperage than your largest load (or possible load) but also within the ampacity of the cable (this should not be a problem if you use 2/0 or 4/0 cable). Your inverter installation instructions should tell you the appropriate size. I use Buss type T-JJS DC rated fuses. This prevents accidental welding or other catastrophic shorts. To get the barrel of the fuse to clear the battery you may need a battery extension post, otherwise there is not room. Just bolt the lug to the fuse, and the fuse goes directly on the extension. Don't forget the Oxguard. You can wrap the fuse with good quality electric tape to minimize the chance of a short when using tools in the battery compartment. You can also mount the fuse in a fuse holder. On vehicles this is often difficult to do and still keep the fuse within a (max) of 18" from the battery. So I generally mount the fuse directly to the battery post on vehicles. On RV's I almost always use a fuse holder, since there is usually plenty of mounting room. Either approach is acceptable.

Route the cables from the battery bank to the inverter either parallel to each other and touching, or twist them around each other. This minimizes interference from the magnetic field that will emanate from the cables.

Hints on Cable Building

When you go to build the cables, build them one at a time - do not try to cut them all to length and then mass-produce the ends. You need to take into account twists and turns in the line - the lugs do not necessarily orient in the same direction. I have found that if you put one lug on and then take the entire cable bundle and lay the uncut cable out in its final position (or approximate it), then place the uncrimped lug on its bolt and actually lay the cable across the lug you will get the exact length you need. Make sure to place an orientation mark on the uncrimped end so you know the angle of the lug on the wire. Otherwise you may find you have a very twisted wire because the lug is rotated into an inconvenient position. Repeat for each cable. Typically, you do not leave extra length in inverter-feed cables. Every foot counts against you for voltage drop, so make the cable runs as short as possible. Also, when building battery cables leave some extra length on the interconnect cables. When you replace the battery bank, the next brand of batteries may have the terminals in slightly different places - you do not want to be forced to build new interconnect cables. An inch or two extra is enough.

Solder or Crimp?

Many people will tell you it is always best to solder, but that is not true. It depends on the circumstances. In dealing with a mobile environment that is full of vibration soldering can be problematic. Soldering a wire makes it stiff and inflexible. It can easily break over time where the solder joint meets the unsoldered wire. Look at marine and aviation wiring - it is rarely soldered.

Soldering is also not UL approved. The issue is that if the wires heat up the solder joint will melt and the joint will fail - often causing a fire.

Here is my advice. Take it for what it is worth. I solder all small wires that absolutely need a good connection - brake wires in trailer brake controllers are an obvious choice for soldering. In this case the benefit overweighs the potential downside. I would never solder a wire bigger than #8, and generally I avoid soldering lugs on any wire. For large solar or high-amperage wires (#6 and bigger) do not even think about using a soldered connection. It is far better to PROPERLY crimp the wire.

Battery-to-Inverter Wiring

Your inverter manufacture will provide a chart that will tell you the requirements for wire sizes from the battery bank to the inverter. They will also specify the fuse size required on the positive line at the battery. You need to follow the manufacturer's instructions carefully. If you under wire this connection you can start a fire or damage the equipment. Don't cheap out and delete the fuse. The fuse protects your RV from fire if the inverter malfunctions. You definitely don't want your RV burning down.

Lets assume you are using a 2000 watt inverter. In this case, you will use 2/0 or 4/0 cable to connect from the battery bank to the inverter, depending on the distance. The shorter the distance, the better, but your inverter can be as much as 10-12 feet (of cable run) from the battery. Use the heavier 4/0 cable if you are even close to the rollover point between the cable sizes. Bigger is always better when it comes to cabling the battery/inverter.

The 300-400amp fuse (use the size specified by the manufacturer) should be mounted within 18" or so of the battery. You can mount it in an appropriate fuse holder, or you can bolt it directly to the positive post of the battery if you have no method to mount a fuse holder. The fuse holder is preferred.

The wires to the inverter will put out a magnetic field that can affect electronics in the RV - especially the AM band on the radio, and potentially TV's. Usually, this is not a problem, but it can be. To minimize the potential for this interference you can twist the cables around each other, or parallel them together. It is usually easier to parallel them together, using a tie-wrap every foot or so. This results in the magnetic fields canceling each other out.

AC Wiring - Interfacing to Your Load Center

There are some major considerations in interfacing to the load center (the circuit breaker box) that can drive the entire system design and complexity.

The first is determining if you will use an inverter with a transfer switch rating that matches the capacity of the main breaker in your load center. In other words, are you using a 30 amp-rated inverter in a 30 amp RV, or using a 30 amp-rated inverter in a 50 amp RV? There is a major difference in system design and capabilities. If you are not installing a sub panel, it is best to match the rated capacity of the inverter transfer switch to the RV AC capacity – a 30 amp inverter with a 30 amp RV; a 50 amp inverter with a 50 amp RV. You pay more for a 50 amp inverter, but you will make it up on ease of installation and system design considerations.

The second major consideration is if you will install a sub panel for the inverter loads. Use of a sub panel to isolate the inverter loads is technically the best design, but practical considerations may lead you away from this implementation. The use of a sub panel isolates the lower power circuits that you will supply inverter power to, from the high amperage circuits that are impractical to support with a battery bank. Typically, these high amperage circuits are the air conditioner's, the electric hot water heater, the converter (if it is left in the system) and any other high-amperage appliance circuits, including the refrigerator. In addition, the 120-volt lighting circuits are usually left in the main panel. Circuits that are moved to the sub panel are typically the wall outlets and the microwave. This includes the entertainment center, since this is typically driven off wall outlets.

AC Wire Types

RV manufacturers all use regular house wire for the AC feeds in RV’s. The exception is the actual shore power cable coming in, which is usually stranded wire. Type NM wire is solid copper wire and is generally what is used for the AC distribution wiring.

However, note that the NEC would require stranded wire to be used in RV construction, but not a SINGLE manufacturer of motorized or towable RVs does so. They use standard residential wiring and wiring techniques for the AC distribution system. This causes "issues" down the road (literally, down the road). Mostly in vibration loosening connections in loadcenters and transfer switches. It is a good idea to add checking these connections to your yearly maintenance schedule.

A better wire is boat wire, which is stranded and tinned. Stranded wire stands up to vibration better, but is much more expensive. It is required for marine use by Coast Guard regulation. In the past I usually continued to use standard house wire because the coach is already wired with it, so upgrading just a portion of it is not usually justified. However, I have recently reconsidered this due to the number of burned and loose fittings I have seen in RVs with solid wire. It is best to use the stranded wiring when retrofitting/adding on.

If you do decide to use solid wire in an RV make sure you tape the wire nuts to the wire (when using wire nuts). A better solution than standard wire nuts for solid copper wire ONLY is the newer "push-in" wire connector shown in the picture. These are available at all the home stores. I use them exclusively for electrical work now. Using tape or the push-in connector will prevent vibration from loosening the connection, which it can, over time.

Grounding

Unless your inverter manufacturer states otherwise, you may directly ground the house battery bank to the chassis. All other DC system ground will be carried back through the chassis ground. There is a DC ground point on the inverter itself ( a safety ground for the case). It must also be grounded to chassis at any convenient point. Make sure you use the proper size wire.

Some inverter manufacturers specify that the battery bank not be directly grounded to the RV chassis. All DC grounding is to originate at the inverter and the DC loadcenter. If your inverter manufacturer specifies this method of grounding, you need to follow it.

Neutral Bonding

Most high-power inverter chargers intended to be hardwired have an AC neutral-to-ground bonding system. This bonds neutral to ground while inverting, and disconnects neutral from ground while on shore power. The purpose is to satisfy code requirements that specify neutral-ground bonding can only occur at one location. The utility power feeding the inverter will have neutral bonded at the electrical panel; therefore the inverter must not have neutral bonded when on shore power. This is the same reason that RV’s NEVER have neutral bonded to ground in the RV electrical panel. Neutral and ground must float in an RV electrical system. When doing the AC wiring to the inverter, do not connect the AC input neutral directly to the AC output neutral; use the separate connection lugs provided. Otherwise, you will circumvent the neutral bonding system.

I mention this mainly because installation of some inverters can cause an anomaly when hooked to shore power circuits protected by a GFCI or AFCI. That is, the GFCI may be tripped by the inverter neutral-to-ground bonding relay. This occurs because the GFCI relay that detects a neutral-ground short (potentially a dangerous condition) is “faster” than the inverter neutral bonding relay. When shore power is connected, power passes through the (normally closed) inverter bonding relay before it can be activated (opened), and back to the GFCI. This causes the GFCI to detect the neutral-ground short and disconnect the power. All this happens in milliseconds, and is typical in “driveway boondock” situations where you may be plugged into a friend’s garage outlet, or on an outdoor receptacle (including those 20 amp outlets in RV pedestals) – all of which are required to be GFCI protected. There is no way to circumvent this, other than to find a non-GFCI outlet. Look at the garage door opener outlet; code does not require that to be GFCI-protected (but it may be anyway). Note that some inverters have provision to circumvent the neutral bonding system. Do not do this unless you know what you are doing.

Installing a Sub Panel

When retrofitting a sub panel to an existing system there are two major issues; locating the panel, and having enough wire length in the existing circuits that you want to move to reach the new panel location. The sub panel is typically protected by a 30 amp breaker in the main panel, and is fed by 10 ga. wire. It can be located anywhere that is practical to reach – in a 5th wheel it is often located in the main storage compartment. It is unusual to have enough slack in the existing circuits wire to reach the new box location – even if the sub panel is co-located with the main panel. If you are lucky, all the wires will feed the main panel from below, and you will be able to pull them down and install the sub panel below the main panel. Often, the circuits to be relocated have to be extended. If the main box has enough room you can simply use connectors and add the required wire to reach the sub panel. You have to extend all the wires, including the neutrals and grounds. Often, the main panel does not have enough room in it to splice in the new wire extensions – in that case you have to add a junction box near the main panel that contains the spliced wires. Make sure that you tape your wire nuts to prevent them from working loose from vibration.

Vehicle Electrical Center 3.gif (17592 bytes)

The sub panel should be sized to handle the number of circuits that you need to move. If you can, get a panel that has at least one extra circuit. It does not matter if the sub panel is the same brand as what is already in your RV. Usually a 60 or 70 amp panel containing four to six circuits is sufficient. When shopping for panels you will have a choice of “main breaker” or “main lug” panels. Main breaker panels contain a breaker controlling power to the entire panel. Main lug panels have connectors for the input wires but no breaker for the input. They depend on a properly sized breaker in the main box to control over-current conditions. The easiest box to use is a main lug box, because it has separate neutral and ground buss bars (or provision for an add-on ground buss bar). Main breaker boxes do not usually have separate buss bars, but have space on a common buss bar for both neutral and ground wires. You must maintain a separate neutral and ground in RV electrical systems. Neutral and ground wires are never joined on the same buss bar, as in residential wiring. When hooking up your wires, make sure that the neutrals are attached to the insulated buss bar.

An additional advantage to using a sub panel is that shore power is not being fed through the inverter transfer switch for your high-draw appliances, like the air conditioner. At least in theory this should prolong the life of the transfer switch, since it is handling less power in normal use. (Remember, all power is passing through the transfer switch for the inverter circuits even when the inverter is not in use.) In practice, it is unlikely to make a difference, since transfer switches are typically tested at 100,000 cycles at rated power.

Use of a sub panel also allows you to “mix” shore power and inverter power use. Even when hooked to shore power, you can flip off the 30 amp breaker that feeds the sub panel and your inverter will then supply power to the circuits in the sub panel, while shore power will supply the heavier loads like the air conditioner. Why would you want to do this? Well, to save power when you are being metered. Or to use your converter, which you wisely left on the main panel, to supplement solar when hooked up to marginal shore power – like at a rally or parked in a friends driveway.

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Powering the Entire Load Center

The alternative to a sub panel – especially in the case of a 30 amp load center – is to power the entire load center from the inverter. This is often called placing the inverter “inline”. In this case, no circuits are moved to an isolated sub panel so it is up to the user to manually “manage” AC power use when using the inverter. This requires that the user either not turn on the devices that draw too much power (such as the air conditioner, or hot water heater), or that the breaker supplying those devices be turned off. The only issue in this design is that you will forget and use a high-power device, and that it will drain your battery bank. If using this technique you need to turn your refrigerator to “Propane Only”, not to “Auto Select”, because most refrigerators will default to AC power if it is present.

In the past this design has worked best on a 30 amp system, because a 30 amp system only has a single power leg and most inverters only support a single power leg. So it is a simple matter to intercept the main shore power line and divert it through the inverter, and then to the load center. The inverter is “inline” before the load center. Everything hooks up cleanly.

The Xantrex RV line of inverters (RV 2000) have a configurable 50 amp transfer switch which allows you to safely support a 50 amp RV. This same inverter has dual AC input connections – meaning that it supports power on both legs of the input line. This means you can power the entire load center of a 50 amp RV, although you are still limited to the specifications of the inverter. This is the first inverter that is designed to handle both legs of a 50 amp RV shore power line. It makes installing an inverter into a 50 amp RV much easier, since splitting the box, reorganizing circuit locations, or adding a sub panel is not required (although a sub panel is always a superior solution, technically). The RS3000 also has a split-phase 50-amp input/output.

Placing the inverter in-line with the shore power requires that you have enough shore power feed wire to insert the inverter. This is rarely the case; usually you will have to add wire to insert the inverter. You can do this 2 ways. The first is to disconnect the main shore power feed from your load center and pull the wire back to the inverter location. You can splice it if you have to by adding a junction box (remember to tape your wire nuts). Then run a new wire from the inverter to the load center. The second method is to disconnect the shore power wire in the load center. Then run two new wires from the load center to the inverter location. The first new wire is spliced to the existing shore power input inside the box and supplies the input to the inverter (splice all hot, neutral and ground wires); the second new wire acts as the output wire from the inverter and supplies the main breaker in the load center. Use the proper size wire for the inverter transfer switch (10 ga for 30 amp, 6 ga for 50 amp).

“Splitting” a 50-ampere Load Center

The process of “splitting” the box refers to taking one leg of the load center, and sending only that leg through the inverter prior to powering the circuits on that leg. (Shore power comes in the shore power line, one leg goes through the inverter, and then to the main breaker. The other leg goes directly to the RV main breaker.) Thus, the inverter can supply power to one (and only one) side of the load center. This is done to avoid the difficulty of adding a sub panel. I do not recommend doing this - but I will describe it for you.

A 50-ampere load center is supplied with two 50 ampere power legs (plus the neutral and ground). Inside the load center the “red” leg supplies one half of the box, and the “black” leg supplies the other half of the box (they may actually both be black wires). There is an attempt made to “balance” the load on the two sides of the box when circuits are attached. That’s why units with two air conditioners typically have an air conditioner on each “leg”, or half of the box. The other circuits are located so that in typical use the electrical draw is approximately the same on each of the legs. This is done because the loads on the hot legs will cancel each other out, and thus the neutral line will carry no load (or a very small load, the difference between the two legs). Notice that the neutral is the same wire size as the two hot lines. If you have a grossly unbalanced system (say 80 amperes on one leg, and zero on the other leg) then the neutral line could be overloaded (it will have 80 amperes returning on it). The reason this is important will become apparent shortly.

So what if one side of your load center does not contain all the circuits you want to power with the inverter? You will have to re-organize the circuit locations in the box so that the circuits you want to have inverter power are on the leg supplied by the inverter. But in doing so, you have to make sure you maintain some degree of balance between the two legs. In a “split” box, just as when you power the entire load center, it is up to the user to manage the electrical loads – don’t turn on high-draw loads or you will kill you battery bank.

The other consideration in splitting a 50 amp system concerns the inverter transfer switch. Notice that the entire load of one leg is going through the inverter transfer switch when on shore power. The inverter transfer switch carries a rating. Most inverters have a transfer switch rated at 30 amps. Some inverters have a transfer switch rated at 50 amps. If you use a 30 amp transfer switch in a 50 amp system you are potentially overloading the transfer switch - you need to use an inverter with a transfer switch rated at 50 amps. Or, if your RV is not using all its capacity (50 amps on each leg) then you could reduce the main breaker sizes to 30 amps. This would reduce your total usable power to 60 amps (30*2)from 100 amps (50*2). This may not work in all RV’s – only you can decide by evaluating your power use. If you do use an inverter with a 30 amp transfer switch in a 50 amp system you must reduce your breaker size or you could overload the inverter AC wiring (or add a sub panel). Some inverters have AC input breakers that can catch this, but many do not. The Xantrex RV 2012 line of inverters have a configurable 30/50 amp transfer switch which allows you to safely support a split box. This same inverter has dual AC input connections – meaning that it supports power on both legs of the input line. This means you can power the entire load center of a 50 amp RV, although you are still limited to the specifications of the inverter. As discussed above, the major benefit of supporting both legs of the input line is that you do not have to reorganize the circuits in your RV load center in order to have the inverter supply power to them. This inverter is very convenient for retrofitting into a 50 ampere RV, but it does have some negatives, such as no ability to turn off the battery charger, and no equalization mode. The RV 3000 has none of these flaws, but costs more.

Note that when discussing the two power legs it is common to refer to “sides” of the box. In reality, a leg does not supply the breakers on one side of the box – the breakers for a leg are “every other one” on both sides of the box. Take a look at an empty load center and you will see. If you do split your box it is best to mark the breakers that can be supplied by the inverter. I use a drop of white paint on them – a bottle of auto touch-up paint works well.

Remember, it is best to add a sub-panel, but if you decide to split your box make sure you understand what you are doing. If not, get help. It is not really that hard, but you can screw things up if you do it wrong. You don’t want to burn your RV down!!

In my opinion splitting the box is a lousy idea. You are far better served by putting in a subpanel.

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Monitoring and Control

There are two capabilities that you need; the ability to control your inverter remotely, e.g. turn the inverter on/off, turn the battery charger on/off, and start the equalization process, and the ability to monitor the electrical state of your battery bank and inverter. The battery monitor tells you what is currently happening with your battery and electrical system, and what happened in the past. You can have the best system available, but if you do not properly monitor it you will still have problems. A good monitoring system will allow you to make usage decisions, evaluate the effectiveness of your system, and create peace of mind based on data, not guesses. Human nature being what it is, the monitoring system is often the place people try to save money. That is a serious mistake. Let me suggest that you view your system a little differently than you might of. Take the perspective that your monitoring system is as important as the inverter and solar regulator. Each of these components contributes equally to the success of your implementation. The availability and choice of the monitoring and control system usually is one of the primary determinants of what combination of solar controller and inverter I choose.

The components of the monitoring and control system can vary, based on the solar controller and inverter you select, and the functions that are required. In most cases you will need a separate battery monitor, in addition to the control instrumentation for your inverter. At a minimum, you need the following:

The ability the see cumulative amp-hours into and out of the battery bank, in DC amps, to tenths (e.g. 13.6 amps used). This “running amp-hour” meter function is the heart of your system. It is the most accurate way to determine your battery DOD (depth of discharge), and to monitor your usage habits. The “fuel gauge” type displays present in most of the monitors can supplement this capability, but is not a sufficient measure. If you don’t design for this feature to be present, you will be buying it later, at a greater cost.
An instant amp-hour measure. How much current am I putting in, or taking out, of the battery bank right now. This is the measure you will probably display on your monitor as the default. It is what we look at first, and it will allow you to measure the draw of all AC and DC appliances, the output of your panels going into the battery bank, and if any lights are left on at night. By watching this measurement you will quickly get a feel for your power usage and will be able to identify and diagnose any problems that might be occurring.
Battery voltage. You won’t use this as often as you might think. State of charge of the bank is primarily determined from the number of amp-hours you have consumed. Voltage is never a good indicator of state-of-charge in a bank that is under load. Primarily, you will watch the voltage being applied to the batteries change as the different stages of charging occur. Once you learn and understand this, you probably won’t refer to voltage very often.
Control functions: you need to be able to turn the inverter on and off, turn the battery charger on and off, and control the genset, if you have one. Usually, the genset is already in place, with its own controls, but if you are adding a generator on the tow vehicle, and want remote start then you need to design for this. Some monitor systems have generator start and management functions.

Link 1000 above, AC line monitor below - click to expand

These are the minimum functions you need, in my opinion. Anything else is optional, but you may feel like you need it. Personally, I like to know everything that is going on, but realistically it is not required. For example, it is nice to have a monitor for the solar controller, but since there is really nothing to control (other than possibly equalization), it is really not necessary. You can see the charge amps and voltage on your battery monitor. The only measurement that is missing is input amps and voltage coming into the regulator. This is interesting if you want to see how much “boost” you are getting from an MPPT controller, or just to see how much current is lost in wire runs. My solar controller display is behind a door, and I rarely look at it.

You need to place the displays where you will see them. They are no good hidden away. Our displays in our current rig are right inside the door, where they are easily seen. In another rig they were in the hallway to the bath. You will find you will refer to them often, especially when you are learning the operational characteristics of the system.

In my opinion, and it is shared with many industry experts, you absolutely need to measure cumulative amp hours of your battery bank. There is simply no other way to effectively know the current state of the bank. If the instrumentation that is available for the inverter does not provide for this, then you need to augment it. If you are going to augment your inverter monitor/control, then buy the cheapest control panel available for the inverter (making sure that it allows control of all functions). Augment the control functions with monitoring functions provided by a TriMetric TM2020 monitoring system. This provides all the monitoring functions you will need for your entire system, and is commonly available for around $150, including a 500 amp shunt. You can check out the TriMetric at Bogart Engineering. You can buy the TriMetric at Solar Seller. If you want to monitor more than one power source individually, check out the new Pentametric battery monitor from Bogart. It can monitor 3 charging sources at once. But it is not cheap. Don't view the purchase of the TriMetric as spending an "extra" $150. If you can not determine the state of your system accurately you will have continual problems down the road.

Recommendations

On a 30 amp RV I would wire the inverter in-line and power the entire load center. This makes installation simple and ensures that the circuits you want powered are available, since all circuits are available when inverting. I definitely would not bother with a sub panel.

On a 50-ampere RV I would add a sub panel if it was at all possible. There are so many advantages to this approach that it makes it worth the extra trouble. This would permit you the flexibility of using a 30 amp inverter if you choose. If a sub panel was not possible, or you choose not to go through the headache of installing it, I would use a 50 amp split-phase inverter that allows for "in-line" installation (if you can find one). As of 2012 I know of no inverters currently on the market that allow 50 amp pass thru on two legs of service. The "60 amp rated" inverters actually restrict EACH leg to 30 amps.

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Using a 30-Amphere Inverter in a 50-Amphere RV

Design Considerations for RV Genset and Inverter Installations

Most modern, large 5'ers and motorhomes have 50 amp shore power (a 50 amp RV actually has 2 - 50 amp lines, each feeding one side of the loadcenter, for a total of 100 available amps). This section assumes the following issue, which is fairly common: If you have a 50 amp RV, how do you safely integrate an inverter, or inverter/charger that has an internal transfer switch rated at 30 amps? This design is specifically done to address the issue of an inverter/charger with a 30-amp transfer capability used in an RV with 50-amp service. The purpose of the design is to circumvent the limitations of the 30-amp transfer switch, and to avoid using a sub panel for the inverter-fed circuits. The best design is always one that isolates the inverter circuits to a sub panel, but that is not always practical in a retrofit implementation. Most people do not need to read this section.

Given a choice, I would always purchase an inverter/charger that has a 50 amp transfer switch. This would allow you to avoid the complexity of TS2 in this design and allow you to place the inverter directly in-line. Some manufacturers of high-powered inverter/chargers now offer a model with a 50 amp transfer switch. So if you are buying new, do yourself a favor and get one - it will simplify the installation.

Inverter with a Separate Battery Charger

The simplest system is shown in the conceptual design drawing to the left. The major difference in this design is that you use an inverter, instead of an inverter/charger, and a separate battery charger.

The separate battery charger is required because you can't run 50 amps through the inverter pass thru relay - the original problem. This stops you from having 120 volt available to the battery charger, since the circuits inside the inverter for pass thru and battery charging both operate on the same 120-volt input.

Inverter/Charger with an External Transfer Switch

In the design below, we use an inverter/charger, but do not use the transfer function, protecting against over current.

This design utilizes the battery charger in an inverter/charger but does not use the power pass thru function. The input AC is fed directly from TS1, avoiding the RV loadcenter. This input AC to the inverter is ONLY used for battery charging, and is never passed on to the RV loadcenter. The reason the input is taken off of TS1 directly is to prevent a back feed situation from occurring if TS2 fails. (If power was obtained from the loadcenter and you were inverting then you would be in a circular input loop.)

In this scenario, the battery charger is supplying the battery bank, and power is passing through the pass thru relay, but it is "stopped" by the TS2 transfer relay, which is set to "prefer" the other input. This way no 50 amp load is ever placed on the 30 amp inverter transfer relay. Power only passes to the loadcenter from the inverter if there is NO power available from TS1 (no shore or genset). In that case, if the inverter is in invert mode, power will pass from the inverter to the RV loadcenter.

The Preferred Design - All Charging Sources Integrated

This drawing provides an overview of the RV electrical system, and identifies major components used to support battery charging via: solar energy, the existing converter, and a new, high-powered battery charger contained in the inverter. It is a more detailed view of the design directly above.

Electrical input sources include a genset (either a portable, an RV mounted or a truck mounted), and shore power sources. Optionally, two main shore power sources are shown, controlled by a separate 50-amp transfer switch (TSO). These are intended to provide for a shore connection at the front of the 5er, and at the rear of the 5er. The existing converter is shown connected to an external power source (other than the RV) for optional use. This should never be plugged into the RV, but only to an external source via an extension cord.

Assumptions:

The RV is wired for 50-amp shore power. This is actually 2-50 amp lines, for a total of 100 amps available at the loadcenter, on 2 legs. All shore power is assumed to be using 6ga wire, except where noted.
All the transfer switches are 50 amps.
The inverter is an inverter/charger with a high output battery charger that replaces the RV converter for normal use. The inverter has a pass thru power capability controlled by a 30-amp transfer relay.

When inverting, the loadcenter is fully energized. It is up to the user to provide manual load management. In other words, don’t turn on the air conditioner, electric hot water heater, or other large loads. Turn the breakers off, if you are prone to forget.

A 400-amp catastrophe fuse is used to protect against an inverter short. It is placed either directly on the battery positive (if not placed in a fuse holder), or as close to the battery as possible, if a fuse holder is used. Use the size appropriate for your inverter. See the Truck Electrical Center page for additional details and sources for the fuse and other components.

The shunt is a 500-amp shunt. It must be placed “downstream” of all loads to get an accurate measure of amps/amp hours. Place it between the distribution hub and the battery negative.

Use appropriate size welding cable for the DC inverter runs. Consult the inverter installation instructions. Do not use less than 2/0. I prefer to use 4/0 in most situations if the inverter is 2000 watts or more. The inverter should not be more than 10 feet from the battery (cable run).

Note 1

Optionally, I show two main shore power cables. When using an external generator (either portable or truck mounted) it is often convenient to have a shore power cable at the front of the rig. You simply use another 50-amp transfer switch – that way you can’t have both “live” at once, or energize the other plug. This is obviously optional, but when wiring the transfer switches and deciding where to break into the main shore power cord you might consider leaving enough slack in the line to accommodate a future transfer switch if you decide not to do this right away.

Note 2

The line designated by Note 2 is the 120-volt AC input to the inverter. The inverter normally would pass this thru the transfer relay and on to the AC loadcenter. The issue is that the transfer switch is rated for 30-amps and the potential load on a single leg of the shore power line is 50 amps. The transfer switch built into the inverter can be overloaded. The purpose of TS1 is to circumvent this issue and still allow use of the high-powered battery charger in the inverter/charger. Thus, AC power needs to be supplied to the battery charger, but the power passing through the inverter must be stopped from reaching the loadcenter. The inverter will pass power to TS2 when shore power is available, but since TS2 is already receiving shore power on the preferred input, the power from the inverter is blocked. Thus the relay in the inverter can never be overloaded, since no load is ever placed on it (when there is shore power available). However, the battery charger is energized, and can charge the battery bank (assuming the inverter control is set to charge). Power is taken directly off the TS1 transfer switch output lugs. The line to the inverter from TS1 is a single 120-volt line, so only one of the hot terminals is tapped (along with ground and neutral). Since the only load on the AC lines into and out from the inverter are from the 1) battery charger and 2) the inverter, while inverting and passing power up to the loadcenter, these lines do not have to be sized the same as the main shore power lines. Depending on the size of the inverter, and the max "surge" of the inverter, you could use 12 ga. I recommend 10 ga., to cover any additional surge capacity various inverters might have. Each inverter will be different.

When wiring the input from the inverter to TS2 you need to jumper the hot lines. The inverter is only passing one hot line through, and you want to energize both of the lines going to the loadcenter. It is permissible to jumper between the two hot lines on the input side of the TS2 inverter line to support this.

Note 3

Distribution hubs are used for DC power connections. The existing house DC wires that feed the DC loadcenter are not shown in the drawing, but they should be moved to the distribution hubs. Typically, a wire goes from the converter directly to the battery, and another from the converter to the DC loadcenter. If you are leaving the converter in place you can remove the converter-to-loadcenter wire, and splice a new wire into the line that goes from the battery charger output of the converter to the battery. (Your converter outputs may be different, but you get the idea...)

The solar input and conventional converter inputs attach directly to the distribution hubs. You should attach all DC power input/outputs here. Nothing should attach directly to the battery except some of the instrumentation and monitoring lines, and possibly the DC catastrophe fuse (if not in a holder). If you have additional DC loads you are adding, you may want to add a small DC fuse center, which would also attach to the distribution hubs. I usually add one to support fusing for the solar lines, and some of the instrumentation lines which otherwise require inline fusing (which is not as neat, and not centrally located).

Instrumentation

In the diagrams, the dotted lines denote instrumentation lines. These are not shown in detail – there are multiple connection points and lines for each instrument. Follow the instructions.

Sometimes the solar controller will have a remote display, and sometimes the entire controller will mount where the display can be seen – it depends on the controller you use. If you have a choice, acquire the remote monitor for the solar controller. It will make the wire run for the solar line shorter. The solar line should be as short as possible to minimize voltage drop. I prefer to use 6 ga for the drop from the roof to the battery bank. Sometimes that means you have to trim the wire where it goes into the terminals on the solar controller in order to make it fit. That is OK. On the roof, I interconnect the solar panels with a minimum of 10 ga.

If the inverter monitor panel has a running amp hour capability (also called cumulative amp hour) then you can eliminate the Trimetric amp hour meter, since it would be redundant. If not, you really need to know your accumulated amp hours (either positive or negative), since that is the best measure of the state-of-charge of your battery bank. You can buy a Trimetric meter, with shunt, for under $175 at www.solarseller.com. If you are using a Heart inverter, the Link 1000 or Link 2000 monitors contain an amp hour meter.

Again, most people do not need to be concerned with this section and it's complexities. This is NOT a preferred design!

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AC Circuit Protection

When revising your electrical system you should consider the addition of an AC power management system. These provide protection from miswired pedestals, high and low voltage conditions and surge suppression. They have a remote display that shows you line loading (so you can figure out exactly how much AC you are drawing on each leg of your box). It is best to use models that are hardwired, instead of external surge guards. Hardwired models are theft proof, and you won't forget to put them out. The time you forget to plug it in will be the time you really need it. They are available in 30-amp and 50-amp versions.

The Progressive Industries model 40240 (50-amp model, $494) is available at Camping World and other outlets. I highly recommend this capability – not only do you know what is going on with your AC loads, but you are protecting your coach AC system. From a design perspective, I prefer placing the device directly “next to” the loadcenter if you are using a sub panel. If wiring the inverter in-line then place the management system before the inverter. This will offer protection to the inverter from surges and spikes.

In my opinion, EVERY RV should have an electrical management system like the Progressive Industries version with the remote panel.

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Sample Systems

The sample systems I've configured here are just that - samples. You may not agree that they are the best combination of components for your particular situation. Everything is a tradeoff when it comes to RV's and this area is no different. However, these should give you a good place to start when you configure your own system. There are also some detailed sample systems (with pricing as of 5/2007) that are a little different in the downloadable Excel file Solar Bill of Materials.

High-end System

For the individual who boondocks a lot, and wants a technically superior system, that is easily expanded. This is a top-end system that no one would be unhappy with. Cost is not the major consideration - it is not cheap. If you really push your system and need the best, then this is (arguably) it. Also see "My Current System" (below) for another take on a high-end system.

Outback FX2012 Sine Wave Inverter 2000 watt/50 amp pass thru
Outback MX60 PV charge controller (MPPT)
Outback Mate Monitor
4- Kyocera KC-130 Solar Panels. Hook them in series/pairs. Best price/size/performance tradeoff. You can add lots more panels with the 60 amp controller.
6 – LifeLine GPL-4C 6 volt AGM batteries (660 Ah rating) or
6 – Trojan L16RE 6 volt batteries (975 Ah rating)

Medium System

For those who want good performance, but price is more of a consideration. The system is not as easily expanded as the high-end system.

Xantrex RS2000 Inverter (on 30 amp system), or RS3000 Inverter (on 50 amp system). Both are Sine Wave.
Xantrex SCP (System Control Panel).
Blue Sky Solar Boost 3024i charge controller (30 amp MPPT). Can handle 24-volt input. Add the IPN Pro-Remote display for battery monitoring. If you want more expandability there is a 50 amp version.
3 - Kyocera KC-130 Solar Panels. Best price/size/performance tradeoff. You can add one more panel with the SB3024i controller. If you need more than 4 panels later, then you can add another controller and network them.
4 – Sam’s Club 6 volt Golf Cart batteries (410 Ah rating). Expand to 6 if you need more reserve, but you will probably want to go with an extra solar panel if you have 6 batteries.

Economy System

For those who want good performance, but price is the major consideration. The system is expandable, but uses lower-cost components.

Heart (Xantrex) 458 Modified Sine Wave Inverter 2000 watt/30 amp pass thru.
Trace C40 charge controller. PWM controller, not an MPPT. Can handle 24-volt input.
Link 1000 Monitor. Has cumulative amp hours.
3 - Kyocera KC-130 Solar Panels. Best price/size/performance tradeoff. You can add three more panels with the C40 controller.
4 – Sam’s Club 6 volt Golf Cart batteries (410 Ah rating). Expand to 6 if you need more reserve, but you will probably want to go with an extra solar panel if you have 6 batteries.

On Our 2010 New Horizons

Xantrex RS 3000 pure sine wave inverter/charger. 50-amp pass thru; wired to a subpanel. 150-amp charge section.
Xantrex XW 60-amp MPPT charge controller.
SCP networked to both the inverter and charge controller. This provides complete control and monitoring of both devices.
Trimetric 2025RV battery monitor.
4 - Sun Electronics Sun SV-T-205 HV panels. Wired as a parallel array. These are the same as Evergreen Panels. They are not UL listed, which gives a far cheaper price. Imp=7.36 A at Vmp=27.90V. MC-4 connectors. Not presently on the rig but everything is set up for them.
AM Solar Large combiner box.
Midnite Solar "Baby Box" enclosure with 2 breakers to isolate the solar controller input/outputs.
6 – Trojan T-105 6 volt batteries (675 Ah rating). In retrospect going with 4 - L16RE batteries would have given me 650 Ah and it is likely a better battery. But the deal on the T105s was too good to pass up. I'll likely replace with the L16s.
5500 watt LP genset.

On Our 2012 New Horizons

Magnum MS2812 pure sine wave inverter charger. 2800 watta with a 125 ADC charge section. ME-RC remote display.
Magnum BMK battery monitor uses the same display as the inverter.
Morningstar Tristar MPPT 60 solar charge controller with remote panel.
4 - Sun Electronics Sun SV-T-205 HV panels. Wired as a parallel array. These are the same as Evergreen Panels. They are not UL listed, which gives a far cheaper price. Imp=7.36 A at Vmp=27.90V. MC-4 connectors. Installed by New Horizons.

AM Solar Large combiner box.

Midnite Solar "Baby Box" enclosure with 2 breakers to isolate the solar controller input/outputs.

Four 8D Lifeline AGM batteries for 1020 Amphours of stored power. Half is usable.
5500 watt LP genset.

The system I had in the Royals International is similar to the economy system, with a Trace C60 PWM charge controller and four KC-120 panels feeding 4 Sam's Club golf cart batteries.

RV Electrical and Solar

AC Wiring - Interfacing to Your Load Center

Neutral Bonding

Instrumentation