Last updated: 08/17/2008

RV Electrical and Solar
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Copyright © 2002-2008 John Mayer. All rights reserved. For reuse policy see Reuse Policy

In this Section:

Introduction to Solar
Solar Panels
Solar Controllers
Xantrex Inverters
The Battery Bank
Installing a 30-Amphere Inverter in a 50-Amphere RV
AC Circuit Protection
Sample Systems
 

Warning:
 
In this section I describe various wiring techniques and electrical designs. These generally conform to code, but it is up to YOU to determine their suitability to your situation. DO NOT take this as electrical advice, only as possible design considerations. If you do not understand basic residential wiring and 12-volt automotive wiring then you should not undertake any of these implementations without further help and advice. If in doubt be sure to get help. Electricity is dangerous. The high amperage DC electricity obtained from the large battery banks described here is suitable for welding and can easily kill you. Do not underestimate the danger involved in working with DC power.

 
I'm assuming that you have a basic understanding of both 12-volt and 120-volt power. There are many excellent tutorials and books on both subjects. I won't repeat the information here, but leave it to you to explore on your own if you don't understand the basics. For basic 12-volt electrical info, try Mark Nemeth Electrical Info which is Mark Nemeth's article on 12-volt power. For wiring techniques and parts check the The Truck Electrical Center section of this site. The information there is oriented to upgrading an HDT truck, but is generally applicable to RV's.
 
For an understanding of various RV and house connectors, and proper wiring, check out http://www.myrv.us. This will give you an understanding of basic RV electrical service, and how it differs from residential electric. You might find  Bob Hatch's website useful as well. He describes upgrading a 30 amp RV service to 50 amps. Even if you are not performing this upgrade, understanding what he did will expand your knowledge of RV electrical systems.

If you have a basic understanding of AC/DC electricity then you should be able to design a reasonable system following the recommendations in the sections below. The system designs and components used are only examples, and need to be modified to meet your needs. You need to complete the entire design before you start implementation or you might find your system unable to meet your future expansion needs.

 
If you need help with system design you can work with a single vendor for most of your system components and they should be able to provide design help. The best thing is to work with someone who understands the special needs of RV's. My first choice would be AM Solar (Greg Holder). Their business is RV solar, they can supply almost all the required solar parts, their prices (for the most part) are reasonable, and their preconfigured systems are very nice; AM Solar. John Palmer (Palmer Energy Systems, Palmer Energy) also specializes in RV solar systems. For parts and design help from the residential market try http://www.thesolar.biz or http://www.backwoodssolar.com. I'm also willing to answer questions if you contact me directly - see the About Us section for our email address.
 
A complete implementation of an RV solar system, including an inverter, and batteries (from scratch) is going to cost in excess of $3000, depending on sizing and components selected. Time spent in the design phase is time well invested. Mistakes can be expensive. If you have the system installed instead of doing it yourself make sure you find a good installer. You want someone who will charge by the hour - not a flat rate. You are more likely to get a good job if the installer is not rushed, or loosing money on the work.

 

 

Introduction to Solar

The ability to dry camp, or boondock, is inherently part of the capabilities of all RV’s. The amount of time one can live effectively “off-grid” is dependent on your water storage capabilities, and the size of your battery bank (or how much you want to run your generator). Most RV manufacturers do not provide advanced boondocking technology as a standard part of their RV’s, so you are usually limited to 2-4 days without hookups. Enhancing the standard RV’s capabilities can allow you to live indefinitely without hookups.

So what do you need to effectively live off grid indefinitely? The heart of your system is the battery bank. You will need enough battery capacity to supply your energy needs. That means translating some of the DC battery power to AC, so it can be used by your normal RV appliances. You do this with an inverter.

Next, you need a way to replenish the battery power you use. That can be either a generator in combination with a modern battery charger, or solar panels in combination with a solar regulator. Or a little of both, which is what most people use. Solar is really an option here. You can live effectively off grid with just a generator and a proper battery bank, but for long term use you will find it most convenient to combine this with some solar panels.

You also need a way to monitor the status of the system. Without monitoring the system you will not know how much energy is available for use, or when to use the generator to help recharge the battery bank. If the battery bank is the "heart" of your electrical system, then the monitors are the "brains". You need them both.
 

To live effectively off grid you also need a way to remove waste water, restore fresh water, and efficiently heat the RV (when required). These last three items are not covered here. This article concentrates on energy-related items.

 
Side Note: typically a "blue boy" is used to remove waste water, either gravity fed or in combination with a macerator pump. The simplest way to restore fresh water is with a plastic water bladder (check Camping World for a nice 45 gallon one that works well). The bladder folds down to a very small size when not in use. Heat is efficiently supplied with a catalytic heater. This uses no power to run, saving your battery power for better uses than running the furnace. It is also 100% efficient in its use of propane. Your furnace is only about 50-60% efficient. For more on these topics see Boondocking Made Easier.
 
I've tried to convey what to look for in each of the areas covered. Although I have made some specific recommendations, you should not assume that these are the best available choices at the time you read this. Electrical and solar components change fast. Manufacturers continually upgrade their products, and introduce new products.  The intent of the information provided here is to help you to identify and select the products that will work for your particular implementation. There are many tradeoffs that need to be made when implementing an alternative energy system for your RV. There is no "right answer" in many of the areas - it is a personal choice with tradeoffs only you can make. The sample systems work well together and should satisfy the needs they are sized for, but they are only samples.
 

Determining Your Needs

First, you need to be realistic about your expectations. If you expect to install a solar system and use power just as you did when hooked to shore power, then you will be disappointed. Despite what some may tell you, living with an alternative energy system in an RV requires conservation. This is because, unlike off-grid home applications, most RV’s cannot store enough batteries to allow a large enough system for unregulated energy consumption. You need to learn to minimize use of high-power-consumption devices, supplement your existing RV systems with more efficient devices (such as using a catalytic heater instead of your RV furnace, which uses great amounts of 12-volt power), and monitor your energy use so you know when you are in trouble. Running out of power when you really need it is not fun. Killing your battery bank because you drew it down too far is even less fun – batteries are expensive.

You also need to examine your motivations for wanting solar. Solar use is a lifestyle decision. Adding an effective solar system to an RV will rarely pay back the costs of installing it. Nor will you recoup your investment when selling the rig. The best (and really only) reason to add solar is so you have the option of boondocking for long periods of time without hookups. If you do not enjoy doing this, then you should reflect on why you want to install a solar system. One or two days of boondocking between sessions of hooking up to shore power does not require a solar system, and its auxiliary systems. You can get by for a couple of days on a reasonable size battery bank. If you need 120-volt power, consider adding an inverter/charger. If you then find you need to recharge the batteries without shore power, you can consider adding a generator – either a small portable one, like a Honda 2000, or a genset that is permanently installed. If you have a motor home, you likely have a genset already. Notice, there is no solar system here. You really don’t need one if you are just overnighting occasionally.

Need to run your air conditioning? Well, a solar system is not going to help you here. It is not realistic to expect to run an air conditioner on a battery bank. You need a properly sized generator to run air conditioning “off-grid”. (Note: small window units could be run for short periods of time off a large battery bank, but from a practical view, this is just not feasible for long periods.)


OK, so you like to boondock for long periods of time. You’ve decided that you can afford to invest $3000+ dollars to make your life more pleasant when boondocking. How big of a system do you need? Only you can answer that. You need to examine your lifestyle while boondocking (or your anticipated lifestyle – you don’t actually have to boondock) and figure out how much power you use. Figuring out power usage while connected to shore power won’t give you your answer, because you are using lots of electric devices you won’t use when you boondock. For example: electric hot water heater, refrigerator on electric, battery chargers plugged in, converter on, lots of lights on, cooking turkeys in the microwave (just kidding).

So, how do you figure your power use? Think about what you have to use and add it all up. You can figure in watts, or in amp hours. Watts is probably easier, but ultimately you will need to convert to amps so I suggest you do your figuring in amps to start with. Look on the electric plate on the various devices and it will tell you what the device uses power-wise. Add them all up for the amount of time you run them. Don’t count any 120-volt lights, because you will only use 12-volt lighting while boondocking. Remember, you can figure watts by knowing the voltage and the amperage that the device is rated at – both are on the electrical plate (and if you are lucky, the wattage is there) watts=volts x amps. Sometimes electric plates on devices list ratings as xxVA (e.g. 40 VA) – this is watts (VA means Volts x Amps). When you work with solar it is best to figure everything in DC voltage, because your battery bank is DC – that means converting all your AC measurements to DC. In electrical stuff, watts is the universal measure. If you have a watt rating on a 12-volt appliance, it can be directly added to the watt rating of a 120-volt appliance to get the total watts consumed. Amperage ratings have to be converted, based on the voltage. Sounds complicated, but some simple math will allow you to get the total DC amps consumed from your battery.

Here are some 12 volt examples: 2 – 20 watt lights for 4 hrs= 40 x 4 = 160 watts DC, refrigerator 2 watts for 24 hrs = 48 watts DC. Now you have to figure your 120-volt loads: hairdryer 1500 watts for 12 minutes = 300 watts. You have to convert to DC – just multiply by 10 = 3000 watts DC. Microwave 1000watts x 5 minutes = 83 watts x 10 = 830. So all total we have (160+48+3000+830) 4038 watts in a 24 hr period. To convert to amps, divide by 12 or 120 – whichever voltage you are figuring watts for. (Getting complicated, right? That’s why you should just use amps to start with.) We did not count TV, satellite receiver, etc. You need to add up everything. Why did we count the refrigerator in our example when it is running on propane? Because, even when on propane, the refrigerator uses 12-volt power for its control circuits.

With an estimation of the number of DC watts you use on a daily basis you can calculate how many panels you need to supply that, and estimate how long you will have to run your generator to fill the “gap”, if generator use is part of your energy strategy. Don’t forget to add in “phantom” loads. For most RVs, these average around 1-2 amps DC (per hour). These are loads that occur when it seems everything is “off”. They come from battery chargers, electronic boards in your propane appliances, propane and CO alarms, etc. You also need to factor in the inefficiencies of converting/using power. There is energy lost when inverting, and energy lost in wire runs. The rule of thumb is 30% lost when inverting, and 20% lost in direct 12-volt battery use. It generally will not be more than this – it may actually be less, depending on your system.

Don’t get obsessed with figuring exactly what you need. Just get close and then usage will allow you to adjust. As a rule of thumb, the average RVer uses between 75 and 100 amps DC per “cycle” (partial day and overnight). Remember, when you are using power during the day (while charging) your instrumentation is not giving you a true count because power is being supplied while you are using it. The nice thing about a properly designed solar system is that you can easily expand it by adding panels (as long as you buy a large enough solar controller initially). For an excellent discussion of sizing your system take a look at Mac McClellan's website Best Fit Recruiting.
 

A Phased Approach

 
If you are not sure you will boondock a lot, or are overwhelmed by all that is required to implement a complete system for extended boondocking, consider using a phased approach. This will allow you to implement portions of the complete system, evaluate your use and needs, and then expand your system if you find it is beneficial to you. Here is my recommended approach: 
  1. Batteries. First I would augment my battery bank by upgrading to at least two 6-volt batteries. (I am assuming you have the typical RV with one 12-volt battery.) This should be able to be done to any RV without too much trouble. It will double the time you can boondock, and the 6-volt batteries will perform better than most 12-volt batteries. See the battery section for recommendations. Cost - $100-$115.
  2. Battery Monitor. Next, I would add a battery monitor - one with cumulative amp hours. This will tell you how much battery capacity is left, and will let you know when the bank is properly recharged. There is no other effective way to accomplish this. Expect to pay around $160 for a Trimetric 2020 and shunt.
  3. Charging. You need a way to recharge your battery bank. It may be that you don't boondock long enough that you deplete the bank - but if you do you need a way to charge. Typically this is a generator of some sort. If you have a motorhome  you probably have one already. If not, look at the portable Honda's and Yamaha's in the 2000 watt range. They will not run an air conditioner, but they will very effectively recharge a battery bank and run a microwave. If you use your converter as the charging source, look into a charge wizard or upgraded charging capability for your converter. Most older converters (pre 2000) do not have an effective battery charger in them. Switching out converters is covered more at the end of the Inverter/Charger section. You will want a high output battery charger to take advantage of your generator. 
  4. Inverter. At this point you should have some experience boondocking and know what size inverter you need. Either you will need a large one to run the microwave, or you can get by with a smaller one that just runs your TV and other occasional small appliances. If you start with the small one and decide to add a larger one later you could use the small one for just your entertainment center, or you can sell it.  Most people who boondock for longer periods will want an inverter of some sort.
  5. Solar. If you boondock enough, and for long enough, you will eventually want to add solar to avoid running the generator. Solar is expensive, so be sure you need/want it.

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Solar Panels
  • Rules of Thumb

There are three types of panel technologies - Amorphous (thin film), Poly-Crystalline (multi-crystal), and Mono-Crystalline (single crystal). Each of these is made slightly differently and has different benefits.

You probably will not use amorphous panels. These are thin, flexible panels and are useful on curved surfaces. They can be directly mounted with no air space. They are not quite as efficient as the other panels, so they take up more room for the same amount of power. They also cost slightly more. But they are useful in certain circumstances, such as in extremely hot conditions, since they deal with heat better. Unisolar is one of the leading sellers of amorphous panels.

I won’t go into how the panels are made; you can search the net if you are curious. All panels have a number of “cells” that individually produce (more or less) 0.5 volts. So, to have sufficient voltage for RV battery charging requires a panel that has at least 36 cells, connected in series. In practice, this 36 cell panel can produce up to 18 (usable) volts, or so. It does vary depending on the manufacturer.

Why do we need so much voltage when we are charging a 12-volt battery? Because 18 volts is the manufacturers “rating” under a certain set of standard test criteria. In practice, we rarely have these “optimal” conditions. To ensure that the required voltage is available at the solar controller and batteries, we need to have a higher input voltage. You need a minimum of about 16.5 (rated) volts to have a decent system in an RV application. Remember, you need to apply somewhere in the range of 14.1 to 14.4 volts to your battery bank to achieve a full charge.

What affects our use of solar panels in an RV environment? First is heat. Panels are tested and rated at 77 degrees. That’s the panel temperature itself, not the air. Think about the top of your RV in full sun – the panels are much hotter than that, and drop voltage because of the heat. They are also mounted about an inch (or so) from your roof, so ventilation is not the best. Second, is solar orientation. The angle of the sun changes seasonally. In the winter, it is very low in the sky. You don’t generally use a sun tracker on an RV, so your panels are generally not optimally oriented for full solar gain. In fact, you may choose to never tilt your panels, leaving them flat on your roof for convenience. Next, is shade. Are your panels partially shaded? Does your air conditioner cast a shadow sometime during the day? Does your crank up antenna? What about all the dust/dirt on the panel surface? Last, is wiring. You are going to drop some voltage due to wire loss between the panel and batteries. These considerations apply to all panel types.

So, how many panels do you need? You have an idea of the total amp hours (or watts) you use daily. To figure how much power a panel produces, take its “rating” (lets say 120 watts, or 7.1 amps) and multiply that by 5. This assumes you are getting 5 hours of sun a day (optimal) and that your panels really are producing at their rated output – neither of which will be true most of the time. But it is good for estimating. Now divide this total output of one panel into the number of watts/amp you use daily. So, for example, I have Kyocera 120 panels (120 watts, 7.1 amps). I’ll use amps to figure the required panels. I know I use 100 amp hours a day (DC), from my previous figuring. So 100 / (7.1x 5) = 100/35.5 = 3 panels (with rounding). But, I know I leave my panels flat, and I know not every day is sunny so I’ll use 4 panels. In my case, I started with 3 and added the fourth later, after I was experienced with our use.

Rules of Thumb:
  • Figure on a max of 5 hours of solar a day, at the panels rating.
  • Most experts recommend one watt of panel for each amp of battery bank. So, a bank of 4 - T105's that are rated around 440 amp hours would be paired with a solar array of around 400-450 watts (about 4 Kyocera 120 watt panels). In this example, you could start with 3 panels and expand later, if needed.
  • You gain as much as 30% by tilting your panels (over them being flat – especially in the winter when the sun is low). The proper way to think of this is actually that you loose 30% of your rated output if you don’t tilt your panels.
  • Most RVers that are regular boondocker's use 100 - 125 amps DC a day. More or less. If you are frugal you will use less – sometimes far less. If you are liberal, you will use more. We know people who use 800+ amps a day.
  • You loose up to 30% of the stored power when you are inverting (heat, inverter inefficiencies, and wire loss).
  • You loose up to 20% of the stored power when you use DC directly.
  • The colder it is, the better solar panels operate. Their rating is at 77 degrees F.
  • The colder it is, the WORSE batteries perform. See the dilemma?

You can download a spreadsheet that helps you calculate your power requirements here.

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Solar Controllers

  • Using 24-volt and 48-volt Panels
  • Solar Controller Summary

Typical solar panels deliver power to the regulator at 15-18 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. Some controllers can also convert voltages – taking in 24-volt or 48-volt DC from the panels and outputting 12-volt (or 24-volt) for the battery bank. To complicate things even further, there are now controllers, like the Outback controller, that can take any voltage in, and output 12 or 24-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.

There are only two types of controller technology worth considering; PWM and MPPT. If you see cheaper “shunt” type controllers ignore them.

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).

MPPT (maximum power point tracking) controllers have microprocessor technology that allows them to convert some of the “excess” voltage (when available) to amperage for more rapid charging of the battery bank. They cost more but can effectively boost the gain from a solar panel array, thus you may be able to save a panel. They are worth the extra expense, in my opinion, if you are building a system that has at least 3 panels and 400 amp hours of storage. For a good primer on MPPT technology check  Northern Arizona Solar - MPPT primer and Blue Sky Solar - MPPT Technology.

Controllers are rated by the amperage they can handle from the solar array. So how big of a controller do you need? First, you may need to “derate” the controller by 20-25% - although the convention in recent years is for controllers to have the derating built into their specs. If they don’t need to be derated they will tell you in the specs. Why derate the controller? Because if you bought exactly the size controller you needed for your array, you might overload it. Remember, panels are rated under standardized test conditions – they can, and do, put out more than their rated power under ideal circumstances. So you have to allow for this when selecting a controller.

Second, you want to be able to expand your system later without buying a new controller. If you are starting with only 2 panels you probably will add a third or even a fourth at some later time. You need to anticipate this, at least a little. I would advise buying a bigger controller than you think you need – it is not that much extra cost. As an example, let’s use 3 – 120 watt panels rated at 7.1 amps (21.3 amps total). You are going to need a 25-amp controller – but what about room for growth? Maybe a 30-40-amp controller would be better. Bottom line: buy more controller than you need today, and you will be happy tomorrow.

For a good solar controller expect to spend $125-$180 on a PWM controller in the 25-40 amp range. If you want a remote display it will be around $100 more. For an MPPT controller, you will spend at least $200, and as much as $500+ if you get a fully featured 60 amp Outback controller. Charge controllers represent a relatively fixed and small proportion of the total system cost. Studies show controller prices do not fluctuate much over time, like panel prices do. So, don’t wait for that great deal, and don’t skimp on capacity.

Controllers usually have the option of having a battery temperature sensor (remember, they are really just battery chargers). This is worth the extra cost if it is not a standard feature. The reason is that the temperature at the battery affects the optimal charging algorithm. Controllers have the ability to adjust the charging algorithm dynamically, based on real time temperature information. The same adjustment can be made (independently) by your inverter/charger. So you will probably have two battery temperature sensors at your battery bank. The sensor either attaches to the battery post, or is taped to the side of a battery. It does not matter which one.

So, which controller should you get? Only you can decide. Trace C40’s are a reasonably priced PWM controller that can be expanded to accommodate almost any RV system. Heliotrope makes a good PWM controller (they pioneered them) as well. On the MPPT side, Heliotrope makes the HPV-22 available for around $240. Blue Sky Energy’s (formerly RV Power Products) Solar Boost series starts around $220 for a 20-amp version (the 30 amp 3024i is an excellent controller that will support up to 4 – 120 watt panels). The Outback MX 60 is an outstanding MPPT controller that can also do voltage conversion – allowing you to use 24-volt or 48-volt solar panels. It comes with an LCD display, and costs about $550. More on voltage conversion later.

It is best to decide if you are going to use a MPPT controller during the design stage. This may influence your choice of solar panel. The reason is that MPPT controllers, by the nature of their operation, work best with higher-voltage solar panels. Solar panel ratings vary some – choice of higher voltage panels would be appropriate for an MPPT controller. You have to trade off the value of the higher voltage with the potentially higher cost per watt of the high voltage panels. Any panel rated 16.9 volts or higher would benefit with use of an MPPT controller – the higher the voltage the better. There are some panels on the market that are 12-volt panels that put out (around) 22 volts. These typically have 44 cells, instead of 36, and in combination with a MPPT controller a 100-watt panel can put out about 7.5 “boosted” amps. They also cost a lot more than a 36-cell panel (about $150 more than a Kyocera 120 watt panel with a 7.1 amp rating). You can do the math and see if you think the cost is worth the gain. Just remember that most of the “ratings” that are either optimistic, or are based on “perfect” conditions. Expect an average of 10-15% gain from an MPPT controller used with a 36 cell panel, as compared to a standard PWM controller. Under ideal conditions (cold ambient temperature, and a depleted battery) you may achieve as much as 30%.

Solar panel prices fluctuate a fair amount. There are deals to be found online. Once you design your system you can look for those deals, knowing that you have the information to make the tradeoffs in price/performance. Personally, I would go with an MPPT controller with the “best-deal per watt” panels I could find. But I would make sure the panels were rated for at least 16.8 volts. Kyocera 120’s can usually be found for $500 or less. (Well, that was true up to the beginning of 2005, but now panel prices have jumped, and availability is becoming limited in some areas.) When doing this assessment, take a look at the 44-cell panels sold by AM Solar . Depending on the street price of conventional 36-cell panels these may be worth the extra cost. You have to run the numbers.

Using 24-volt and 48-volt Panels

You can buy panels in 24 volts, or you can connect two 12-volt panels in series, just like you do with batteries, to increase the voltage to 24 volts. Why would you do this? Because voltage loss over distance is reduced as voltage is increased. You can send 24 volts twice as far as 12 volts over the same wire size (at a specified rate of loss). This means you can use smaller wire, or perhaps use the wire your manufacturer all ready ran in the “solar prep” package if it is otherwise too small. Or, if the solar regulator is far from the solar panels you can minimize voltage loss by sending 24-volts to the regulator instead of 12 volts. Use the table below to calculate drop, or this interactive Voltage Drop Calculator.

 
Chart accounts for wire runs in both directions.
wirelengthchart.jpg
 

To use 24-volt panels you need a solar controller that can convert 24-volt input to 12-volt output. There are a number of controllers that can do this. They tend to be higher-priced controllers with advanced features, so you have an overall better package. I particularly like the Blue Sky Energy 30-amp 3024i MPPT controller, about $279.

 

Obviously, you need to decide if you want to go 24-volts during the design stage. If you go this route, look for 24-volt panels. 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. Use of 12-volt panels also means you have to have pairs of them (three panels won’t work). The price per watt is always the determining factor for me. If the price was close I’d go with the 24-volt panels.

What about 48-volt systems?  It is the same as principle as 24-volt, with the same benefits. If you have long wire runs, consider stepping up to 48-volt panels and regulator. Many of the solar regulators that handle 24 volts also handle 48 volts. Just make sure that the 48-volt regulator will output 12 volts. Some will only output 24 volts. This is because residential alternative energy systems often run on 24-volt batteries. This is not convenient in most RV’s, because your DC house systems are all 12-volt. (Unless you have a bus, then you are usually 24 volts.) The disadvantage of 48 volt panels is that you often pay a higher cost per watt. And it is much harder to find a regulator that will take in 48-volts and output 12-volts. Only you can make the tradeoffs required.

 

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. It is not worth saving the (about) $100 difference.
  • Wiring: Make sure you have big enough wires. Use the charts - do not guess. It is unlikely that an RV manufacturer supplied heavy enough wires for a proper solar installation. Make sure you check. 
  • Voltage: consider the benefits of using panels rated at 24 volts, or series combining to get 24 volts. You need to buy a regulator 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. Solar regulators 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.

 

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 modified 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 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 usually not required. With inverter/chargers you usually get what you pay for – so beware the inexpensive high wattage inverter.

 

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 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. Our modified sine wave inverter runs a Dell LCD monitor and a Viewsonic LCD monitor with no problems.

 

Most clocks will also not run properly – 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, 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. 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: 

  1. 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 and laser printers often require pure sine wave. Almost all other devices do not.
  2. 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 plenty of charger output.
  3. 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. If you have a 50 amp RV you need to choose an inverter that has a 50 amp transfer switch unless you are using a sub panel. If you plan to remove the inverter when you sell the RV you might want to choose a 50 amp inverter now, even if your current RV is only 30 amps.
  4. 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).
  5. Does the instrumentation and 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.
  6. 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 regulator needs to have it. It is a little easier if the inverter has it, but either will work fine.
  7. 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.

 

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.

 

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.

 

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 connection 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, and use your inverter to supply your coach. 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 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 pull 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 good 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. 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 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.


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. 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. On a 50-amp RV I would use the RS3000. The RS2000 list price is $1600; the RS3000 is $2000, but you can find the RS2000 for $1125 and the RS3000 for $1395.  The SCP sells for around $205.

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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.

 

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 true deep cycle batteries. Both SLI batteries and Marine batteries are usually rated in CCA (cold cranking amps).

 

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.  True deep cycle batteries have solid lead plates and are quite heavy. “Golf Cart” batteries are not quite true deep cycle batteries like L-16 or industrial batteries, but are somewhere in between. From a practical perspective they are considered deep cycle. 


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 3-6 months. The main argument for the use of 6-volt batteries is that they perform much better. It is difficult (but not impossible) to find true 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 true 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.

 

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. True 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). The 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.

 

When comparing batteries in places like Wal-Mart or Sam’s Club you can determine the better battery by its weight. Assuming that they are the same “group” then the heavier battery will be better, 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 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. You get what you pay for in batteries. 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 are now 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.

 

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, so it was worth the risk.  They did charge to specifications and load tested to spec. They lasted almost 3 years.


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 regulator provides the primary charging source for your battery bank. This provides the best charge, since it is multi-stage and slow. Most solar regulators 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 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.
 

batterychargestages.jpg

 

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 the inverter. Voltage rises during this phase until it reaches the bulk charge voltage set for the battery type. For flooded cell batteries this is typically 14.4 volts, 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.

 

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 regulator 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 will destroy them. Inverters and solar regulators 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. 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 optimal recharging is one watt of solar for one amphour of battery capacity. So four T-105 class batteries (400 Ah, rounded) are optimally charged by 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 5th 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 5th 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. 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.

 

KSBattbox2_x.jpg (62563 bytes)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 probably 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 80o F:

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

  1. DC wiring from the panels to the rooftop wiring hub, or between the panels if not using a wiring hub.
  2. DC wiring from the wiring hub on the roof, to the battery bank, which goes through the solar controller.
  3. DC control wires that connect your instrumentation to their sensors.
  4. DC cables that interconnect the battery bank, and connect the battery bank to the inverter.
  5. 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.

KSelecricalcent1ax.jpg (76855 bytes)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 electrical center in my Kountry Star. It is placed on a piece of 3.4" plywood, which makes it convenient to mount components.



 


Rooftop Wiring 

For the purposes of this article, I will assume you are connecting your rooftop panels in parallel, and that you are using 12-volt panels (nominal rating). Just like batteries, solar panels come in 12, 24 and 48 volts. 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 24 or 48-volt 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. Use of the distribution hub helps in several ways. First, each panel’s wire runs directly to the hub, 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 distribution hub, but it does increase the cost slightly, and complicates the initial installation a little. Locate the hub centrally, near the panels. This will minimize the “spaghetti” effect on the roof. You can then run one larger wire from the hub down to the controller. Look at the voltage drop tables in the Solar Regulator section to calculate the wire size required. Or, just use #6 wire for the run to the solar regulator, which is sufficient in almost every case.  

Double Distribution Block - 4:1
distblock1.jpg
 
 

You can find various types of 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 the distribution hub 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 item above. Position them in the box so you can tighten the set screws. Epoxy them into the box, when you are satisfied with the layout. The alternative to building your own is to buy a ready-made combiner box from AM Solar. Check the product section for combiner boxes. I prefer using the CB Combiner Box.

Combiner Box 2.JPG (106485 bytes)

 

I try to bring the wires in from the sides of the box that will face the sides of the RV. 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.

 

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 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 a minimum of 6 gauge wire to run down to the controller. Unless you have an unusual distance from the hub to the controller this is usually sufficient. 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 6 gauge wire and abandon the manufacturer’s wire.  Wire size and connector quality are particularly important when using an MPPT regulator. Heat and bad wire connections will cause an MPPT regulator to operate far below its rating, negating any advantage to using it instead of a non-MPPT regulator. 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 3% or less voltage drop. 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 6 AWG wire, depending on the length of the run. There is a fuse installed in this line. I use automotive ATO fuses or "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. If you need more capacity than the #6 wire will give you and don't want to use heavy cable, you can run two #10 wires and achieve a similar result. Fuse each of the "positive" lines individually. This is not a great wiring practice, however.
 

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.
 
Information on how to build cables, types and sizes of fuses, sources for fuses, appropriate lugs to use, and how to lay them out is in  The Truck Electrical Center section of this website.

 

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.