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RV Solar – Picking the Right Battery Bank

Choosing Batteries That Will Fit Your Budget and Needs

Are you thinking about a solar charging system for your RV? Whether it is a large “run the air conditioner” type system or just some simple panels to keep the batteries charged on a long weekend, there are a few simple steps that will help your get started in finding the best system to fit your needs.

There are a lot of factors that need to be calculated before you can even think about how many watts of solar to put on the roof. Continue reading my multi-part series on the DIY solar install I did to our unit in our transition to full-time RV living.

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Lets Talk About the Bank

If you have read my first two blogs about properly sizing an off-grid power system for your RV you should know that throwing a few batteries into a box or two may not be the best way to go about things. You should be able to calculate what your daily usage is and what your maximum usage may be. But before we can pick the batteries, we will talk about sizing options, Amp Hour ratings and then I will discuss the battery chemical types and some of their pros and cons.

Battery Dimensions

Batteries are typically referred to in group numbers for their physical dimensions. The sizing standard comes from the Battery Council International (BCI). The BCI size does not specify the storage capacity of the batteries, but the average Amp Hour ratings normally go up the larger the physical case is. Below is a list of some of the typical group numbers and their physical dimensions.

Type Use Length” Width” Height”
24 Auto 10.3 6.8 8.9
27 Auto 12.1 6.8 8.9
31T Commerical 13 6.81 9.43
4D Commerical 20.75 8.75 9.8
8D Commerical 20.75 11 11
GC2 6V Golf Car / Utility 10.37 7.18 10.87
L16 Utility 11.64 6.95 15.73

Amp Hour Ratings

Most deep cycle batteries are measured by the manufacture in a classification of Amp Hours. The measurement needs to also specify the total number of hours that the battery was tested at and most will list the 20-hour rate. The link provided is for a calculator that allows you to calculate your specific capacity based on your load.

It is not as simple as being able to double the amp-hour rating and half the time because lead-acid batteries deal with a scientific law called Peukert’s Law. This is when the battery’s rate of discharge is accelerated based on an increase of load. In order to get the correct output for the calculator, you will need to know at least two different hourly rates. Most manufactures will be able to provide you with a 10hr, 20hr, and 100hr rate. Once you know that, the calculator will be able to provide a Peukert’s Constant value. With that now, you can enter your average load to be applied and the calculator will tell you approximately how long your battery bank should last.

12 volt Vs 6 volt

The lead-acid batteries are available in several nominal battery voltages. The two most common for RV use are the 12 volt and 6-volt batteries. For deep cycle batteries, it is often preferred to have two 6 volt batteries wired in series over two 12 volt batteries. You will likely get better amp hours out of similarly sized batteries because the plates are constructed a little heavier so they are better for higher amp deep draws.

Battery chemical properties

Lead Acid

One of the oldest forms of batteries is the flooded lead-acid battery. They are comprised of lead alloy plates submerged in an electrolyte. By changing the alloy composition and varying the plate number and thickness, a battery’s performance can be customized to provide either a high current output required for a starting battery or allow for a deep discharge battery for uses such as golf carts or RV house batteries. Some side effects for the deep cycles include a greater need to equalize the batteries and having to replace water lost during the internal chemical reactions. Proper discharging and charging practices also need to be followed as deep charging causes elevated amounts of sulfates on the negative plates. This build-up can be reduced by increasing the charge voltage and ensuring the battery is returned to a full charge regularly but that same increased voltage will also cause corrosion to build on the positive plates. The average life cycle of a typical lead-acid battery could be 200-1000 cycles depending on the average depth of discharge.

Crown AGM Sealed Lead Acid Battery
Click here for latest pricing

Sealed lead-acid batteries began appearing on the market in the 70s and got their name because there is no water servicing required and the battery cases are sealed. Being sealed is however a little misleading as they still have the ability to off-gas. They are constructed utilizing less liquid electrolyte, instead of utilizing a membrane between the plates that is impregnated with the electrolyte mixture. This mixture is allowed to stay moist due to a chemical reaction of the hydrogen and oxygen gases under pressure to return to water. AGM(Absorbed Glass Mat) and Gel-type batteries are a form of sealed lead-acid. The charge rates for the AGM and Gel are faster than flooded lead-acid due to lower resistance internally and they allow higher instant load draws. One of the drawbacks to these batteries is that the charging parameters need to be closely monitored to ensure that the batteries do not reach the level where they begin to off-gas. They also require the float levels to be slightly lower than their Flooded Lead Acid alternative.

Pros and Cons of Lead Acid Chemistry

The flooded lead-acid battery is considered to be the cheapest upfront battery option. You can build a pretty substantial battery bank for $125-$150 per battery using flooded lead-acid batteries available at places like Costco or Batteries + Bulbs. You can find some pretty reputable name brands such as Trojan, VMax, and Fullriver batteries in the AGM line for a little more. The AGM batteries are going to provide faster-charging rates without the required battery watering maintenance required of the flooded lead-acid batteries. What has to be the biggest drawback to the lead-acid batteries is the weight of each battery. The average weight of a GC2 type flooded lead-acid battery is going to around 65lbs. So for a 690 amp hour battery bank, you could end up with 350lbs just in the battery bank.

Lithium Batteries

So why would you want to go with lithium batteries? Between long-term cost savings due to extended life expectancy, long usability between charges and fast recharging times, and reduce load weight, there are plenty of reasons. A question we hear answered more often is how one can justify not having them. 

Above all, the long-term cost savings are already documented and obtainable. When comparing the costs broken down by total watt-hours of useable power, you can calculate the number of batteries that would be required to match a single lithium battery. Because of the high cycle counts that can be obtained, one would have to replace multiple Flooded Lead-acid or AGM batteries before the end of the life cycle of the lithium battery.

The amount of available power from the lithium batteries between charges is one of the greatest instantly noticeable gains when installing the lithium batteries for the first time. Having the available power for as long and steady as the lithium batteries are noticeable with every additional light you turn on and load you apply. Gone are the days that you turn on the vent fan and see the lights dim.

The lithiums will be able to hold a much flatter voltage output resulting in a more efficient operation of anything being powered by the 12volt system. Even the larger loads like your inverters will cause only a minor drop in battery voltage through most of the battery cycle. When the batteries do need to be recharged, they will take large charge rates all the way to 98% state of charge. This will greatly reduce the amount of generator run time as well.

Most RVers are well aware of the weight limitations we face when it comes to load capacity of the RV. With the increasing lengths and sizes of tanks that people are looking for, sometimes it makes it a little more difficult to stay underweight. With the weight demands that are also being added to the RV for the solar setups, it is nice to have an option that allows for the required energy storage in a lightweight package. As a standard drop-in replacement battery will weigh less than half the weight of a comparable amp hour lead acid and be able to provide twice the amount of usable energy. This will allow for your banks to be smaller to fit your needs.

Finally, lithium batteries come in several different chemical makeups. I will not get into the many details other than to say that if you want to get into repurposing salvaged EV batteries, make sure you do your homework. There are some well-thought-out and closely monitored systems out there. But if you are not that tech-savvy, there are some good manufactures of drop-in lithium batteries, that are safe and reliable.

Check out our other blogs to help you design and build your own Solar Energy solution, check out the links below.

Thinking About Solar?
A brief writing discussing the initial considerations when first looking for a solar power solution.

Solar, Do You Know What You Really Want?
Once you know your energy usage and needs, you can look at some equipment the can help meet those needs.

It’s All About the Bank
Helping to find the right bank to meet your needs and your budget.

Please make sure you come back and continue the series on how to design and build your own RV solar system. Also when you do start purchasing your equipment please remember to utilize our affiliate links as a way to show your appreciation. Check out what we currently use in our rig.

We can help you!! If you need DIY Solar Consulting, we’ve got you covered! CLICK HERE to get signed up for Justin’s solar consultation.

8 thoughts on “RV Solar – Picking the Right Battery Bank”

  1. Why do you use 2 inverters and 2 solar charge controllers? I am just in the learning phase and have a Grand Design Imagine 2970RL with 50 amp service. I have much to learn before I start. In a system like yours, is it feasible to start with 3 batteries, a smaller solar array and 1 solar controller then “grow into the second half of the system to spread out the costs of the system?

    1. The two inverters allow me to power both legs of my 50 amp service. When plugged into 50 amp service, I can charge the bank with 240 amps. The dual charge controllers allow me to get the best use out of my panels. When in the stowed position on my roof, the panels are not all facing the same direction. Having the two MPPT charge controllers allows me to match the panels and the controller will be able to optimize their efficiency.

      1. I see the battery disconnects before the inverters which I do not know if yours are readily accessible by code, but I do not see where you put the solar panel disconnect before the charge controller.
        Did you locate that in a different place?

        1. Thanks for your response. I have made some changes to the system to replace the Outback solar charge controller with two Victron solar charge controllers and mounted the disconnect switches/circuit breakers on the front panels right below the applicable charge controllers. Would you be able to send me the applicable code that you are referencing that specifies accessibility or inclusion of any solar components utilized in a recreational vehicle?

    2. My setup can easily be built in stages. Because everything is stackable, it can easily be added too as the budget allows.

      1. So if I understand this correctly, you have 2 solar arrays. Each solar array has its own controller. What is the total power output of each array and what is the voltage of each array? IE 12 v, 24V Ect.

        1. I have eight 12 volt nominal panels total. They are each 175 watts. I have them set up so that each controller will handle four panels for a total of 700 watts per controller. I have one set wired in a series parallel set up and the other is all series.

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