It’s about this time in the processes of creating our solar system that I realize the solar system batteries really dictate the solar system’s size, not the solar panels. This is because at night when the solar panels are doing nothing the only thing supplying power to our rig is the batteries. So, if they aren’t large enough to support the electrical system through the night then the panels can be massive but it wouldn’t matter because we would quickly damage the solar system batteries and be out thousands of dollars… Good batteries aren’t cheap!
Quick Links for this Truck Camper Solar System Series
- Solar System Introduction
- Solar Panels & Charge Controller
- Solar System Pieces
Solar System Batteries: Size and Weight?
My tactic was to figure out the largest batteries I could put into our truck camper and not shoot ourselves in the foot with too much battery weight or take up unnecessary space. Our camper has an access panel to the bed. I originally thought this area would be an ideal spot for the solar system batteries. They would be out of the weather but well ventilated. I wasn’t, however, taking into account how to strap a battery down in this area or how to even get a battery weighing over 100lbs into that hole and then wired. Luckily, I had a backup spot picked out inside the camper under the bench seat for the solar system batteries. This would not be a suitable area for a wet cell battery. See the battery section below.
Keep in mind that the batteries, even on a small system like ours, add a lot of weight. One of my priorities was making sure that this new weight was strategically placed in front of the back axle. Even weight distribution on a rig is important and should be taken into consideration.
Not all Deep Cycle batteries are made the same. Here is a quick synopsis of the different options available:
LiFePO4 (Lithium Iron Phosphate)
- Pros: About a fifth of the weight of equivalent useable amp hour batteries, can be stored in any orientation, lifespan is 15+ years, and can be used to 0% of capacity without damaging the battery.
- Cons: Most expensive and lithium technology isn’t 100% stable. They are adversely affected (explosive) when subjected to temperature extremes. However, LiFePO4 batteries are designed specifically for RV lifestyles and are very reliable with internal battery monitors built-in that regulate them during extreme temperatures. This includes not allowing them to take a charge if their internal temperature exceeds the high temperature or drops below the low.
AGM (Absorbed Glass Mat)
- Pros: No maintenance, can be stored in any orientation, won’t vent unless damaged, and slightly lighter than wet cell (flooded) batteries.
- Cons: More expensive than flooded batteries, can be damaged if discharged to less than 20% of capacity or not fully charged regularly. Even maintained well the maximum lifespan is only 4 to 5 years.
- Pros: No maintenance, can be stored in any orientation, and won’t vent unless damaged.
- Cons: Heavier than AGMs, more expensive than AGMs, and if not recharged properly the battery can lose life cycles (battery life). Maintained well the maximum lifespan is about 4 to 5 years.
Wet Cell (Flooded)
- Pros: Cheapest of the deep cycle batteries, most readily available, & long life expectancy.
- Cons: They must be mounted upright, need regular maintenance, and can vent toxic gases so they should not be stored inside the living areas. Typical life expectancy is 3 to 5 years depending on build quality and good maintenance.
Our Battery Choice
After collecting the above information my choice of the type of battery we needed for our truck camper solar system was relatively easy. Wet cell batteries are out because of the toxic gasses. Lithium was out because of the prohibitive cost… nearly 4 times the cost for the same amount of power. The gel is eliminated because they have no advantages over AGMs, despite being more expensive. So AGMs are the winners for us in 2017.
2021 UPDATE: In the past few years the technological advancements in LifePO4 batteries have made lithium more reliable and cheaper. If I were making this choice today I would choose BattleBorn’s LifePO4 batteries. The good news is that while these batteries are a different technology BattleBorn states that our Trimetric Solar Controller is capable of being used to charge them. No changes would need to be made in our design in order to implement the LifePO4 batteries.
What Size Voltage?
The next decision to make on the battery side of our truck camper solar system is voltage and configuration. Basically, when creating a system it can be wired in series to create a 12V, 24V, 48V, etc system or it can be wired in parallel. As batteries are added the voltage stays the same, but the capacity, rated in amp-hours (Ah) increases. A higher voltage system allows for more wattage to travel across a wire without increasing the amperage. However, most 12V devices can only accept a certain range of power before it burns them out. A combination of both parallel and series wiring can be used to create a system with a higher Voltage and a higher amp-hour rating.
Wiring a battery in series adds the combined voltage of each battery. To wire a battery in series you simply wire the negative pole on one battery to the positive pole on the other. The rest of the system is wired off of the other two poles. Batteries should be the same model matching voltage and amp-hours. While the voltage doubles in this type of configuration, the amp-hours remain the same. Two 6V batteries wired in series creates a 12V system. Two 12V batteries wired in series creates a 24V system.
Wiring a battery in parallel adds the amp-hours together while maintaining the voltage. Three 100Ah, 12-volt batteries wired in parallel would create a 12-volt system that has 300Ah. To wire, a system in parallel the batteries’ negative poles are daisy-chained together (one battery to the next) and the positive poles are daisy-chained together. The positive lead wires into the overal electrical system off of one side of the battery cluster while the negative wire goes off the opposite site. See the diagram for clarity.
Two 6V Solar System Batteries
I ultimately have a very small camper so I choose to use two are 330AH 6V batteries wired in series to get my 12V system. Most devices designed to run off batteries are built for 12V power. 6V batteries tend to be designed with taller casings allowing the footprint to be smaller while maximizing amp-hours. This allows for two “smaller” batteries to be wired together rather than one larger, heavier battery. Ultimately, I get more amp-hours out of the system than I would if I used a single 12V battery since they are smaller and not rated for as many amp-hours. Again two 12V batteries could have been wired in parallel to double the amp-hours if I had space. This is all about preference and usability for our rig.
So with that, I set out to find my AGM batteries. I took Handy Bob’s recommendation and looked at Crown and landed on the 6CRV330. The 330 is the number of amp-hours that can be discharged over a 20 hour period. However, that is misleading because again you don’t want to drop below 20% of an AGM’s capacity—it can damage these expensive batteries. You also want to make sure that you can recharge the batteries regularly to 100% as over time batteries will lose capacity if they aren’t fully charged through a process called sulfating. This isn’t true of Lithium. While I am left with 264 usable amp-hours without risking damage to the solar system batteries I prefer to stay above the 50% window to give myself a lot of cushion.
What does 264 amp hours mean?
Basically, what 264 amp-hours means is that from the time the sun goes down at night and my solar panels stop producing power I have about 264 amps that I can use until the sun returns to start charging again. This of course assumes that the solar system batteries are at 100% when the sun goes down. These numbers come in handy later on once the solar charger and battery monitor are installed. After 3-years we have yet to drop our truck camper solar system below 45%.