You need to decide how many cloudy days you want to go without moving. What I mean by this is – if it is cloudy, overcast, or winter you will not generate much solar power. If you are not moving the van you don’t generate power from the alternator. Batteries are rated in Amp Hours. So calculate the number of amp hours per day. Then multiply that by the number of days you want to go without generating power.
I have decided on a 3000W inverter and 300 AH of lithium batteries. I will have room for a 4th battery if I find that it is needed. Now, I will use lithium batteries which will provide the full 100 AH of power. If you use AGM of gel batteries you can only use half their capacity. Using more than 50% of their capacity could damage the batteries. Lithium (Lipo4) is also smaller and lighter.
There are many website that will tell you the difference between Lead-Acid, Gell-Cell, AGM, and Lithium batteries. I am not going to list the features of them all. I am using Lithium (LiFePO4 batteries, because they are more efficient and cost effective. They are very expensive. But their life span and capabilities are higher than any other type of battery, they are simply better. Sometimes spending more money is actually the frugal way! They are actually cheapest over time.
Compared to other batteries lithium LiFeP04 are: Lighter, smaller, need no venting, and can be fully discharged without harm.
The negative of lithium batteries – High cost, and temperature. They cannot be charged below 32 degrees. So, if you travel in freezing temperatures you should have a heating pad for the batteries.
Lithium batteries have a BMS or battery management system. This BMS ensures safe use of the batteries. The BMS monitors and regulates battery voltage, current and temperature. If the battery gets to cold, the BMS will not allow it to be charged.
All batteries are not the same. Some brands are better built, with better quality management systems.
When connecting batteries use the same size and length battery cables. This ensures the same resistance, so that all batteries are handling the work load equally. Batteries can be connected in series or parallel.
If connected in series the voltage will add up, and the current remain the same. If connected in parallel the current capacity adds up, and the voltage remains the same. I will connect my batteries in parallel, maintaining a 12 volt system.
A battery charger goes through several stages when charging a battery. These are called bulk, absorption, and float. Bulk charging sends high current to the battery. Absorption slowly reduces the current over time. Float maintains the full charge of the battery. Lithium batteries do not need the float stage. It is important to have the correct charging stages for the type and manufacturer of the battery you are using. Different batteries require different charging profiles to maintain there health.