How to wire up a battery bank:
Wiring batteries in parallel is hooking all the positive lugs together and all the negative lugs together. It does not change the overall voltage of the battery bank, it adds amperage and storage of amperage.
Wiring batteries in series is hooking the positive of the first battery to the negative of the next battery. It adds the voltage value of the second battery to the first one. Meaning two 6-volt batteries wired in series would equal 12 volts.
You can use both parallel and series wiring in the same battery bank.
See the drawings below to get a better understanding.
Two 6V, 100Ah batteries wired in Parallel will have a total storage capacity of 200Ah at 6V.
Battery banks consisting of 12V batteries wired in parallel are often seen on boats and in RVs. Such banks are simple to wire up and require a minimum of cabling. However, the wiring must have the capacity to deal with a full battery bank.
When two 6V, 100Ah batteries are wired in Series, the voltage is doubled but the amp-hour capacity remains 100Ah.
Battery banks wired in Series-Parallel are even more complicated. Here, four 6V cells are wired in two "strings" of 12VDC that were then wired in parallel. Using 6V, 100Ah batteries, this system will have a storage capacity of 200Ah at 12V.
Since such a system has more wiring, it is very important to group "strings" logically and to label everything. Furthermore, it is a very good idea to fuse every "string" of series-wired batteries to ensure that a problem in one part of the battery bank does not take the whole bank down.