What is the best way to combine multiple batteries (flooded lead acid)?
To make a 24v system from 12v batteries, you need 2 in series. You have 4 batteries, so you will have 2 paths in parallel. So the only remaining option is whether you have this optional link, indicated by a switch, in place.







simulate this circuit &ndash; Schematic created using CircuitLab

Switch closed, you have two sets of parallel batteries in series. Switch open, you have two strings of series batteries in parallel.

In normal operation, with balanced batteries, and equal lengths of cable connecting them, the switch will have zero volts across it if open, and carry zero current if closed. In normal operation then, it really doesn't matter.

When that link does become significant is for balance fault conditions. What do you want your system to do should one battery fail open circuit, or short circuit, or wear out, or have one cell fail short circuit? 

My preference is to have the switch open. It means one fuse per series string will protect all the batteries. It simplifies the wiring, having fewer connections, and guaranteeing that any wire sized for one battery is not going to accidentally carry two battery's worth of current. It simplifies debugging, as a DVM placed across the switch position will show a big voltage if there is an unbalance problem.

However, if you are (unwisely) using a bunch of batteries of mixed age, then the balance currents flowing through the switch may make for a better overall battery performance. If you're going to do this, place one fuse per battery, to protect against all the possible fault conditions.