Originally Posted by 5picker
IMHO, the better way to connect two 12v batteries in parallel is with a selector/parallel/cut-off switch. You eliminate the parasitic drain of the weaker battery by not having them both always hooked in parallel, you can drain one and still have fresh one, or if needed, connect them both. Recharge times are slightly better too.
The problem with your scheme is reconnecting the batteries in parallel if they are not equally charged. Been there, done that, have the melted wires and battery tops with holes to show for it.
Say the battery voltages are off 1/10 of a volt when you reconnect them in parallel. And you have #6 wiring connecting the batteries, each wire 1ft long (what I had). The wire will have 0.0008 ohms by AWG tables. We'll estimate the effective internal resistance of the lower-charged battery at 0.1 ohms - which will dominate. The theoretical current at reconnection with a 0.1 volt difference is 10 amps - tolerable, but a waste of battery power.
In reality, I probably had more like a 1 volt difference, because I only recharged one of the batteries. Let's say I get 100 amps flowing between the batteries for a couple of minutes. The lower charged battery is boiling furiously, and the combination of venting hydrogen and current in the wire melts the insulation. The venting hydrogen produces a nice blue flame near the glowing red bare wire. And blew a couple of holes in the tops of both batteries. A pair of lineman's pliers cutting one of the wires put the fire out. I was very glad the batteries were in a plastic box on the a-frame, and no acid leaked out.
Needless to say, I have switched to two 6V golf cart batteries in series. I get more capacity at less cost, and am not likely to repeat my electrical fire. Parallel battery banks must be kept as close to identical as possible to avoid problems. I failed to do that.
Just my experiences. If my wife had seen the event, she would probably never camp with me again.
2014 Rockwood A122 A-frame