Originally Posted by glarior
What do you mean wired properly in parallel?
If you connect two 12V batteries in parallel and hook up the charging circuit and load to the + and - terminals of one of the batteries, you end up placing a larger load on the battery that those wires are hooked up to. The second battery has additional wire resistance between it and the load/charging circuitry which causes this. I doubt that you'd see much difference with a 1-3A load but when you start drawing 20, 50, or 100A then the resistance of the wires between the batteries starts making a lot more difference.
The correct way to wire two batteries in parallel is to attach the load + to one battery and the load - to the other battery. This equalizes the wire lengths to each battery and better balances the load on each. A similar approach needs to be done when using more batteries which can quickly lead to complicated wiring structures. Of course, with the load balanced, charging using the same terminals will also be balanced.
I have an interior 12V outlet in my Georgetown, wired with #10 wire. I also have a portable 80AH 12V deep cycle battery I can plug into it to add a third battery to the two that came with the rig. With less than a 10A load on the battery system, the inside battery will show over 3/4 volt higher voltage than the manufacturer installed house batteries. This difference is caused by the resistance of the 10' of #10 wire between the spare battery and the house batteries.
Question about replacing one of a pair of 6V batteries:
If one cell of a pair of 6V batteries fails, I'd replace both batteries. Batteries have a service life which should be the same for two identical batteries. Random differences in the batteries will always make one fail first and when this happens, you have no idea low long it'll be until the other one fails.