As a general rule if both batteries are the same age I would replace them both, but you can just replace 1 if the amps are the same.
I have 4 6volt batteries in mine 2 paired in series giving me 2 sets of batteries @ 12 and the 2 pairs are in parallel.
2 6volt batteries in series will give you 12volts and amps stay the same,but when you put 2 sets in parallel the voltage will stay at 12v and you the double the amps.
I can go 24 hrs with lights fridge and tv before I need to start generator or plug in.
Most 6volt batteries have a higher amperage rating then 12v
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