I’ve always stored them inside for winter too. My 31’ however had 7 batteries (3 for inverter) and not in friendly to reach places. I just checked the water a few times a year, that alone was a feat of contortions.
and plugged the boat in a few times over the winter. A few hours at a time if I could.
I replaced them the 1st or 2nd year i bought the boat. But Never had a problem during my 7 years of owning it. I think if you get 4-5 years out of a battery you’re doing good. Mainly because the different usage than a car battery.
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