WE had a auto type
battery charger that the previous owner installed for a two bank set of batteries. It cooked the $400. battery but the other two series 27 batteries are still in good shape. In short, don''t use auto charger in a marine environment. That said, we installed a marine 20A smart charger {they can be had from $100 up} and all the batteries stay charged, stay cool and aren''t cooking. Nuf-said.
Good luck --
Shaman