Until I started reading about the care and feeding of batteries in marine usage, I knew very little of the details. Keeping it fairly generic and simple:
It turns out that all batteries will self-discharge even if all external loads are removed. The rate of discharge depends on lots of factors, but the bottom
line is that you cannot expect to leave any battery for long periods without taking care that they remain reasonably well charged.
The longer a battery stays below its optimal charged state, the more the process of sulfation can occur. Left too long in a partially drained state, the battery is permanently "robbed" of some of its original capacity.
It sounds like a lot of extra expense and effort to ensure that your batteries are always clean and properly charged at the appropriate voltage levels, but the experts in the field say the payback in battery reliability and life is worth it. Some recommend an appropriately sized solar panel to keep them topped off with no risk of overcharging.
I''m not trying to say that Sailmc''s methods are ill-advised. Those batteries may be coming through a winter OK. I''m merely summarizing what the experts are saying.
Good luck.
Duane