Originally Posted by smurphny
Although it is a major PITA, I always remove all three HUGE batteries and take them home where I can keep them warm and charged. I don't want to have to worry about a solar panel failing and the batteries freezing. Of course it gets to -30 around here so it's probably more a concern than points south. If batteries freeze, they're ruined and expensive to replace. The other thing that is always a concern is having any potential source of ignition in an unattended boat. I am surprised that any boatyards even allow batteries to be on a boat all winter.
A fully charge battery will not freeze until it's colder than -70F. I was born in Alaska and my folks were there for years, in Fairbanks, and they only had flooded batteries back then. Freezing batteries was not an issue but tires that went flop, flop and cracked steering wheels were when it hit -50 - -60F.. The batteries survived just fine but had less cranking amps so many cars had bigger batteries then they shipped with.
Keep in mind that in warmer temps the rate of self discharge & the chemical reaction rate/sulfation accelerates so keeping the batteries "warm" can have less benefit to storing them in a cooler location at your house. In cold temps sulfation is virtually non-existent and this is why ours have always been stored on-board every winter. Many boat yards require batteries to be "disconnected" from the vessel, if left on-board, as they should. They allow top up charging while you are there but generally disallow batteries to be left on-charge..
If leaving them on-board they need to be 100% disconnected and charged them to full...