@andyfender Not sure if this is an option, or even necessary for you, but here's something else to consider.
If you live in an area that drops well below freezing for extended periods of time, it
is possible to freeze a battery. Especially standard lead-acid batteries. A frozen battery is usually toast and ready to be recycled.
My boat is stored inside my garage, so I have the privilege of leaving it uncovered all winter. But my garage is not heated or insulated (yet), so it does still get pretty cold during January & February.
What I do to aid with keeping the batteries from dropping too far below freezing for too long is to place an older style electric heating pad (one of those made for sore muscles) on top of them and turn it on high, and then close the bilge lid to retain the heat. I've been doing this for several years now and I still have the same AGM batteries that I bought back in 2010.
I turn it on when I know the temps are going to be severely cold and it seems to have helped me.