Sounds like the battery charger had a failure. Most 2 amp chargers just go to a trickel charge once the battery is fully charged and just maintain.
I know you should winter your battery over in a location that won't freeze but I use my boat year round and leave all three batteries in the boat year round and have never have had a problem. I do make sure I charge them as soon as I come home and then just leave them till next time. A fully charged battery should not freeze but a discharged battery can freeze.
When I lived in NC I also left the battery in the boat year round with no problems.
Garry
