I live in Minnesota where it gets -30 and never take them out because
it's too much of a hassle. I top them with water, charge them up and put
them to bed for the winter. On one boat I put one of those 600 milliamp
maintainer/chargers and the other one gets a regular charge every month.
I clean the terminals in the spring and have never had a problem.

Don't try this with a marginal battery, though. If it loses charge it
could freeze and now you have a real mess with battery acid all over.

Capt. Jeff