During the winter, a lot of folks around here have moved on
from battery blankets and plate heaters to onboard maintenance
chargers. Most ship with a hardwire adapter for battery terminal
clamps that can be used in place of the usual alligator clips.
I’ve found that it fits nicely in front of the battery and I just plug
it into the same multi-outlet 14/3 arctic cord that supports my
engine heaters. If I’m plugged in at -30, I’m charging up.