I bought an led side marker type unit and put alligator clips on the two wires.
I attached them to my boat battery to put a small constant drain on it.
Then at the first of each month u til spring, I'll attach the battery charger to bring the volts back up.
Also bought a battery blanket for the Highlander to supplement it's block heater.
When I hear it's going to drop to 15F or lower, I attach the extension cord.
If I plan on going out I'll throw the outside plug switch about 45 minutes in advance.