To my mind, a sealed battery and a dumb converter is a bad combo to leave plugged in for months at a time. I’d pull the ground cable, as discussed above.
BTW, just FYI, a fully charged battery will not freeze until it hit’s minus 80 degrees F. A completely discharged battery will freeze at 20 degrees F. A partly discharged battery will, of course, fall somewhere in between, depending on how far it’s been discharged.
I don't like to make plans. They cause the word "PREMEDITATED" to get used in court!
DON'T FEED THE VULTURES!
My Body is a Temple! Ancient, Crumbling, Probably Cursed . . .