You've never been to my house - I didn't have heating for the two coldest years in the history of the universe of the united kingdomz of Britishland and I bet your garage isn't sub-zero.
Anyway, the cold only really messes with NiMH batteries. Li-Ion batteries are pretty good in the cold.
You sure you don't have a short circuit? My Shiv sits idle for months in the off-season and still holds its charge.