Rene, in the absence of a power outlet and sunlight, I would also recommend the battery kill switch. My father uses it on his 2001 +8 (a lot of Kriechstroeme...) whenever he gets out of the car. Also a great theft protection! However, I have a Jan 2018 Plus 4, and only after six weeks in very cold winter has it left me standing.

Something like this: Battery switch