Trickle charger / Power supply options

[Raindoh]

Have been scouring the internet on options that might fit my needs figure I would reach out to the community. I have an e92 M3 I don't get to drive very often and I'm starting to get the low battery warning. I want to get a proper trickle charger to keep the battery topped up and on condition. The problem I have is I live in an apartment building and my car is parked underground and don't have a 120V source close to plug in the charger. I've been trying to think of ways to possibly bring portable battery power and plug in the charger that way and charge the battery source once a week or however often needed. Does anyone know of a self contained unit that you can charge separately then use to charge the car battery completely wireless?

[DRLane]

Goal Zero Yeti series - Unfortunately pricey and probably quickly stolen.