Solar powered battery maintainer for automobile

Changed jobs, and now commute by bicycle. After a month or two sitting in the back driveway, I frequently find that the battery on my car is too low to turn the engine over, or even power the headlights.

Because that location is far from any outlet, I can't easily plug in a battery charger in the car. Does anyone know of any solar powered battery maintainers that I could place inside the car, and that would keep the battery charged? Especially in Wisconsin winters.

Thanks!