
I have a cheap solar charger that I keep in my car.  I purchased it via
some mail order catalog when the 4 year old battery in my Oldsmobile would
run down during Summer when I was riding my bike more than driving my car.
Knowing I'd be selling the car in a year or so, I purchased the charger.
Believe it or not, the thing worked.  The battery held a charge and
energetically started the car, many times after 4 or 5 weeks of just
sitting.

Eventually I had to purchase a new battery anyway because the Winter sun
wasn't strong enough due to its low angle.

I think I paid $29 or $30 for the charger.  There are more powerful, more
expensive ones, but I purchased the cheapest one I could find.

I've never used it on the bike because I have an E-Z Charger on it and
keep it plugged in all the time the bike is garaged.