I bought one from a marine supply place for, if I remember correctly, about $15. Installed it on the rear deck of my Saab 900S hatchback and routed it through the ignition switch, as you've got to get the current into the battery and on Saabs, when the ignition is turned off, the cigarette lighter doesn't work, so by routing it the way I did, the cell's power flowed directly into the battery, regardless of whether the car was turned on or off. The one I bought was sold as a sailboat battery minder and produced 12.8 volts.

At the time I was fairly senior, which meant that for long periods of time, the car would sit in Memphis, unused. One February night, about 1:30 AM, I ended up in Memphis, searching the Democrat lot for my car, which I hadn't seen in perhaps 5 months. Just wondering the lot was a huge frustration, and the thought of the car not starting was also, in my mind, an issue. Oh, and it was about 20 degrees F and I was freezing my butt off wondering those many rows of cars.

Eventually I found the car and low and behold, it started on the first turn of the key. So, Yes, they work and work quite well. It does however depend on where you mount the solar cell. Mount it where it will get the most light throughout the daylight hours, and you should do fine.

JJ