This battery/solar powered 12volt cupholder fan is constructed using a 12volt battery that I took out of an old Power Wheels (R) 4 Wheeler and a 15 watt solar panel that I was using to keep my Harbor Freight Power Pack charged. (I now have the HF Power Pack wired up to my 65 watt solar panels).
The cup holder fan was purchased at Wal-Mart for $15.00.
The solar panel is tucked away behind the truck seat when traveling or not in use. The battery is also stored on the floorboard behind the front seat. The solar panel can be placed either inside on the dashboard or outside on the windshield.
As long as the fan is running I shouldn't have to worry about the battery being overcharged.