Skatebiker has the right idea, except I would use a solar panel that outputs 12 volts at full sunlight so that if it is somewhat cloudy it may still provide the 8.5 volts to the car charger. The 12 volt solar panel is very standard and used to keep a small charge on your car battery when parked for months at a time. For about $40 you can buy a 2.5 watt panel which will give you 200ma charge current.