Amazon has the Ohuhu 18V 5W Solar Car Battery Maintainer and Charger for $27.99 right now. Use the code C9A6EMBP at checkout, though, and the price drops to only $14.99!

• 5-WATT ENERGY: With an output current of 400mAh,the 12-volt batteries charge and create 5 watts of energy to maintain your car battery
• WEATHERPROOF SILICONE PANEL: Mono-crystalline silicon panel is weatherproof and splash proof for added durability and enhanced sight exposureunl
• BUILT-IN PROTECTION: blocking diode can prevent reverse discharge and electrocution
• DURABLE CLIPS: Built with thickened clip lines for increased durability as compared to thin clip lines regularly sold
• EASY TO USE: Works with automobile, motorcycle, tractor, boat batteries and more.Lightweight makes it portable and suction cups makes it easy to place onto the front screen of vehicles