The lid to the SolLight Lightcap 200 Solar Cap Lantern has a solar-cell charging system built right in. The cells power up a rechargeable Ni-Cad battery in a waterproof sealed unit which in turn lights up the water bottle with either a white or red LED. You can get up to 18 hours of light from one charge with the red LED (10 with the white). The solar cap only adds a negligible amount of weight to your water bottle. Now when you can hiking or camping you will always have a fully charged camping lantern ready for you at night. A great idea!