Obsessed by the improvements of StarTrek and the fourth millennium, while feeling restricted by the limits of today's technology or that boring second law of thermodynamics? Just let your imagination flow - you needn't be Scotty to beam your thoughts in here!
There are solar powered battery chargers all over the internet but, a battery that charges itself when left in the sun in a very interesting innovation. The problem is that batteries don't like the heat of the sun. Also, solar cells need mostly direct sunlight and if your solar cell is wrapped around the battery the sun can only hit a small portion of it. You might have to use this on rectangular batteries like Burgess batteries. You can buy both at Radio Shack and build a prototype in minutes. If you buy the flashlight that uses the square Burgess battery and attach the solar cell to it -- PRESTO!...you have a prototype.
oops,... forgot to tell you to make sure you get enough solar cells to connect in series so that they put out a little more voltage than the battery and, connect positive to positive and negative to negative.