From this discussion, it seems that any source of 5-volt power could be used to recharge the average cell phone battery as long as you knew which contact was which... Three 1.5 volt batteries, a solarcell array, a bunch of lemons, copper pennies and zinc washers...

Now I'm picturing some little LED+variable resistor device that will light and indicate polarities up when you are feeding it 5 volts from some jury-rigged system.

My boss really needs to give me more work.:)

-Blast