Researchers from the University of Pennsylvania have designed a backpack that converts motion into electricity for mobile devices. It work similarly to self-winding wristwatches, only much much bigger and bulkier. From Reuters:
The backpack is deliberately designed to shake around a bit. The up-and-down movement of the backpack's cargo compartment against the frame of the pack turns a gear connected to a generator…Humping along just under 85 pounds (38 kg) of weight in the backpack can produce up to 7 watts of electricity, Lawrence Rome and colleagues at the University of Pennsylvania report…
"Metabolically speaking, we've found this to be much cheaper than we anticipated. The energy you exert could be offset by carrying an extra snack, which is nothing compared to weight of extra batteries," Rome said.
"Pound for pound, food contains about 100-fold more energy than batteries."
Link to Reuters article, Link to press release