4 Kinetic Energy Start-ups

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Walk, talk, ride, recharge

If you happen to be viewing this on your iPhone or some other power-hungry device, chances are you are about to run out of juice. Soon, though, you may never have to plug in again. After an early wave of enthusiasm for bulky backpacks with solar-powered chargers, the new bet is on kinetic energy, or energy that can be generated from movement, including the stride of a walker or the turn of a bicycle wheel. In February, Nokia patented technology for a phone able to charge itself from the energy produced by a moderately paced walk. Here's a look at four young companies that are also hopping on the bandwagon.