Gray Matter: Batteries Out of Thin Air @ Popular Science

A battery that runs on air? Why, that’s almost as good as a car that runs on water! Those cars are fantasy, but batteries that run on air are actually quite common, especially among older people. Tiny zinc-air batteries are widely used in hearing aids, where they have replaced toxic mercury-based batteries in providing a small but steady stream of power. They supply more energy for their size than any other battery, because they draw some of their power straight from the air.