US researchers say they\'ve developed an inexpensive rechargeable battery that works by the oxidation of iron plates -- more familiarly known as rusting. The air-breathing battery uses the chemical energy generated by the oxidation of iron plates exposed to the oxygen in the air, scientists at the University of Southern California reported Wednesday. \"Iron is cheap and air is free,\" USC chemistry Professor Sri Narayan said. \"It\'s the future.\" Currently the batteries have the capacity to store between eight and 24 hours\' worth of energy and could be used to store energy at solar power plants for a cloudy or rainy day, the researchers said. As California moves toward more renewable energy, solar- and wind-power plants will need an effective way of storing large amounts of energy for use during such times, they said. Regular sealed batteries are not rechargeable, and lithium-ion batteries like those used in cell phones and laptops, which are rechargeable, are at least 10 times as expensive as iron-air batteries, they said. The researchers said their work is ongoing to make the batteries store more energy with less material.