Alkaline and zinc-carbon batteries are common in many self-powered devices. However, once a battery is discharged, it is no longer usable and is discarded. According to estimates, nearly 15 billion batteries are produced and sold worldwide annually. Most of these end up in landfills and some are salvaged for valuable metals. However, although these batteries are not usable, there is usually a small amount of energy left in them. In fact, about half of them contain as much as 50% energy.