A convenient store of energy is the electric cell or battery. These are used everyday to power torches, radios, toys and many other appliances. The most common form of battery contains carbon and zinc separated by a solution of ammonium chloride. When the battery is connected to an electrical circuit, its stored chemical energy is changed into electrical energy. The battery continues to produce an electric current until all the chemicals have reacted with each other. Then, the battery is said to be ‘flat’. Batteries containing nickel and cadmium (Nicads) can be recharged by passing a small electric current through the battery for several hours. This makes them last much longer.