Batteries are chemical devices that provide an electric potential. They do so through two chemical reactions. Consider the "wet" battery that is used in your car or as a storage battery in a photovoltaic system. Metallic plates are immersed in an acid. In one of the reactions, electrons are removed from the anode plate(s) and go into the surrounding solution. At the cathode plate(s), a different reaction absorbs the electrons. These reactions are not sustained unless there is a continuous source of electrons to the anode and a continuous drain from the cathode. The terminals of any battery are connected to the anode and cathode plates. When a device, such as a light bulb, is connected to the terminals the cathode can supply electrons to the anode through the completed circuit. Check out the following sites for more details.
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