Batteries and similar devices absorb electricity, store it, and release it as needed. Batteries, like many other everyday energy sources, use chemistry in the form of chemical potential to store energy. For example, wood logs store energy in chemical bonds until the energy is converted to heat by combustion. Gasoline is the chemical potential energy stored until it is converted into mechanical energy in an automobile engine. In order for a battery to function, it must be converted into a form of chemical potential before it can be easily stored. A battery consists of two electrical terminals, a cathode and an anode, separated by a chemical called an electrolyte. A battery is coupled to an external circuit to absorb and release energy.