A flywheel battery is a mechanical energy storage system that operates by spinning a mass, known as a rotor, at a very high speed. . Wouldn't it be better if you could somehow store that energy when you stopped and get it back again the next time you started up? That's one of the jobs that a flywheel can do for you. First used in potters wheels, then hugely popular in giant engines and machines during the Industrial Revolution. . When energy is extracted from the system, the flywheel's rotational speed is reduced as a consequence of the principle of conservation of energy; adding energy to the system correspondingly results in an increase in the speed of the flywheel. When excess electricity is available, it is used to accelerate a flywheel to a very high speed.