Classicly

A flashlight battery supplies electricity without moving mechanical parts. It converts the chemical energy of its contents directly into electrical energy.

Early direct conversion devices such as Volta’s battery, developed in 1795, gave the scientists Ampere, Oersted, and Faraday their first experimental supplies of electricity. The lessons they learned about electrical energy and its intimate relation with magnetism spawned the mighty turboelectric energy converters—steam and hydroelectric turbines—which power modern civilization.

We have improved upon Volta’s batteries and have come to rely on them as portable, usually small, power sources, but only recently has the challenge of nuclear power and space exploration focused our attention on new methods of direct conversion.