In the early days of electronics, vacuum tube (called valves in British contexts) devices (such as radios) were powered by batteries. Each battery had a different designation depending on which tube element it was associated with.

Initially, the only such device was a diode with only a filament (cathode) and a plate (anode). Following the direction of electron flow, these electrodes are identified as "A" and "B", respectively and thus the associated batteries are referred to as the "A" and "B" batteries respectively. Later, when the control grid element was added to create the triode tube, it was logically assigned the letter "C" and supplied from a "C" battery. Subsequent addition of further internal elements to improve the performance of the triode did not require an extension to this series of batteries – these elements are either resistively-biased from the existing batteries, connected to ground or to the cathode.

This nomenclature is used primarily within North America. Different battery names are used elsewhere in the English-speaking world.