A power source used in older vacuum tube-based radios to provide power for the tube filaments, in situations where alternating current (AC) power was either unavailable or undesirable. Its use was the origin of the terms "A+" and "A-", to denote the filament voltage in a receiver.
The first batteries used for this purpose were 6-volt automobile storage batteries, since the tubes of the time (up to roughly 1929) contained filaments that required 5 volts. A new series of tubes that required only 2 volts came into use in late 1929, and their filaments could be lit by using standard 1.5-volt cells wired in series/parallel, with a rheostat to reduce the voltage; or with a 2-volt "Air Cell", a type of rechargeable storage battery.
Just before World War II, a new series of battery-powered vacuum tubes appeared on the market. These types required only 1.5 volts to light their filaments, thus a single 1.5-volt cell (or group of cells wired in parallel) could be used as a filament power source. These more-efficient tubes also required less filament current, so A batteries tended to last longer than in previous receivers.
In the 1950s, as vacuum tubes began to be replaced by transistors that required far less power, the use of the A battery as a filament voltage source passed into radio history. Smaller-voltage batteries became the radio’s sole source of power.
SOURCES
Ghirardi, Alfred A., Modern Radio Servicing. New York: Radio & Technical Publishing Co., 1935.
Kendall, Lewis F, Jr., and Koehler, Robert Philip, Radio Simplified. Philadelphia: The John C. Winston Company, 1925.