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 vacuum 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 were identified as "A" and "B", respectively and thus the associated batteries were referred to as the "A" battery and "B" battery, 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 were either resistively-biased from the existing batteries, connected to ground or to the cathode.
This nomenclature was used primarily within North America. Different battery names were used elsewhere in the English speaking world.
An A battery is any battery used to provide power to the filament of a vacuum tube. It is sometimes colloquially referred to as a "wet battery". (A dry cell could be used for the purpose, but the ampere-hour capacity of dry cells was too low at the time to be of practical use in this service.) The term comes from the days of valve (tube) radios when it was common practice to use a dry battery for the plate (anode) voltage and a rechargeable lead/acid "wet" battery for the filament voltage. (The filaments in vacuum tubes consumed much more current than the anodes, and so the "A" battery would drain much more rapidly than the "B" battery; therefore, using a rechargeable "A" battery in this role reduced the need for battery replacement. In contrast, a non-rechargeable "B" battery would need to be replaced relatively infrequently.)
A batteries were initially 2 volts, being lead acid accumulators, but with the introduction of all dry battery radios, 1.4 volts became more common. Other voltages can be encountered. For example, 7.5 volt batteries were sometimes used to power a series connected set of 1.4 volt valves (tubes).
In Britain and some other countries, the "A" battery was known as the "LT" (low tension) battery if dry, and simply the "accumulator" if wet.
A B battery is any battery used to provide the plate voltage of a vacuum tube. It is sometimes colloquially referred to as a "dry battery" (although there's no reason why a "wet" battery of suitable voltage couldn't be utilised for the purpose).
The filament is primarily a heat source and therefore the A battery supplies significant current and rapidly discharges. The B battery experiences comparatively little current draw and retains its stored capacity far longer than an A battery. Early B batteries used with bright emitter tubes were 120 volts, but these quickly became obsolete as they were replaced with examples having voltages of typically 45 volts, 67½ volts, or 90 volts as more efficient tubes became available. Some examples had taps every 22½ volts.
Even when the plate voltage rail is fed by a power supply rather than a battery, it is generally referred to as the "B+" line in American schematics.
Because plate voltages can be as high as 300V DC, multiple B batteries may be connected together in series to additively provide the required operating voltages. The much higher available voltage of B batteries means that they must be handled more carefully than other battery types due to their ability to shock and/or burn the person handling them.
In Britain and in some other countries, the "B" battery was known as the "HT" (high tension) battery.
In electronics, a C battery may refer to any battery used to provide bias to the control grid of a vacuum tube. Until the early 1930s this was common practice in valve (tube) radio sets but was largely superseded by grid leak resistors or voltage divider biasing. Because the tube grids drew no current, the C battery provided the bias voltage with no current draw. The battery's life in the radio was essentially its shelf life.
Grid bias batteries are still manufactured today, but not for radio use. They are popular in schools and colleges as a convenient variable voltage source in science classes. The most popular battery is the 9 volt type with taps every 1½ volts that accept banana plugs.
A rare form of "C" battery was the bias cell, a button-size miniature battery designed to deliver a constant voltage with no current drain. These were briefly popular between 1936 and 1945 as the bias cell was less costly than a resistor/capacitor bias network.
In Britain and in some other countries, the "C" battery was known as the "GB" (grid bias) battery.