Common battery system

The common battery system is the telephone arrangement in which a single electrical power source, usually a string of rechargeable electrolytic cells kept at the central office, energizes many circuits and instruments at once. Before 1891 in the United States each telephone needed its own battery for the transmitter and a hand generator to signal the exchange, and the cost of maintaining that equipment was a large part of why early telephone service was expensive. During the 1890s central office common batteries became prevalent, which reduced the complexity of the equipment placed at each subscriber, because every telephone now drew its power over the line from the exchange. The switch hook replaced the hand generator as the way to signal that a line was in use or free. The usual nominal supply is minus 48 volts direct current, often floated across a rechargeable battery fed by converted commercial power, which is also why landline telephones can keep working when the regular electrical grid is down.