telecommunications Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...

, a common battery is a single electrical power source used to energize more than one circuit,

electronic component An electronic component is any basic discrete electronic device or physical entity part of an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singula ...

, equipment, or

system A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its open system (systems theory), environment, is described by its boundaries, str ...

. A common battery is usually a string of

electrolytic cell An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would otherwise not occur. The external energy source is a voltage applied between the cell's two electrodes; ...

s and is usually centrally located to the equipment that it serves. In many telecommunications applications, the common battery is at a nominal −48 VDC. A central office common battery in the battery room supplies power to operate all directly connected instruments. ''Common battery'' may include one or more power conversion devices to transform commercial power to direct current, with a

rechargeable battery A rechargeable battery, storage battery, or secondary cell (formally a type of energy accumulator), is a type of electrical battery which can be charged, discharged into a load, and recharged many times, as opposed to a disposable or prima ...

floating across the output. Before 1891, telephones in the United States always needed a separate power source to supply the subscriber line at each end. Each telephone needed a battery to power the transmitter and a hand generator to help signal the central office to manually connect the line to other subscribers and disconnect the line at the end of each call, and the cost of maintaining all this equipment was a large part of why early telephone service was so expensive. During the 1890s, common batteries became prevalent in telephone company central offices across the United States. They helped drive down the cost of phone service by reducing the complexity of the telephone equipment to be deployed to each subscriber, since each telephone would now draw all the power needed from the central office. The telephone hook replaced the hand generator as the primary method for signaling the exchange for when the line was in use or no longer in use. The presence of a common battery at the central office also explains why

landline A landline is a physical telephone connection that uses metal wires or optical fiber from the subscriber's premises to the network, allowing multiple phones to operate simultaneously on the same phone number. It is also referred to as plain old ...

telephones can often still work when the regular

electrical grid An electrical grid (or electricity network) is an interconnected network for electricity delivery from producers to consumers. Electrical grids consist of power stations, electrical substations to step voltage up or down, electric power tran ...

is down.

See also

References

Further reading