A common battery is usually an electrolytic device 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 an electrolytic battery floating across the output. Common battery operation largely replaced local batteries in each telephone in the early 20th century.