The telephone company maintains large battery systems that supply DC line voltage for the operation of analog telephone service at customer locations. The voltage supplied is a compromise between operational needs for reliable service and safety precautions for customers and service personnel. The length of the line to a customer telephone interface presents a resistance across which the central office voltage experiences a drop and therefore the voltage at the customer site may vary. The nominal battery ("system") voltage is 52.1 V, based on a 24-cell lead-acid battery. The voltage at a subscriber's network interface is typically 48 V between the ring and tip wires, with tip near ground and ring at -48 V.