Batteries often used in battery rooms are the flooded lead-acid battery, the valve regulated lead-acid battery or the nickel–cadmium battery. Batteries are installed in groups. Several batteries are wired together in a series circuit forming a group providing DC electric power at 12, 24, 48 or 60 volts (or higher). A battery room is a room that houses for backup or uninterruptible. The rooms are found in , and provide standby power for computing equipment in Telephone system central offices contain large battery systems to provide power for customer telephones, telephone switches, and related apparatus. Terrestrial microwave links, cellular telephone sites, fibre optic apparatus and satellite communications facilities. Battery rooms are found on diesel-electric, where they contain the lead-acid batteries used for undersea propulsion of the vessel. Even nuclear submarines contain large battery rooms as backups to provide maneuvering power if the nuclear reactor is. • • Battery rooms are also found in electric and where reliable power is required for operation of, critical standby systems, and possibly of the station. Often batteries for large switchgear line-ups are 125 V or 250 V nominal. Since several types of give off if overcharged, ventilation of a battery room is critical to maintain the concentration below the lower. The number of air changes per hour required to prevent unsafe accumulation can be calculated from. • Kusko, Alexander (1989). Emergency/Standby Power Systems, pp. 99–117. New York: McGraw-Hill Book Co.,.• National Fire Protection Association (2005). 'NFPA 111: Standard on Stored Electrical Energy Emergency and Standby.