Battery room

A battery room is a room in a facility used to house batteries for backup or uninterruptible power systems. Battery rooms are found in telecommunication central offices, and to provide standby power to computing equipment in datacenters. Batteries provide direct current (DC) electricity, which may be used directly by some types of equipment, or which may be converted to AC by uninterruptible power supply (UPS) equipment. The batteries may provide power for minutes, hours or days depending on the electrical system design, although most commonly the batteries power the UPS during brief electric utility outages lasting only seconds.

Battery rooms were used to segregate the fumes and corrosive chemicals of wet cell batteries from the operating equipment; a separate room also allowed better control of temperature and ventilation for the batteries. In 1890 the Western Union central telegraph office in New York City had 20,000 wet cells, mostly primary zinc-copper type, in use.

Telecommunications

In the application of stationary battery systems for telecommunications equipment, the equipment is operated on DC power (typically 48 volts in a central office - main telephone switching center or remote office). During normal operation when utility power is available, the telecommunications (load) equipment is operated from the DC power supplied from the rectifiers, which also serve to maintain full charge on the battery systems. In the event of a utility failure, the load is supported from the battery until an emergency power source can be applied, such as a generator. The typical design of a battery in the telecommunications application is to provide DC power to the load equipment for a minimum of four hours if a generator is also installed at the site. If a generator is not installed at the site, the typical design of a battery system is to support the load equipment for eight hours. If a utility outage appears to be for an extended duration (such as during a hurricane or ice storm) a mobile generator would be mobilized to a site (that does not have a generator installed) to recharge the battery system and support the site until utility power can be restored.

For outages longer than a few seconds, the batteries must continue to provide power until an emergency power source can be started. Facilities that have no emergency power source follow a written manual protocol that dictates an orderly shutdown of the highest electrical demand equipment to extend battery life, followed after a specified interval by a complete power-down before the batteries are exhausted. As mentioned earlier, the batteries most frequently provide power for a UPS during brief, transient electric utility outages lasting at most, seconds. When electric utility outages last over 15 seconds to one minute, the UPS or the Automatic Transfer Switch (ATS) recognizes this as a long duration power outage and signals an emergency power source to start. After the emergency power source is started and is allowed to stabilize, an Automatic Transfer Switch disconnects the facility from the electric utility and connects the emergency power source to provided replacement AC electricity to the facility, including the UPS. Once the emergency power source, such as a diesel engine-generator (genset) or gas turbine coupled to a generator, is online, the UPS ceases drawing power from the battery room, and recharging the batteries begins with DC power supplied by the UPS or a free-standing battery charger. After electric utility power is restored, the batteries are again called on to supply power during the very brief period while the Automatic Transfer Switch disconnects the emergency power source and reconnects the electric utility. Recharging the batteries can add considerable additional load to the emergency power source, potentially overloading it. To avoid this, most UPS systems large enough to require a battery room have, as part of their electronic controls, a signal wire from the Automatic Transfer Switch that the ATS energizes when emergency power is active. When this signal wire is energized, the UPS recharges the batteries at a preselected reduced rate. When electric utility power is restored, full rate recharging resumes.

The interval the battery room provides electric power is known as ride-through, and a battery room is rated by the maximum amount of ride-through time it can provide at maximum rated load. An approximately reciprocal relationship exists between the ride-through time and the electrical load on the batteries. Thus, a battery room rated to provide 15 minutes of ride-through while delivering a maximum 900 amperes at 48 volts will provide more than 30 minutes of ride-through if the actual demand is only 450 amperes. The total capacity (product of current and time, or ampere-hours) increases at lower current discharge rates, an effect called Peukert's law.

The type of battery most commonly employed in battery rooms is the flooded lead-acid 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). Usually there are two or more groups of series-connected batteries. These groups of batteries are connected in a parallel circuit. This arrangement allows an individual group of batteries to be taken offline for service or replacement without compromising the availability of uninterruptible power. Generally, the larger the battery room's electrical capacity, the larger the size of each individual battery and the higher the room's DC voltage.

Electrical utilities

Battery rooms are also found in electric power plants and substations where reliable power is required for operation of switchgear, critical standby systems, and possibly black start of the station. Often batteries for large switchgear line-ups are 125 V or 250 V nominal systems, and feature redundant battery chargers with independent power sources. Separate battery rooms may be provided to protect against loss of the station due to a fire in a battery bank. For stations that are capable of black start, power from the battery system may be required for many purposes including switchgear operations.

The world's largest battery is in Fairbanks, Alaska, composed of Ni-Cd cells. Sodium-sulfur batteries are being used to store wind power.

Submarines and ocean going vessels

Battery rooms are found on submarines of the diesel-electric type, where they contain the 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 shutdown. Batteries in surface vessels may also be contained in a battery room.

Battery rooms on ocean-going vessels must prevent mixture of seawater with acid, since this will produce toxic chlorine gas. This is of particular concern on submarines.

Design issues

Since typical secondary batteries may give off hydrogen gas especially if overcharged, ventilation of a battery room is critical to maintain the concentration below the lower explosive limit.

The life span of secondary batteries is reduced at high temperature and the energy storage capacity is reduced at low temperature, so a battery room must have heating or cooling to maintain the proper temperature.

Batteries may contain large quantities of corrosive electrolytes such as sulfuric acid used in lead-acid batteries or potassium hydroxide used in nickel-cadmium batteries. Materials of the battery room must resist corrosion and contain any accidental spills. Plant personnel must be protected from spilled electrolyte. In some jurisdictions, large battery systems may contain reportable amounts of sulfuric acid, a concern for fire departments. Battery rooms in industrial and utility installations typically have an eye-wash station or decontamination showers nearby, so that workers who are accidentally splashed with electrolyte can immediately wash it away from the eyes and skin.

References

• Kusko, Alexander (1989). Emergency/Standby Power Systems, pp. 99–117. New York: McGraw-Hill Book Co., ISBN 0-07-035689-0.
• National Fire Protection Association (2005). 'NFPA 111: Standard on Stored Electrical Energy Emergency and Standby Power'