An uninterruptible power supply (UPS), also known as a continuous power supply (CPS) or battery backup, is a device that maintains a continuous supply of electric power to connected equipment, such as, for example, computers or telecommunications equipment, by supplying power from a separate source when a normal power source (e.g., AC wall outlet) is interrupted or not available. A UPS differs from an auxiliary power supply or standby generator, which does not provide instant protection from a momentary power interruption.
A UPS can be used to provide uninterrupted power to equipment for a certain duration, for example, thirty minutes, until a generator can be turned on or until the normal power source is restored. Integrated systems that have UPS and standby generator components are often referred to as emergency power systems. A UPS system may remain idle until a power failure occurs and then quickly switch from utility power to its own power source, or may continuously power the protected equipment from its energy reserves stored in a battery while simultaneously replenishing its reserves from another power source. When the battery's energy reserves are depleted, or partially drained, the battery will need to be recharged. With batteries that contain a number of serially-connected battery cells, draining and recharging of the battery often results in a voltage imbalance among the cells.