A backup battery provides power to a system when the primary source of power is unavailable. Backup batteries range from small single cells to retain clock time and date in computers, up to large battery room facilities that power uninterruptible power supply systems for large data centers.

<operating system> ("back up" when used as a verb) A spare copy of a file, file system, or other resource for use in the event of failure or loss of the original. The term commonly refers to a copy of the files on a computer's disks, made periodically and kept on {magnetic tape} or other removable medium (also called a "dump"). This essential precaution is neglected by most new computer users until the first time they experience a disk crash or accidentally delete the only copy of the file they have been working on for the last six months. Ideally the backup copies should be kept at a different site or in a fire safe since, though your hardware may be insured against fire, the data on it is almost certainly neither insured nor easily replaced. See also backup software, differential backup, incremental backup, full backup. Compare archive, source code management. (2004-03-16)