Backup batteries are used in uninterruptible power supplies (UPS), and provide power to the computers they supply for a variable period after a power failure, usually long enough to at least allow the computer to be shut down gracefully. These batteries are often large valve regulated lead-acid batteries in smaller or portable systems.
Higher-end battery backup units will also often feature LCD screens that show how charged the batteries are, how much power it's using, how many minutes of power are left should power be lost, etc. The rear of the UPS will feature one or more outlets that provide battery backup.
The battery backup sits between the utility power (power from the wall outlet) and the parts of the computer. In other words, the computer and accessories plug into the battery backup, and the battery backup plugs into the wall.
Standby service is a battery application where the battery is kept in a charged state and ready to provide backup power in case of mains power failure, such as in UPS systems or emergency lighting. The battery remains on standby most of the time, only discharging during power outages.
The battery keeps all necessary items running for between 30 minutes and 3 hours. Large aircraft may have a ram air turbine to provide additional power during engine failures. Backup batteries are almost always used in burglar alarms. The backup battery prevents the burglar from disabling the alarm by turning off power to the building.
Data center UPS backup batteries may be wet cell lead-acid or nickel cadmium batteries, with lithium ion cells available in some ratings. Server-grade disk array controllers often contain onboard disk buffer, and provide an option for a "backup battery unit" (BBU) to maintain the contents of this cache after power loss.
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. Small backup batteries may be primary cells; rechargeable backup batteries are kept charged by the prime power supply.