UPS, or Uninterrupted Power Supply is a device to keep the output power supply on for a limited period in case of input power loss. The battery is actually a part of UPS.
UPS commonly take in and supplies single-phase AC (110/230V AC) and essentially consists of
The Battery is connected at the DC part so that in case of loss of input power, the battery supplies the DC for DC-AC conversion. With input power. available, the battery gets charged parallelly.
There are different types of UPS available. The easiest to understand variants are
The size of the battery decides how long a backup will be.
A central battery can be useful as a backup but it needs to be charged by a separate battery charger circuit and need to have its own inverter to be used as a general power supply.