When there is a power failure, power is drawn from these batteries to feed the loads in the distribution.
High Voltage UPS systems have been designed using the lithium ion batteries or the lead acid batteries. Manufacturers claim that these sytems have an efficiency of about 99.6%.
The batteries are charged by a rectifier-converter unit which converts the AC into DC for charging the batteries when the power supply is available. When the power supply is not available, it converts the DC supply of the battery and converts that into AC for the supply to the load.