UPS means Uninterrupted power supply. It has a rectifier circuit, a battery bank and an inverter circuit. The battery bank is connected in parallel to the circuit joining rectifier and inverter. Normally the battery will be charging, when power fails, dc power from battery is inverted to AC and supplied to the load.
Coming to the online UPS, the batteries are always connected to the inverter, so that no power transfer switches are necessary. When power loss occurs, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When power is restored, the rectifier resumes carrying most of the load and begins charging the batteries, though the charging current may be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte. The main advantage of an on-line UPS is its ability to provide an "electrical firewall" between the incoming utility power and sensitive electronic equipment.
The basic technology of the online UPS is the same as in a standby or line-interactive UPS