Basically , there are two types of UPS.

In case of offline UPS the AC mains supply supplies the power to the load and at the same time charges the Battery. On AC mains failure the load is transferred to the Inverter mode. The transfer of load from Mains mode to Inverter mode on mains failure and retransfer from Inverter mode to mains mode on resumption of mains power is automatic. Normally , this transfer time should be less than 5 mili seconds for computer loads, otherwise the computer systems will re-boot.

Unregulated Charger: The output voltage of unregulated charger varies as the input AC mains varies. At low AC mains voltage , it does not charge the Battery fully and hence lesser backup and the life of the battery will drastically reduces if the low voltage situation prevails. When the AC mains voltage increases, the charger output voltage also increases and over charges the battery resulting in the follwing.

Regulated Battery Charger: The output of this kind of charger is constant for variations in the AC mains supply in a particular range for which it is designed. Hence the Battery gets charged fully to the desired voltage prescribed by the Battery manufacturer even at low AC input voltage and does not get overcharged at high AC input voltage. The charging current in a regulated charger will cease when the Battery gets fully charged. However a few milliamps or amps trickle charging current depending on the capacity of the Battery will always be maintained in order to keep the Batteries in healthy condition.

Apart from having all the regular features of offline UPS , a Line interactive UPS also works as an solid state stabilizer. It maintains output voltage between 200 volts to 240 volts which is considered as the safe working voltage range for most of the electrical and electronic equipments. The load current is supplied by the transformer within the UPS. Hence care has to be taken to see that the load connected to the UPS does not exceed the capacity of the UPS even during the AC mains mode.

The control circuit in a line interactive UPS continuously monitors the input AC mains voltage and switches between the mains and inverter mode automatically during low and high mains voltages.

These UPS with external Batteries can support for 2 to 3 hours and the backup hours can be customized as per the customers requirement.

This kind of UPS are mainly used for computer application.

Built In Battery UPS: Known popularly as BIB, this is a Line interactive UPS with Batteries inside it. Normally this kind of UPS will have backup of 15 -45 minutes depending on the capacity of Battery used and load on it and are suitable for computers.

In an online UPS systems the AC input voltage is rectified into DC voltage using the rectifier which supplies the DC voltage for the inverter and at the same time charges the Battery connected to the UPS systems. The Inverter then converters the rectified DC voltage into AC voltage and supplies power to the load connected to the UPS system during the AC mains. When the power fails or during abnormal AC input voltages , the Inverter derives its DC input voltage from the Batteries and continues to feed the load without any interruptions.

Basically Offline UPS , Line Interactive UPS and Online UPS systems differs in their construction and working principle. The following table wherein only few differences are listed will help you understand better the difference between these two kinds of UPS Systems.