In the previous tutorial, Online UPS was assembled from its basic building blocks like Lead Acid battery Charger, Square Wave Inverter and Modified Sine Wave Inverter. Now in this tutorial, offline UPS will be designed. The Offline UPS is one of the topologies of UPS (Uninterruptible Power Supply). In UPS system, the term topology refers to the switching mechanism of the UPS system.  Contrary to online UPS, in offline UPS, the load circuit is initially directly powered by the AC mains. On the failure of power supply, the power failure is detected by a switching circuit and the power is provided to the load through the battery and inverter after the power failure has already occurred. So, in the offline UPS, the load experiences the power cutoff once before it receives the power backup from the UPS.

In this project a mobile/battery charger is made by using voltage divider rule. Voltage divider is also known as a potential divider. Potentiometer having two resistor in it is the best example of a voltage divider rule. This charger  has reference voltage lower than the input voltage. Using voltage divider any voltage can be derived by it by passing high voltage than the voltage we want. This is because output voltage which comes out is always lesser. Voltage dividers are used for adjusting the level of a signal, for bias of active devices in amplifiers and for measurement of voltages. A Wheatstone bridge and a multimeter both include voltage dividers. A potentiometer is used as a variable voltage divider in the volume control of a radio, in finding temprature, in battery charger, in loud speaker etc.