This book presents a new topology of the non-isolated online uninterruptible power supply (UPS) system consisting of 3 components: bridgeless boost rectifier, battery charger/discharger, and an inverter. The online UPS system is considered to be the most preferable UPS due to its high level of power quality and proven reliability against all types of line disturbances and power outages. The new battery charger/discharger reduces the battery bank voltage, which improves performance and reliability, while a new control method for the inverter regulates the output voltage for both linear and nonlinear loads. The proposed USP system shows an efficiency of 94% during battery mode and 92% during the normal mode of operation.