An uninterrupted power supply system incorporates a plurality of lithium batteries as a back-up power supply to an AC power supply. In the event that there is an interruption in the AC power supply, a power output switch is automatically activated by a power control system to draw power from a lithium battery pack. In an embodiment, a battery pack is monitored by a battery management system that is coupled to battery monitoring modules couple to each battery. The battery management system may be configured to monitor the voltage and/or temperature of each battery. In another embodiment, an uninterrupted power supply system comprises a battery unit balancing system that maintains each battery within a battery unit below a threshold voltage value. A charging circuit and discharging circuit are used to maintain the batteries in a ready state of charge when not being used for the output power.