A method of charging a battery in an uninterruptible power supply system, the method comprising the steps of: a) initializing an indicator (Discharge AH indicator) indicative of the amount of charge supplied during a discharge cycle; b) making a determination as to whether the battery is being discharged for a first discharge cycle; c) setting a low battery shutdown voltage at a high level different from a non zero value if the battery is being discharged for the first discharge cycle; d) setting the low battery shutdown voltage at a level different from a non zero value lower than the high level different from a non zero value if the battery is not being discharged for the first discharge cycle; e) determining if the battery is still in discharge mode; f) monitoring an amount of charge supplied by the battery during a discharge cycle; g) suspending discharge of the battery if a voltage across terminals of the battery drops to the low battery shutdown voltage; h) determining if the battery has been discharged for a predetermined time interval if the voltage has not dropped to the low battery shutdown voltage; i) updating the indicator with the amount of charge supplied during the discharge cycle; l) determining (426, 104) if back-up power is still required; m) repeating steps a) to 1) until back-up power is no longer required; n) charging the battery during a subsequent charge cycle; o) monitoring charge supplied to the battery during the charge cycle; and p) ending the charge cycle and placing the battery in a standby mode responsive to the charge supplied to the battery during the charge cycle exceeding the amount of charge supplied by the battery during the discharge cycle. a) initializing an indicator (Discharge AH indicator) indicative of the amount of charge supplied during a discharge cycle; b) making a determination as to whether the battery is being discharged for a first discharge cycle; c) setting a low battery shutdown voltage at a high level different from a non zero value if the battery is being discharged for the first discharge cycle; d) setting the low battery shutdown voltage at a level different from a non zero value lower than the high level different from a non zero value if the battery is not being discharged for the first discharge cycle; e) determining if the battery is still in discharge mode; f) monitoring an amount of charge supplied by the battery during a discharge cycle; g) suspending discharge of the battery if a voltage across terminals of the battery drops to the low battery shutdown voltage; h) determining if the battery has been discharged for a predetermined time interval if the voltage has not dropped to the low battery shutdown voltage; i) updating the indicator with the amount of charge supplied during the discharge cycle; l) determining (426, 104) if back-up power is still required; m) repeating steps a) to 1) until back-up power is no longer required; n) charging the battery during a subsequent charge cycle; o) monitoring charge supplied to the battery during the charge cycle; and p) ending the charge cycle and placing the battery in a standby mode responsive to the charge supplied to the battery during the charge cycle exceeding the amount of charge supplied by the battery during the discharge cycle.