Uninterruptible power supply system and method for reconditioning an exhausted battery module, wherein a terminal voltage of the battery module is determined and compared with a predefinable threshold value after starting a network-side supply, where after a waiting time has elapsed, when the threshold value is fallen below, an inverse polarity protection unit is activated for a time pulse, such that when communication between the base unit and the battery module occurs, the inverse polarity protection unit is permanently switched on and a voltage drop is determined and compared within the battery module with a predefined minimum voltage, and if at least one determined voltage drop falls below the predefined minimum voltage, then a predefined charging voltage is applied to the battery module until either a predefinable period of time is exceeded or a predefinable current value is surpassed by a charging current determined in the base unit.