An uninterruptible power supply unit comprising: an input/output terminal which is connected in parallel with a power supply line supplying electric power from an external power supply to a load device; a battery unit including a first battery pack and a second battery pack, each of which has a rated voltage equal to the voltage of the external power supply; a charging circuit configured to boost the voltage of the external power supply to a charging voltage of the first battery pack and the second battery pack, and further configured to charge the battery unit with the boosted voltage; a discharging circuit which is configured to, at the time of power failure of the external power supply, discharge electric power from the battery unit to the load device via the input/output terminal; and a control unit which is configured to control the charging circuit and the discharging circuit, characterized in that wherein, during charging of the first battery pack, and until, after the first battery pack is charged to a fully charged state, the voltage of the first battery pack is lowered below an upper limit voltage of the load device, the control unit is configured to prohibit discharging of the first battery pack from the first battery pack to the load device via the input/output terminal and is further configured to allow discharging of the second battery pack from the second battery pack to the load device via the input/output terminal, and during charging of the second battery pack, and until, after the second battery pack is charged to a fully charged state, the voltage of the second battery pack is lowered below the upper limit voltage of the load device, the control unit is configured to discharge the second battery pack from the second battery pack to the load device via the input/output terminal and is further configured to allow discharging of the first battery pack from the first battery pack to the load device via the input/output terminal. an input/output terminal which is connected in parallel with a power supply line supplying electric power from an external power supply to a load device; a battery unit including a first battery pack and a second battery pack, each of which has a rated voltage equal to the voltage of the external power supply; a charging circuit configured to boost the voltage of the external power supply to a charging voltage of the first battery pack and the second battery pack, and further configured to charge the battery unit with the boosted voltage; a discharging circuit which is configured to, at the time of power failure of the external power supply, discharge electric power from the battery unit to the load device via the input/output terminal; and a control unit which is configured to control the charging circuit and the discharging circuit, characterized in that wherein, during charging of the first battery pack, and until, after the first battery pack is charged to a fully charged state, the voltage of the first battery pack is lowered below an upper limit voltage of the load device, the control unit is configured to prohibit discharging of the first battery pack from the first battery pack to the load device via the input/output terminal and is further configured to allow discharging of the second battery pack from the second battery pack to the load device via the input/output terminal, and during charging of the second battery pack, and until, after the second battery pack is charged to a fully charged state, the voltage of the second battery pack is lowered below the upper limit voltage of the load device, the control unit is configured to discharge the second battery pack from the second battery pack to the load device via the input/output terminal and is further configured to allow discharging of the first battery pack from the first battery pack to the load device via the input/output terminal.