A battery pack updating method updates firmware stored in internal control circuit memory via an update signal sent from a main device that supplies power. When battery pack memory is updated, an AC adapter is connected to the main device, and power is supplied to the main device from the AC adapter. The updating method transmits a charging and discharging blocking signal from the main device to the battery pack control circuit via a communication line. The charging and discharging blocking signal stops discharge from the battery pack rechargeable battery, and stops charging of the rechargeable battery. With rechargeable battery discharging and charging to and from the main device stopped by the charging and discharging blocking signal, the updating method transmits update data from the main device to the battery pack control circuit to update memory.