A battery device includes battery module, each including an assembled battery and a battery monitoring circuit. The assembled battery comprises battery cells. The battery monitoring circuit measures voltages of the battery cells, current and temperature levels of the assembled battery. A control circuit is connected to the battery monitoring circuits through a communication line and supplies power to the battery monitoring circuits. The control circuit communicates with the battery monitoring circuits through the communication line. Each battery monitoring circuit is configured to monitor signals transmitted on the communication line for a predetermined time period after its startup, and then to select a communication ID to one that does not conflict with any other ID used to communicate through the communication line during the predetermined time period. The selected ID is then transmitted on the communication line.