A battery management system comprising: a power source constituted by a plurality of battery modules (5A to 5L) having battery cells; battery module status sensors (12A to 12L) mounted on the battery modules (5A to 5L) on a one to one basis, adapted to detect voltages and temperatures of the battery modules, and mutually connected in series by a communication line for transmitting numbering data, and each battery module status sensor adapted to assign itself an ID code on the basis of ID information received from an upstream battery module status sensor, and adapted to transmit the ID information as well as the ID code to a downstream battery module status sensor; and a control unit adapted to judge statuses of the battery modules on the basis of data detected by the battery module status sensors (12A to 12L), connected to the battery module status sensors (12A to 12L) via the communication line for transmitting numbering data and a communication line for transmitting various data except for the numbering data, and adapted to locate an abnormal battery module on the basis of detection information received via the communication line for transmitting numbering data and the communication line for transmitting various data except for the numbering data, characterized in that each battery module status sensor (12A to 12L) is adapted to: compare a current ID code with a previous ID code; upon the current ID code and the previous ID code being the same, each battery module status sensor (12A to 12L) transmits to the control unit information that its ID information has been correctly assigned with the current ID code, upon the current ID code and the previous ID code being different or no previous ID code being assigned, each battery module status sensor (12A to 12L) stores the current ID code in its ROM, checks whether or not the ID writing has been normally completed, and transmits information to the control unit, wherein the information transmitted to the control unit comprises, upon the data writing has been normally completed, information that the ID information has been assigned with the correct ID code or information that the ID code has been normally updated, and upon the data writing has not been normally completed, information that the ID information has not been assigned with the correct ID code. a power source constituted by a plurality of battery modules (5A to 5L) having battery cells; battery module status sensors (12A to 12L) mounted on the battery modules (5A to 5L) on a one to one basis, adapted to detect voltages and temperatures of the battery modules, and mutually connected in series by a communication line for transmitting numbering data, and each battery module status sensor adapted to assign itself an ID code on the basis of ID information received from an upstream battery module status sensor, and adapted to transmit the ID information as well as the ID code to a downstream battery module status sensor; and a control unit adapted to judge statuses of the battery modules on the basis of data detected by the battery module status sensors (12A to 12L), connected to the battery module status sensors (12A to 12L) via the communication line for transmitting numbering data and a communication line for transmitting various data except for the numbering data, and adapted to locate an abnormal battery module on the basis of detection information received via the communication line for transmitting numbering data and the communication line for transmitting various data except for the numbering data, characterized in that each battery module status sensor (12A to 12L) is adapted to: compare a current ID code with a previous ID code; upon the current ID code and the previous ID code being the same, each battery module status sensor (12A to 12L) transmits to the control unit information that its ID information has been correctly assigned with the current ID code, upon the current ID code and the previous ID code being different or no previous ID code being assigned, each battery module status sensor (12A to 12L) stores the current ID code in its ROM, checks whether or not the ID writing has been normally completed, and transmits information to the control unit, wherein the information transmitted to the control unit comprises, upon the data writing has been normally completed, information that the ID information has been assigned with the correct ID code or information that the ID code has been normally updated, and upon the data writing has not been normally completed, information that the ID information has not been assigned with the correct ID code.