To make a battery pack (2) capable of connection over a communication network for the sake of maintenance of the battery pack (2). The battery pack (2) is provided with a network interface (12) given a unique IP address, which is connected to a LAN along with a network interface (11) of a PC body (1). Even if the PC body (1) is suspended, the battery pack (2) can be accessed over the LAN. A computer for maintenance, arranged on the LAN, can thus access the battery back (2) to acquire the operating status of a rechargeable battery (3) and data stored in a memory (8), such as a serial number, thereby performing maintenance.