Battery system

A battery system comprises: a battery module which is formed by serially connecting battery cell groups each having serially connected battery cells; integrated circuits which are provided so as to correspond to the battery cell groups of the battery module and which detect and diagnose the terminal voltages of the battery cells of the corresponding battery cell groups; and a battery controller which issues commands to the integrated circuits and receive the result of the detection and the result of the diagnosis which are performed by the integrated circuits. The battery system is provided with writable non-volatile memories, and the writable non-volatile memories hold data which represent the use environment of the battery modules including the maximum voltages or the maximum currents of the battery modules, and also holds history data which are prepared on the basis of the use process of the battery module.