A battery monitor apparatus and method for an automotive battery system including a battery for supplying power to in-vehicle electrical equipment and sensors for detecting battery voltage, the charge/discharge current and the battery temperature are disclosed. The capacitance and the internal actual resistance of the battery are calculated at the time of starting the engine. Further, the theoretical internal resistance of the battery corresponding to the ambient temperature and the battery open-circuit voltage during the stationary engine state are detected in advance. Based on the change in the battery open-circuit voltage, the battery change is provisionally determined. After that, battery change or degeneration can be determined based on battery capacitance, the actual and theoretical internal resistance values of the battery, the battery open-circuit voltage and the provisional battery change determination value.