A battery performance monitoring system anticipates the sudden fall of battery output. The system measures the battery voltage at a time point when the discharge current of a battery becomes smaller than the reference current value or at a time point when the current to the load which is connected to the battery becomes smaller than the reference current value, and judges the battery to be abnormal if the difference of the measured battery voltage from the approximate formula in regard to the current-voltage relation in the former half of a same discharge event becomes out of the reference range.