The charge state of a battery powering a starter of a motor vehicle is evaluated to determine whether the battery voltage is sufficiently high to re-start an internal combustion engine during a stop/start operating mode. The engine is operated to drive a generator so that the generator output voltage is at a level below a setpoint value for a time period. At the end of the time period, the battery voltage is measured and if it exceeds a threshold voltage the battery charge state is determined to be sufficient to re-start the engine. The threshold voltage is approximately equal to the rated battery voltage. If the measured battery voltage is below the threshold voltage value, indicating that the battery will not have sufficient charge to re-start the motor, automatic shut-off of the engine when the vehicle stops is suppressed.