Checking the generator voltage (Opel Corsa C)

8. If the battery does not charge or charges insufficiently while the vehicle is moving, the generator voltage must be checked.

9. Connect a voltmeter between the positive and negative terminals of the battery and start the engine. The voltage at start-up should drop to approximately 8 V (at an ambient temperature of +20°C).

10. Increase the engine speed to 3000 rpm - if the generator and regulator are working properly, the voltage at the terminals should be from 13 to 14.5 V.

11. To check the voltage stability, turn on the high beam and repeat the measurements at 3000 rpm. The measured voltage should not increase by more than 0.4 V from the previously measured values.

12. If the readings go beyond the nominal value, the generator and regulator must be checked in a specialized workshop.

(Material copied from this website OpelBook)