FOn a full charge, a 12V battery’s voltage will read 12.9 volts. If we are talking about a 12 volt car batter, when the car is running it should read between 13.7 Volts and 14.7 Volts. In case you don’t have a multimeter and want to know whether the battery is fully charged, turn on the headlights. If the lights are dim, then the battery is running low.