Nobody likes an overachiever and the same goes for car parts. The second most important part of a car is the battery and sometimes it can be too energetic. Just like overcharging a phone, your car battery voltage can be too high. High voltage can be damaging to your battery and your vehicle. How do You Test Battery Voltage With a Voltmeter?
If the voltage is too high, the alternator may be disengaged so it no longer produces a charge, or the battery circuit may be isolated to protect the battery from damage. In this case, you may find that the vehicle won’t start or it may loose power when driving.
Up to this voltage and at normal temperatures, the battery will only gas very slightly. Above this, and it will gas increasingly with increasing voltage. This will damage your plates by causing flaking of plate material and shortening your battery's life. Gassing is undesirable. Note, by gassing, I don't mean boiling.
Turn on your voltmeter and make sure it’s set on the “voltage” setting. Place the red sensor on the positive terminal and the black sensor on the grounded (or negative) terminal. Check to see the reading and if it is over 12.9 volts, your battery may have excessive voltage. 12.6 to 12.8 is the ideal voltage level for your battery.
A regulated charger will hold the voltage to 13.5 to 13.8 volts once the voltage has peaked, to prevent gassing and overcharging, and the current will drop to almost zero in a healthy battery. Gassing due to overcharging also "uses up" the water in your electrolyte, and in sealed batteries will fairly soon ruin the battery.
Voltage goes beyond 14.7: The alternator is overcharging the battery. Ideally, the voltage remains at 13.5 to 14.5 with the engine running. This is the perfect amount of power to recharge the battery while providing support for other electronic equipment in the car. In nearly every case, the voltage regulator is to blame.