What Stops The Alternator From Overcharging The Battery. Make sure the alternator is the problem. I thought about putting a Zener Diode in the circuit so that it would sent the voltage back to the motor but this raised two questions.

Make sure the alternator is the problem. Yes but it is likely that the new one will not last long either. An overcharging alternator provides a consistent voltage above the standard operating Voltage for your cars electronic system.
And eventually the whole electrical system in a car will fall.
Set the multimeter Dc voltage and set it above 15 volts. Begin with your car start and let the alternator start the battery. An overcharging alternator provides a consistent voltage above the standard operating Voltage for your cars electronic system. Yes but it is likely that the new one will not last long either.

