What damages the regulator causing the alternator to overcharge

An alternator starts with 14 volts then drops back as the battery charges up. You may have a bad regulator in the alternator.

Sounds like a faulty regulator rectifier on the alternator borrow a compatible alternator to test the charging system
It usually is cheaper to replace the whole alternator than have them repaired

Change the alternator

Don't know much about your vehicle but sounds like you may have an external voltage regulator...need to find out no matter how many alternators you change if the regulator is allowing too many Amps and voltage out..you could damage the entire charging system including the battery..confirm what your car has can checked to see if the alternator you bought has an internal reg also have your auto store go in computer an see if in deed you can order an external one for your car..if available you know it does have one or not.. But overcharge is all in the regulator..good luck..

Over charging could be the result of several things including:

Faulty Voltage Regulator
1. A car's battery will overcharge if the voltage regulator isn't working correctly. A voltage regulator is usually part of the alternator, and is used to keep a steady flow of voltage to the battery. If the voltage regulator is defective, it will send either too little or too much charge to the battery. If it sends too much, the car battery will overcharge. The voltage regulator is easily replaceable at an easily affordable price, sometimes for as little as twenty dollars, as of 2009.
Faulty Alternator
2. Sometimes the alternator itself can be at fault. The alternator is the device that converts the mechanical power of the engine into electrical power to charge the battery. When an alternator breaks, it usually stops creating electricity for the battery, which will then eventually die. However, if the wrong alternator is placed in the car, or if the alternator is running at the wrong pace, it will create too much energy for the car battery, causing it to overcharge. The alternator is another easily replaced part.
Incorrect Charger Use
3. If a battery charger is used to charge your battery outside of your car, improper use of the charger can result in overcharging. If a battery is placed on the charger too long, it can result in overcharging, and a significant decrease in your battery's lifespan and efficiency. This is why it is important to read about your specific battery and understand how long it needs to charge to be effective. Too much charge will lead to problems.
Faulty Battery Chargers
4. Sometimes chargers can be faulty. Their settings may be wired incorrectly, or the charges labeled incorrectly. As a result, your battery may be getting overcharged, even if you are carefully monitoring your charging. This is a problem that is hard to avoid, as manufacturing mistakes can happen anywhere, anytime without warning. It is a good idea to test your charger regularly to see if it is running correctly.
Heat
5. Extreme heat in the summer can also have an adverse effect on the car battery. If the battery has been previously overcharged, extreme heat can increase the problems caused by overcharging, and exacerbate any other problems with the battery. This problem can be hard to avoid if you live in a warm climate. The only real way to ensure safety against this problem is to avoid overcharge in the first place.

You need to bypass voltage regulator to be certain alternator is charging properly. When you full field alternator it eliminates voltage regulator. If it indeed is good it will overcharge. If it overcharges as it should when you bypass/full field alternator then the regulator, or its connection or the place it attaches to is rusty & wont conduct a ground.

If overcharging to much can damage the computer and burn up the battery, if the battery is a service free battery then it could boil the water out and if their is a spark it could cause the battery to explode. Most free maintenance batteries are not maintenance free, they can still have water added to them and need cleaning from time to time as the posts corrode up.

Did you wire in the Cummings PCM ?

Never saw a charging system problem cause anyone to fail inspection. But, overcharging will cook the water out of your battery and can damage the computer system if not taken care of. Overcharge can be caused by either a grounded alternator field wire or a bad regulator. Since the condition appeared after replacing the alternator it too is suspect as an internal grounding can also cause that.