Why is the alternator overcharging the battery?

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

i would say pcm.these. are notorious for getting spiked from ignition sys.have ya had any perfomance problems in the past?

yes it is possible

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.

If you have a fully charged battery when you installed then 14 volts is way to much charge for the battery which would mean that the alternator probably has a faulty regulator and the alternator needs to be changed otherwise you will continue to install new batteries. If the battery was in an uncharged state then it is possible for the alternator to charge at full capacity to charge up the battery.

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.

this can be caused by a shorted cell in the battery, has the battery tested? and clean the battery cable's, corrosion can also cause this condition.