If Larry's advice checks out and you do happen to get the proper voltage output from the alternator, then it's time to see if you have a small electrical leak.

Disconnect the negative battery terminal and connect an ammeter that can read milliamps between the negative battery terminal and the cable clamp.

Watch the meter. You should get a reading below about 60 mA. If it is above 100 mA, you need to start pulling fuses to see which circuit is drawing the most power. Also, remove all the wires from the back of the alternator to determine if the alternator has a small leak.

When you find the culprit, the ammeter should get down below 60 mA, or about.