The most common cause of alternators failing is the alternators diode and/or voltage regulator.
The alternators diodes are A part of the rectifier assembly that converts the alternator's AC output to DC.
A bad or leaky diode also can allow current to drain out of the battery through the alternator when the vehicle is not being driven.
A faulty voltage regulator can also cause charging problems with the alternator.
A bad alternator diode can also allow AC current to leak into the vehicles electrical system and cause electrical problems with the vehicle.