Alternators often fail from age and misuse. Disconnecting the battery when the car is running can cause loads on the diodes that will cause premature failures. Overzealous charging of the battery can also cause diode failure. Replace your alternator with a new one from Pelican.
If your charging system is giving you troubles you should check both the alternator and voltage regulator. The voltage regulator regulates the voltage within the car's electrical circuits and can fail if overloaded.