I used to change out the alternators on my first car every couple of years. My father-in-law would take his alternators apart and change out the diodes, but I didn't have time for that - it was easier to slap in a new alternator and I got quite good at it. If the battery is over 3 years old, it could have a bad cell. Of course, it could be both - if the alternator is flowing too much juice, it could be burning out the battery.
The essential American soul is hard, isolate, stoic, and a killer. It has never yet melted. ~ D.H. Lawrence