You can buy new diodes, new bearings and new brushes for the original alternator. If you replace these parts, you have the equivalent of a remanufactured alternator for a fraction of the cost and you get to keep the original part with your car. It's not a difficult job. At the most, its two hours on the workbench if you take your time on it as a new puzzle.

Bad diodes or worn out brushes will cause the alternator to not charge. So can voltage regulator or wiring issues. Brushes seem to go at around 75K miles. A rare cause of failure would be shorted or open turns inside the alternator. In that instance, you're only option is to replace it.