There are nine diodes in an alternator. Six are configured as a three phase full wave bridge rectifier. The other three form a half wave bridge to supply the field winding via the regulator, and rarely go bad.
If any one diode in the full wave bridge rectifier is open, output current available will drop by 37%. This usually shows up at idle with the alternator light coming on if the headlights are on, or at least the headlights will dim at idle. The headlights will brighten as the engine speeds up and the voltage comes up to the point where the battery is charging again. This type of problem will leave you stranded on a rainy night.
If a diode is shorted or leaky, the battery will drain while parked.
