The component that has the highest failure rate on most alternators is the bridge rectifier. The bridge rectifier (aka rectifier) is the component that converts the Alternating Current (AC) that an alternator makes to Direct Current (DC) for use by the vehicle. Over time or under high demand situations the diodes in the bridge rectifier can become weak, they don't do as good a job as they did when they were new. You can tell your bridge rectifier is going bad when you first start a cold vehicle and it charges better at first compared to after it warms up. As the diodes in the bridge rectifier heat up they don't work as well. In high demand situations the more AC power the alternator makes, based on demand, the more power the rectifier has to convert to DC leading to a greater chance of diode failure.