[RichardJ]

An alternator is an AC generator. There are three windings in the Stator and they produce phased AC voltages like that shown in the first image. There are two diodes per winding with a total of six. The diodes chop off the negative portions of the AC signal and you end up only the positive 12 volts DC, with the ripple as you see in the second image.

If you lose one of those AC signals, the other two pulses can still produce a DC voltage that is above battery voltage. In other words, enough to charge the battery. If you lost that one AC signal because of a shorted diode, that diode will act as a short to ground and drain your battery when the engine is off.

The third image doesn't give a DC reference voltage indication, but the scope pattern in (B) can still be above 13 volts and charge the battery as if nothing were wrong.