Here's my guess Matt...

The belts were slipping when the voltage was below 12V because the alternator's load is at its maximum just after starting the car. Your belts were slipping without making noise or you would have mentioned the typical squeal.

Once it began to charge your reading was OK. If the Bentley were to quibble about millivolts here, it would specify the temperature and where your probes belong. Check for voltage drop between B+ terminal and battery positive post; alternator frame ground bolt and battery negative post. This, and temperature of the regulator module is where the millivolts matter.

This chart just shows how the temp affects voltage...





--
Art Benstein near Baltimore

Real Engineers consider themselves well dressed if their socks match.
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