Hey Guys-
Long time absent for me, but whom ever posted these charging voltages on this forum, below, I just want to thank you.
I was getting high voltage readings after startup at the battery, but then after several minutes (5), things came back to normal
after a recheck
'07 ADV 5SP 87234 MI

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12.6v resting
11.4v cranking
15.1v startup
13.6v after 4 minutes( jumped to 13.9v when I turned accessories/lights on)
12.8v after 10 minutes (with all accessories off)
My GBCM is working properly.

If you research your dic error, you will see many posts about GBCM bypass, I don't recommend!

High Output: A Look at Today’s Charging Systems

The GM regulated voltage control system can vary from 11.5 –15.5 volts. As with other systems in use today, the GM system has a fuel economy mode, which is designed to lower the charging system voltage by reducing the alternator field strength.Technicians often diagnose this mode as a faulty charging system, only to find out that the vehicle operates the same even with new parts installed. Simply applying maximum load to the vehicle’s electrical system will cause the PCM/ECM to respond, raising charging system voltage.

GM also designed this system to identify battery sulfation. In response, the PCM/ECM will command the charging system’s voltages to increase to as high as 15.5 volts, even when there’s no electrical load. Sulfation mode will typically last less than five minutes at a time, so, as the timer expires, the system will return to normal charging operation.