Bad battery
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Neil, I first saw your post in the tech forum and answered in more detail there. Here is a copy of that info but to summarize what you are seeing is perfectly normal.
Copied from my other post in the tech forum....
There is nothing wrong or abnormal with 12.9 volts in a modern GM vehicle with the engine operating. The alternator output level commanded is part of the fuel efficiency strategy and also a way of increasing battery life through heat/overcharge management. When the system senses the battery is low it will command higher alternator output via greater field excitation resulting in higher system voltage and a faster charge rate but once the battery is at/near full charge the voltage will ramp back to the mid to high 12 volt range. My 2016 Z06 behaves this way as does my 2014 Cadillac ATS and my 2008 CTS was the same.
If you monitor voltage you will see even greater swings on a long trip as it goes from heavy charge to a very light system maintenance level.
Copied from my other post in the tech forum....
There is nothing wrong or abnormal with 12.9 volts in a modern GM vehicle with the engine operating. The alternator output level commanded is part of the fuel efficiency strategy and also a way of increasing battery life through heat/overcharge management. When the system senses the battery is low it will command higher alternator output via greater field excitation resulting in higher system voltage and a faster charge rate but once the battery is at/near full charge the voltage will ramp back to the mid to high 12 volt range. My 2016 Z06 behaves this way as does my 2014 Cadillac ATS and my 2008 CTS was the same.
If you monitor voltage you will see even greater swings on a long trip as it goes from heavy charge to a very light system maintenance level.
