Normally the voltage in the 2015 GT reads > 14 volts indicating that it is being charged properly but yesterday it went to 12.8 (battery voltage). Today it was OK (back to > 14). No faults codes or the battery light appeared.

I assume it is being done on purpose either to save fuel and/or to cycle the battery like a battery tender does. My other car does it occasionally as well.

Anybody else notice this? Can anyone confirm my theory?

 

yes. modern cars optimize their charging strategy based on engine load, electrical demand, and other things. we're long past using dumb bi-metal strip voltage regulators; your car's PCM controls the alternator's field. if the display said your car's supply voltage was 12.8, then that means the demand on the electrical system was low and the alternator wasn't called into doing anything.

 

Thanks for the quick and detailed answer!