If the car detects that your battery is healthy it will opportunistically stop charging and let the battery support the entire electrical load of the car at idle (car stationary, engine running 800 RPM). This means your ECU, spark plug coils, infotainment, etc. When this happens, voltage can sag to below 12 under that load, but that doesn’t mean that the state of charge of the battery is that of a no-load battery showing 11.x volts.
A smart charging alternator tries to opportunistically charge the battery when you are decelerating or otherwise not needing to use extra gas to keep the alternator spinning. At idle, it doesn’t want to charge because that means giving more gas to the idling car to maintain idle speed.
Headlights, parking lights, turn signals, automatically boost the charging back to 14+ volts, so daytime driving charges the battery less than night-time driving. For that reason, many of us have started driving with our parking lights or headlights on during the day, just to force the alternator into full charge mode, especially for short trip driving, or infrequent driving.
When the car turns off, if you immediately check the voltage, the car is still somewhat “awake” and still drawing power and will show some voltage sag. If the fob is beyond detection range of the car, after some period of time (maybe 20 minutes?) the car will go to normal sleep – still a small parasitic drain remains for starlink, but you might see voltage rebound to say 12.3 volts, which is still undercharged but not 11.x volts.
If you use the infotainment system “factory mode” to look at battery voltages, just realize that this is not no-load resting voltage. It’s the voltage of the system either fully energized by the alternator, partially supported by the alternator, or completely depending on the battery itself. Yes the alternator may stop working on purpose even if the engine is spinning.
