@gasman Yes, the alternator/internal regulator combination is also controlled by the engine management system. As the ECM (or ECU if you are a Ford guy) got faster and more powerful, the engineers figured out that they could squeeze out better fuel economy, better performance, and lower emissions (the holy trinity of automotive engine engineering) by controlling the alternator. You could give it higher output power but only have it work under certain conditions. This makes the “ALT” or “BAT” warning light function more complicated. It knows the required voltage at all conditions and moves the threshold continuously. Most cars have it all integrated into the Check engine light.
Just for fun, hook a good volt meter on your car battery so you can see it from the driver’s seat and go for a drive. You will be quite surprised. The voltage is not constant.
And – VoilĂ ! – Better performance, better fuel economy, lower emissions.
Hug the next engineer you see.
@jerv You’re correct. I should have specified AA Alkaline batteries in my example.