Basically all cars before say 2010, the alternator VOLTAGE is regulated to 13.4 to 14.5 volts and the battery draws as much current as it need to, to charge.
When the battery is fully charged it draws only a small current. The exact voltage the alternator is regulated to actually changes a little with temperature, up a little in cold down in hot. That battery university web site has a lot of good detail.

BUT,

I did hear that some newer cars have some weird mode where the computer sets the alternator to a lower voltage as a fuel saving measure. That may be what the poster was seeing.

What year was the car you saw this behavior?

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