Battery Light

13.9 is the voltage. But does not get into peak amperage. You could have nearly no amps at 13.9 volts. 99% of the time the battery light is on, replacing the alternator is the needed repair. The other 1% is when the wiring connector is compromised.

It is pretty common for the light to stay on even if it is charging well. There are multiples of things that can cause the problem, wiring,alternator, and computer The usual fix is to replace the alternator. Most likely someone has replaced it with an aftermarket replacement

May have alternator diode problem but still charges ok. dale45 is correct. How do you know is charging? You checked it or shop with test equipment? How did you check it?