The voltage regulator has failed. You must have a later model vehicle that has a computer controlled alternator, you may need a new computer.
The old alternators had an internal voltage regulator. Years ago I had this problem on my 82 Firebird TransAM. The voltage was 18 volts. I bought the regulator for a few dollars and replaced it myself. Removing the bolt that holds the pulley on is the tricky part. You need an impact wrench for that. Reinstalling that nut is the tricky part. I've seen guys in the auto parts stores do this for a customer.