As Trichard says - get it fixed quickly before the over-voltage evaporates the battery acid and/or does other damage.

The cause may be that the battery is not taking the charge, BTDT. Get the battery checked ASAP.

The only control/feedback that alternator has is to vary the output voltage. If the battery won't take a charge (in my case a terrible rough rocky road 20-mile drive had knocked the lead oxide off some of the plates and it fell to the bottom of the cell, three cells were killed), the voltage regulator raises the output voltage.

In my case, once started (rolled down a hill) the car ran, but the headlights were super-bright and the little voltage regulator for the dash instruments was buzzing like mad. Trying to restart gave symptoms of a dead flat battery.

Good Luck,

Bob

:>)