Have your alternator bench checked. It could be good and you have a voltage regulator problem. More than likely it is the alternator. If you buy a replacement alternator, have it bench checked before you leave the store. Also check the pulley to make sure it is the correct one.
I purchased a Dodge truck alternator, had it bench checked, all good. Installed it and the pulley was not the right one. I had to take it off go back to the store and replace pulleys.