Check Charge System

I Just recently had some work done on my 95. Got A new Alternator and belts and a new ac Compressor put on. Now my check charge system and battery light is on. Any ideas on what this could be thats making this happen? Thanks

what voltage are you indicating when the engine is off and then when at 2000 rpm.

With the engine off its at 12.4 volts. With it on, driving or parked its at 14.4

I also am having a problem after replacing my alternator and serpentine belt. Some of the time when i drive my car the voltage decreases and increases repeatedly ranging from 15.4 to 12.4 this causes all lights to dim. Sometimes the voltage will just stay under 12 volts usually 11.8.