Battery overcharging
 

Ok so my Alternator went out on me so I replaced it yesterday. So then today when I was coming home from work I was getting on the highway I punched it then my volts Meter shot up to 18 volts the come down when I let off. Need help I had the alternator and battery tested all it good.

It only goes up to 18 volts when the car is up in hight RPMs like after 4,000rpms

The rebuilt alternator has a bad regulator. I had the same problem with a reman alternator in my wife's Celica. Take it back and make them replace it. The tester obviously doesn't rev the alternator up enough to trigger the fault.