Battery guage confusion...i think?

Whenever I give the Stang gas....the battery guage goes all the way over to the right...(18Volts) and the battery light comes on?? Any guesses why?
I have an 800CCA Optima but I am not sure which alternator.....could the alternator be too powerful?
 
The regulator is on the back of the alt IIRC (External).

You can have a parts store bench test the alt if you desire.
Check all wiring, fusible links, etc first (there is a voltage sensing wire which goes from the power cable to the gauge).

Good luck.