I have a 2000 A4 1.8T. Usually my volt gauge reads around a 14 but just today when I really step on my gas after a few secs my volt gauge drops down to 12 and the battery light comes on. Is that because of my battery or my alternator? Someone help me.

Sounds like your alternator is going- Any Autozone can testyourbothfor free. I have an A6 and had the same symptoms a few weeks ago and needed to replace my alt. approx. $500 to fix.