Hey nice site! Just picked up a 00 GT 5speed 68k on clock.

When I get in thegas...the battery light starts to blink.

Bad battery? Bad tensioner? or something else?

 

welcome!
It sounds like your voltage regulator and possibly your alternator is shot.
The regulator is in the alternator... I had to replace mine and upgraded to a 3g in the process.
I don't know what kind of alt. 00 mustangs have though

 

Well it also has the orginal battery and is almost 8 years old. Think I will start there. but wasn't sure if this was a normal thing as far as it flashing...

 

Check the battery voltage with the engine off. Then start the car and check it again. have someone rev the engine a few timesand see if the voltage drops. Its an easy way to check the charging system. 8 years is a long time for a battery, but it could be something else. I,ll check my 99 sometime and will have good data to usein future posts.

 

99% of the time that light comes on it means the alternator is toast. well then again there are times that the belt could be loose, and not that the alternator is defective. if it's not on tight enough the car will run off the battery, and trigger that light because it isn't functioning. i don't even know if the light would come on if the battery was done. everytime in every vehicle i've owned or driven when the light came on it always involved replacing the alternator. i can think of 6 different vehicles i drove while the light was on. 4 of my own.