need help!! with alternator, and car cant keep a steady idle...

well today we replace the allternator after it went out while on my way to a carshow =(
but the problem is that my battery light still comes on. after replacing it, it wouldnt be able to keep a steady idle it kept going from 700-1200rpms. idk what can be the cause of that. any help would be greatly appreciated. thanks.

Did you check your voltage with a dvom and made sure your battery is fully charged prior to changing the alternator? They say if your alternator goes bad, it can also kill your battery. Make sure your connections/ grounds are good. Another way to test your alternator while in the car is putting a load.
*connect your dvom on your battery terminal
*start up your car and turn your headlights on with the high-beam on
*check your voltage on your dvom, should be in the 14
*Rev the car and if your voltage starts to drop, you have another bad alternator!
*IMPORTANT (make sure your battery is fully charged before doing this test)

My evo8 recently wouldn’t hold a charge thinking it was a bad alternator but to find out my terminal had broke off. G/L

thanks for the info chanseevo8. yes the battery was fully charged after we replaced it. not i realized that the belt also snapped!