1999 Chevy Venture Charging system

I have a 1999 Chevy Venture and the charging light is now coming on. Replaced the battery (battery was tested and was bad), but the same problem is there. If I idle up the motor slightly the light will go away, but at an idle the light will light back up. I have had the alternator replaced approx. 3 years ago (again tested and was said to be bad). warranty and extended warranty now out of the question so I am on my own, any help would be appreciated. I can do most of the work myself just need some suggestions.

Thank you,