1999 dodge durango electrical problem

I have a 1999 Dodge durango that isnt charging the battery. When you get the car started it runs for a few min and then dash lights start coming on and the headlights dim and finally the car dies. I removed the alt and had it tested at my local auto parts store they say it is good. The Battery is good also. Surpatin belt is good also. What else could it be and how would I check it.

Thanks
Scott