1993 Nissan Sentra altenator will not charge battery

Any help please,
I have a1993 nissan sentra xe car. The other day I notice that the batteryu would not stay charged. I had battery tested and it was a good battery, so I tried the only thing I know to test the altenator myself. I started car and remove one of the battery cables from the battery and the car cut off. I was told that my altenator could be bad because the car cut off when battery cable was removed. Also I can jump car off cause I have ran battery down and car will run for 2 minutes or so and it will cut off and will not start again unless jumped off. I took altenator to a parts store to have tested and was told it was bad. Being that I not intirely trust astore that s in business of making money when possible I took altenator to another parts store and they tested it 3 times and said it was a good altenator. Does anyone have any suggestion on what could be wrong?