96 hyundai accent battery does not charge.?

okay i had the alternator tested it is good. I just changed the battery (with a used one) and it works fine as long as it's fully charged. i tested for continuity between the positive and ground. with the key off and i get high resistance (3000 or more) with the key on 10. I unplugged the radio fuse as it is the only draw on the battery and retested, no continuity at all. I tested for continuity between alternator and battery and i get 0 resistance. I can push start my car and with it running i get 9v and 1 amp but i may be on the wrong scale. also the volts will steadily drop untill thry get to 6v the the car dies.