1999 Dodge Intrepid Charging Issues.

I have a 99 dodge intrepid 3.2 liter V6 that is right now sitting with a totally dead battery. Apon boosting vehicle the car will start and run and I have left vehicle running for over 30 minutes, drove vehicle to increase power and charging amps per say and get back home and cut car off and try to restart and nothing. No interior lights, no click apon turning key, just totally dead. I have boosted vehicle and removed positive connection on battery and vehicle dies instantly. I have also removed alternator and taken to Local auto parts store for a test of alternator and they ran 4 test with me looking and every time the alternator passed. The employee that performed the test said that output from the alternator was possibally too low to run vehicle. The battery that I have is an Orbital 6 Cell High Performance battery rated at 1280 cold cranking amps. Also before alternator issues I had about 5 occasions where others using my vehicle would operate my stereo system and I have 1500 watts at least of running off my vehicles electrical system at any given time and I use a Switch up front to turn amps on and off and they left switch on a few times causing battery to die. I fear this may be part of the problem as to what may have caused a more servere problem. Any help with a solution would be greatly appreciated.