problem: my battery light is coming on and eventually my battery dies. battery is a 2 year old interstate battery.

what i have done:
-i brought the car to advanced auto and they tested the battery and alternator on site, both tested fine with 14+ volts when the system was under distress(brights, high fans, etc) battery tested at over 12v.
symptoms persisted...
-i took the alternator off and brought it to oreilly auto for a bench test. again it tested fine. i put it back in and i made it 3 days without the battery light coming back on or any issues.
after 3 symptoms returned.
-replaced serpentine belt and cleaned all connections, the car made it 3 miles down the interstate before the battery light returned and the car eventually lost all power.

i have drive 12 miles of interstate a day. typically since the issue arose i have been able to get to and from work. after i get home from work and turn the car off i have been able to turn it back on and drive the remainder of the night without the battery light coming on and i seem to have a full charge.