alternator problems, need help
 

I have an '06 GMC, Duramax, 4X4, CC, 2500HD, with Banks Big Hoss package, and Air Lift w/ on-board compr. A few weeks ago, couldn't start truck as battery power was too low, even after a jump start, not enough juice to put it in gears, had to be towed 80 miles, ouch!
Figured since I had original batts, they didn't hold charge well, so bought two new batteries.
But the problem didn't go away, the red battery light kept coming on now and then, and when I ran a diagnostic on truck thru the banks IQ, and got codes P0622 and P0621, (alternator circuits L and F), so i figure alternator is bad. Get a re-manufactured one from Auto zone. Install it , but it appears it stops functioning after just a few minutes, and output drops to 5 or 6 volts. So I get another, from NAPA auto parts. Put it in this am, with the same disappointing result, put out around 13.96 to 14.3 volts for abt. 3 to 4 mins and then dropped to 5.6 volts, and red battery light came on. So, obviously i'm burning up alternators. Very frustrating, up to now truck has been a dream. What to do? I haven't got a clue, anyone had a similar experience, if so please help!!!

Seen this problem on a Dodge.Wound up being in the PCM that regulates output voltage of the Generator/Alternator.

I solved my own problem
 

I decided to get to the bottom of this problem, as it was really bothering me, and I couldn't use the truck, (had to borrow my wife's Toyota FJ Cruiser, and it ain't Turbo Diesel..)

I went thru a lot of the wiring, especially things that draw a fair amount of juice, such as: Head lites, Fog lites, Starter, Fans and blowers, Interior lites, Radio, CD player, and a couple of after-market gizmos I installed myself, an 800W Inverter, and a set of Big Rig long air horns..
I finally found that I had a slightly under-tightened connection between alternator hot lead and the batteries, (inside the little red plastic box), and that the inverter negative wire had in a few places, the insulation thickness reduced due to chafing, and that in those locations, it was creating a negative grounding, which sucked a great deal of juice out of the batteries, which was not being replaced by the alternator due to the inadequate positive to-batteries connection.After all this was corrected, the alternator is again working properly, and the Banks IQ vehicle Diagnostics shows no more codes (before P0621 & P0622).

Thanks for your suggestions