2003 Trailblazer not charging, battery light.

2003 Chevy Trailblazer with \~225,000 miles. It's got the 4.6l inline 6 cylinder.

A few days ago, my wife said the battery light came on and the voltage indicator went down to 9 or 10, instead of being in the middle around 14. Tuesday evening, the engine wouldn't start. I jump started it and it seemed to run OK. No battery light or voltage issues (at first). The weather had turned cold that day (\~22° f), so I figured that the cold snap killed the battery.

We drove to AutoZone, they put the tester on and said the battery and alternator are OK. On the way home (\~1 mile), the light came on and voltage went down, so I went back and got a new alternator. On the way home from that, the voltage got worse, the headlights and dash lights all went out, and I barely made it home.

I got the new alternator installed, and the engine started OK, then the battery light came back on and voltage went down. Headlights and other lights wouldn't come on. So I went back and got a new battery, and installed that, with new side post bolts so it's nice and tight. The cable ends are clean. The issue remains, the gauge still indicates low voltage and the battery light comes on. Not right away, it comes on after running for a minute.

Also that D321 on the trip odometer. I didn't notice it yesterday, I don't know if that's significant. And I know the dash is dusty, I'll get right on that. Thanks for any help or suggestions you can provide.