[Biturbos4] Brewing electrical problem?

[Chris Newbold]

The voltmeter has acquired a new behavior in the last few days that I
suspect is foretelling some electrical issue in my near future...

Cold start : Turn key "on" : almost 12v -- about normal given ambient
temps around 20degF
Start engine : cranks normally
Engine running : ~14v on the gauge -- this seems normal
Voltage stabilizes around 13.5v -- normal, too
After ~10 minutes : voltmeter gradually starts dropping from ~13.5v -- uh oh
After ~20 minutes : voltmeter barely at 12v when RPMs > idle
Barely ~11v at idle -- yikes!

Turning off all electrical consumers results in a slight up-tick, but
still under 12v.

No other signs of trouble (e.g. battery warning light, etc.). Engine
still seems to crank normally.

I suspect that something is about to let go, but what? Battery?
Voltage regulator (is this separate or built-in to the alternator)?
Alternator?

-Chris
2001.5 S4 Avant 6spd