Unplug the alternator (wire that go to alternator) and turn the headlights on and try the test again.

That'll at least confirm if it's the alternator circuit or the headlight circuit.

One of our club members could use some help.  On our forum, he writes that with his headlights on, he can barely make it up his driveway but "If I turn them off, it is like a nitro boost and I zip right up the hill."  The car is a 998 equipped with an automatic transmission.  He says that the car starts fine and that the alternator voltage to the battery was at the lower limit but passed.  Any thoughts??