2nd Update, as I think now the charging system is operating normally.

I checked. It drops to something between 11.8 and 13 volts *momentarily*.

After the slight drop, it rises pretty quickly (3-10 seconds?) to something
between 14 and 14.5 volts.

When I turn off the loads, it stays up over 14 volts for a little while. (I
didn't wait for it to drop.)

If I then turn off the car, then turn it back on (no loads), the voltage is back
to about 12.7 volts (here in the summertime).

I can repeat the above events merely by turning on the headlights, though it's a
bit more dramatic with more electrical loads.

I'm buying this.

My best guess as to what was happening in February (when unloaded, the battery
terminal voltage was 14.4 volts) is that it is somehow related to the
particularly cooler temperatures then compared to now.

Darn tootin'. I'm sure as heck not going to replace an OEM, dealer installed
alternator after a lousy five years and 50k miles... :-) That puppy better make
it to 100k miles or eight years, AFAIC.

I hope!

Autozone Aside: I drew on the "expertise" of the fine folks at Autozone today.
They did what they said was an "alternator test." The guy hooked up to the
battery terminals a sophisticated-looking electrical tester. (IOW, it does more
than my $15 Radio Shack digital voltmeter.) He read off voltages while operating
the throttle control manually (under the hood). He got the same sub-13 volts I
got. He never changed the electrical loads. His conclusion: Buy a new
alternator. My conclusion, "All he did was what I pretty much did, so not so
fast." I thought he'd at least take some readings at the alternator terminal.
But one gets what one pays for, I suppose. This check was free.

Firestone (where I bought my new Insterstate battery) aside: After buying and
installing (in the Firestone parking lot) my new battery Monday, I checked the
voltage and saw no change from my previous readings of about 12.7 volts. I
popped into the Firestone showroom and asked the guy if he'd give me his
opinion. He said I was wrong about the 14+ volts being usual when the car was
running. I said okay and left, not wishing to start a row and knowing this was
my problem, anyway. At this point, I feel the guy was speaking with some
legitimacy.

Lastly: Thanks, Jim.

Do Honda's have voltage regulators? ISTR that a dead voltage regulator
could lead to the appearance of a dead alternator in 80s GM cars, but I
may be misremembering.

Abe

John Ings wrote:

theyre worth replacing, regardless. along with the connection to the
alternator. if the readings still dont improve, its new rebuilt
alternator time.

Randolph,

I understand what you're saying and so checked the voltage at the battery
terminals first thing this morning, before the engine had anywhere near warmed.
Voltage at the battery terminals remained about 12.7 volts, whether idling or
not.

I am not sure whether the engine was warmed up or not when I checked and got
proper voltage readings this past February. But regardless, in the future I
agree this is something to consider.

The car continues to run fine. I drove it with the headlights and radio on in
darkness for about 50 minutes last night and another 50 minutes this morning.

Thanks Jim, Eric, Rex, Mark (= spike), John, Abeness, and Mike for your input.
All has been considered. I can't reject any of the theories and am bearing all
in mind as I continue to work on this. I have made progress. See below.

John, I'm going to go work on cleaning up the battery cables, especially the
ends, a bit more this afternoon.

Mike, I'm looking into replacing as many battery cables as I can easily do. Some
of these wire harnesses/bundles seem like a bit of a morass to get into. I guess
there's an argument that (low load?) wires age with time and so should be
replaced every so often, but I wonder whether the amperage load through these
wires is low enough that they tend to last a very long time. They don't see the
same sort of load (via fatigue?) that spark plug wires see, I reckon.

Abeness, yes, Majestic Honda online indicates this alternator has a regulator
(assembly).

Further Update:

This afternoon I started the steps at
http://www.honda.co.uk/owner/ConcertoManual/62sk301/16-66.pdf and got to about
#6. The first thing I noticed was that the alternator connector's plastic
fitting (see step 2) was partly broken on the female (= alternator) side. This
breakage is around the piece that snaps to hold the connector very securely in
place. It looked like maybe someone had got rough with it and pried the
connector free, breaking the plastic in the process. (I don't recall
disconnecting the alternator connector in the past, but maybe it was in the way
and I did disconnect it at some point. Or maybe I broke it today while
disconnecting it! It's kinda hard to see down there. It came free too easily,
based on my experience with similar connectors where a little screwdriver may be
used to free up the plastic clip. I suppose the techs who put in the new
alternator several years ago may have broken it, too. Anyway, it's now busted
plastic... )

I went ahead and checked for an open electrical connection (black/yellow wire
from alternator to fuse box) per step 2 and found none.

I perused steps 3-6 and saw where the procedure was going, but I didn't have
enough equuipment to fully test the regulator. Nonetheless, I did check the
voltage difference between alternator terminal B and ground for different loads.
Results:

Under no load, and I guess somewhat warmed up (I'd been driving but the car had
been sitting for about an hour. The engine was hot to the touch.), voltage from
terminal B to ground = 14.4 volts.

With headlights, defroster, cabin fan, and hazard lights on, the voltage from
terminal B to ground = about 13 volts.

The readings at the battery terminals under these conditions were just a little
lower, like 14.3 and 12.8.

So now the car is getting over 14 volts at the battery when idling(!)

My latest theory is that the alternator connector had come loose, possibly
because of the broken plastic fitting, possibly because of all the monkeying
I've been doing with the car in general in the last several months.

I will keep checking and, if you don't hear further from me, for the sake of the
archives, assume I (my car) was bamboozled by a somewhat busted alternator
connector, allowing it to come partly loose.