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.

Caroline,

The voltage regulator has two settings. Under normal operation the
output voltage should be 14.5 V +/- 0.6V. At idle with a warmed up
engine (and a few other requirements) the voltage is dropped to save
fuel. Try measuring the voltages again, but this time step on the brake
pedal when you measure the voltage at idle (stepping on the brakes will
force the voltage regulator into the 14.5 V mode). When you measured in
February, perhaps the engine wasn't fully warmed up?

1991 Civic LX 4-door sedan, 1.5 Liter, manual transmission, no air conditioning,
156k miles, here.

Voltage at the battery terminals when the car is
-- idling = about 12.7 volts
-- stopped, ignition off, also about 12.7 volts

In February, these numbers were 14.5 volts and 12. 4 volts.

Internet sources say a voltage when the car is idling of around 14 volts or so
indicates a properly operating charging system. So something is wrong, IMO.

I put in a new battery (Interstate) today, replacing the old one (4-years-old;
Diehard; wrong climate design as I moved from up North to the Southwest in the
past year?). The voltages above didn't change.

The car is on its second alternator (OEM). This 2nd alternator is 5 years and
50k miles old.

I installed a new alternator belt in June. I originally had the belt too loose,
as indicated by a squeal at cold startup. I tightened it and the squeal stopped.
Unfortunately I did not at this time check the battery terminal voltage.

I did shake out quite a lot of white powder (indicating some corrosion) from the
battery's positive terminal's cable connector while changing the batteries.

I'm going to do the checks at
http://www.honda.co.uk/owner/ConcertoManual/62sk301/16-66.pdf this weekend.

Meanwhile, has anyone seen a condition like this? If so, what was the fix?

I think this is not critical unless I do a lot of driving with the lights on
(which I do not, as I drive mostly during daylight hours). But I do feel my
battery isn't going to be maintained at optimal charge and so, as winter
approaches, I need to fix this.