does the voltage drop under load?  you know, fans, lights, defrost etc.,
on?  if it drops to say 10V, then you have a charging problem.  if not,
and it kicks up to 13/14V when loaded, everything's working just fine.

the alternator should have a regulator circuit that determines whether
it needs to "charge hard" or not.  if the battery is charged and there's
minimal load, the alternator is not required to produce max output, and
indeed it shouldn't in order to not fry the battery.

alternators generally fail when the diodes go.  semiconductors have a
limited lifetime at high temperatures, and hot climates and/or full
electrical loads will keep them nice & toasty.  oem alternators can last
a good long time when treated conservatively.

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.