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.

Clean your battery cable clamps with a round wire brush. Check the
cable itself under the insulation near the clamp, make sure it isn't
corroded under there. Check both cable connections at the other end,
especially the ground cable. Unbolt them, clean with a file and
reconnect. Half an ohm resistance in a 115 volt circuit is nothing.
Half an ohm in a 12 volt battery cable is a big deal.  Do the math
E = IR  If the alternator wants to put 20 amps into the battery,
.5 X 20 = 10 volts drop.

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.