Lately my charge indicator light has been partially illuminating when both the engine rpm is fairly high (20+ mph in 1st, 70+ mph in 4th) and the voltage is low (~13.25 V).

That the charge light comes on when the voltage is low is not a surprise, of course. I'm mainly curious as to how the voltage gets that low.

Checked the voltage regulator; brushes seem fine, although that doesn't rule out some internal problem there. The alternator works (the charge light is registering a voltage difference between the low battery and the alternator voltage), but not well enough to keep voltage up with the headlights on (see below).

Voltages measured through cigarette lighter during a 20-mile drive:

Car off, 13.5
Idling voltage, 14.25
At 70 mph with nothing electrical on, 14.5
At 70 mph after having headlights on for some time, 13.25 (charge light illuminated here)

Being unfamiliar with charging system performance in these cars, I'm curious whether any of this is substandard. I've read that the system should provide 14.4 V or so while the engine is running...but how much of a charge dip is within spec with headlights, radio, &tc. on? And what does the problem here sound like?

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'Betsy the Getaway Car' : 1978 MB 240D, 582k
'The Mistress of the Sea' : 2001 Giant Boulder, 10.30k