For the past few months, and seemingly getting progressively worse, I've had a charging problem: both battery and alternator test good (checked via charger with alternator test mode and higher-end test equipment at Advance Auto), but the battery doesn't charge. It doesn't drain when left sitting, but each time the engine is started, it gets harder and harder to start, until it won't start at all without a jump or push. Charging with the battery charger makes it work again for a while, but then it gradually gets harder to start.

Battery/charging system voltage seems reasonable, over 12V with engine off, and around 13.5V running.

Tested for any loads causing a drain just in case, but there don't seem to be any - battery charger drops to "float" mode and stays there after battery is charged.

Vehicle is 1992 Honda Civic.

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I'd been suspecting the alternator cable, especially after smelling what smelled like burnt insulation a few times since the problem began. The first few times I measured voltage at the alternator vs at the battery terminal, there seemed to be less than a 1V drop, perhaps slightly higher than it should be, but not enough I'd consider it conclusive. Today I tested resistance across the cable, and first was getting readings of 20 (!!) or more ohms. After pulling on the protective cover a bit trying to get a better reading, it started showing only about 0.8 ohms, but by that time I figured it was almost surely bad.

After replacing it, I'm getting 14.3V at the battery terminal, and it looks like the battery is going to stay charged.

Anyway, moral of the story is that "alternator testers" can show false negatives for a charging problem if the voltage is higher than nominal battery voltage but the current is minimal due to bad cable or bad alternator.

Update: Battery is not staying charged. It looks like the wear on the alternator cable might have been a symptom of a battery problem rather than the root cause. I've posted a follow-up question: Battery only charges with parallel load - why? recoverable, or replace? ... and indeed, the battery was bad.

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    Just shows that doing all the correct tests will give the real problem - most manufacturers have a thorough list of things to check, which includes the cables and earth or grounding straps.
    – Solar Mike
    Commented Apr 27, 2019 at 7:22

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