It ain't over till it's over.
In the 12 days following the battery replacement, I drove it once, went away for a week, then drove it twice more, for a total of maybe 30 miles. In that last drive, I took out out to the highway at midnight for some exercise, getting up to 5,000 RPMS in 3 or 4 gears. (Wanted to make sure all 3 digits of my digital speedometer still worked.) At some point, the battery light went on.
I took it to the shop this morning, and they looked it all over, and ran another load test. They determined that the alternator is working, but not at full bore- UNTIL you rev it past 3,000 at which point it charges well. (So I'm thinking the battery light measures roughly what's going on in the overall system, more than measuring the battery as a standalone.) They told me the alternator is on the way out and will need to be replaced eventually. Perhaps it was damaged in the struggle to keep my dying battery alive longer. It's been decades since I nursed along a weak battery, but I remember that going on for months and months, making sure to park where I could bump start it. This one stopped starting the car in the middle of a day of driving, and 5 months prior it made it through weeks of sleeping at zero degree temperatures without showing any weakness. When they replaced the battery, they told me they tested the alternator, regulator, and starter, and it was all good.
Okay, fine. But, if that's what happened, two mysteries remain. If it charges better when you get past 3,000RPMs, why did the light first go on when pushing it to 5,000? And if the light means anything, why did it not go on when the previous battery was dying and behaving erratically? I don't necessarily expect answers- it all seems more like an expression of the gray areas involved in an electrical system tied to an ECU, and maybe it should all be read as a metaphor for existential quandaries in general.
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