This morning, the car wouldn't start. Same symptoms. I put the jumpstart battery on and started it, then drove it a few blocks to my mechanic, described the symptoms, and said, "Fix it please." We agreed that the most likely culprit was the alternator.
They called me a few minutes ago. "The charging system seems just fine. Tell us again what happened?" In addition to what I was describing above, the various lights (dash, headlights) brightened and dimmed cyclically at idle today, as they did just before I replaced the battery.
So I have two guesses:
- Intermittently-bad charging system
- Something draining the battery
I'm guessing the latter, which makes me grumpy.