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Car: BMW M3 (2006)

I went to a track day (HPDE event where I drove the car on a road course at its limit for the entire day) and at the end of the day the car wasn’t cranking. I push-started it and drove back home. On my way home, I noticed the alternator red light flickering and assumed my alternator was bad. The next day I jump started the car to bring it to a mechanic but the red battery light was not flickering anymore and the battery recharged. Now everything seems to be normal.

I have a couple of theories but I would like some help in understanding what is going on.

1- Could the serpentine belt be slightly loose and slip only at high RPM ? If that’s the cause then I don’t understand why the alternator light was flickering during my slow drive back home.

2- Could the alternator be going bad and provide just enough current to charge the battery at low RPMs but the higher ignition current required at very high RPM has slowly drained my battery throughout the day ?

Any other theories ?

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My first thought would be that the carbon brushed in the alternator are worn down and not making a good contact on the rotor slip rings some of the time.

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  • It’s a possibility. But the light flickering only happened while driving back from the track (and it happened in an obvious way, a bunch of times, on the drive back). I was able to drive the car for about 80 miles afterwards (the next day) without any sign of flicker and I was paying close attention.
    – user3829017
    Commented Jun 18, 2022 at 21:13

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