ok, I'll chime in... This is an exercise of trying to fix something that isn't broken.
The voltage output of the alternator cannot keep up with the demand required when the headlights are on. Most low power alternators do not provide the rated amperage until higher in the RPM range. In other words, the output is small at idle.
Higher output alternators will correct the perceived issue, not because there is something wrong now, but because they provide a higher output at idle.
Unless it really bugs you, there is nothing to do as nothing is broken.
Have you ever see the same symptom on a motorcycle???
Scott
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