Touran- Battery light comes on over 80MPH on long journeys

2004 VW Touran 1.6FSI

Only on a long journey (> 2 hours motoring) the Battery light will come on if you exceed 80 miles per hour. Slow down a bit, and it goes off again.

As far as I can tell the battery is charging okay so it's certainly not telling me that it's not charging.

Could it be trying to tell me that it's over charging? battery over voltage?

The only way I could think of to test for this is rig up a volt meter in the cab, and see what the battery voltage is when this happens, but since it's only on a long journey, it might be some time before I can test that.

Anyone encountered this "fault" before, is this just yet another quirk of a rather picky and fussy computer, and just another little quirk to accept and just ignore? or might I actually have a problem?

check to see if the ribbed belt tensioner is vibrating badly on full lock!
if so it could be the alternator pulley
they seem to go at around 80_100 k