I seem to have a strange fault - on the way home last night my battery light came on - and stayed on - for about 100 miles - the car starts fine & runs fine, but the battery light is on.

I connected up the IID tool and it says 12.2v at rest, then drops to 11v when I start the engine.

All the other electronics work OK - I ran for 2 hours with lights, zircon, music etc - no problems and no strange warning messages.

I was wondering if whatever it is that measures the voltage has a problem - but odd that it seems to get the resting battery voltage correct.

Any ideas - I would have thought a dodgy alternator would kill the battery pretty quickly.

your belt could be slipping, the idle voltage should be around 14 volts so thats worth checking with a multimeter.