I have a 2007 TDV8 RRS which has covered 80K miles and is on its third alternator.

I bought the car with 44K miles, which I was told by Guy Salmon that a new alternator had just been fitted.
18 month and 20K miles later I had an intermittent battery warning light and subsequently new alternator fitted. Another 18 months and 20K miles later I have the same intermittent battery warning light.

When I start the car from cold, after about three miles on an dual carriage way doing 60-70mph and the engine half warm, the battery warning light comes on, when I turn off the dual carriage way and slow down to a stop at a junction, the battery warning light goes off. When I regain speed to 60-70mph the battery warning light does not come back on.
That is my usual drive to work. If I take an alternative route to work, through town, which involves 30-40mph drive, then the battery warning light does not come on.

I have accessed the diagnostics page via the sat nav screen and the voltage does drop from 14v to 12.5v when the battery warning light is on.

When I had the new alternator fitted, it did clear the problem for about a year, but the problem is becoming progressively more frequent.

The garage will tell me it is a faulty alternator because the computer says so, but I think there must be more to it than that.

Has anyone experienced this before? Could it be a loose wire or faulty voltage regulator? I am quite sure the battery is not at fault.
If it is the alternator, does anyone know what could be the cause of the fault?
I would be grateful for any suggestions.

Do you know which brand of alternator was fitted both times?

There have been issues with spurious aftermarket ones

Hi Disco_Mikey,
thanks for the response.

The alternator that I had fitted was, as far as I am aware, an original part. It was fitted by an independent garage, but was told parts were genuine LR. The receipt shows part number LR010514.

I would also have expected Guy Salmon to have fitted an original part for the earlier alternator, which showed the same symptoms.