Had this very issue with the wife's car, battery light kept coming on, then air bag light then others finally battery died, and after it being in the garage a whole week with diagnostic guys as well they got to the bottom of it.
And if you live anywhere near Woodford green I'd use this garage
Roberts Motor Works
020 8989 1829
cause there just superb and have been so helpful. I've used them for thirteen years now with all of my family's cars.
What they found was the alternator is quite unique that it is recognised by the bsi and if you don't keep the original one it won't charge even with a bran new alternator,
The answer was to get the original alternator refurbed which took less than 24 hours and cost less than the new alternator.
They tried five different alternators from various years while tracing this fault not one worked.
If you've already scraped your original alternator, it's probably going to have to be a dealer job or a scrapyard to find one of the same year, bear in mind the one from the scrapyard can be refurbed and they come back like new.
Russell
September 2016