Alternator warning light

I have a 1988 XJ-S with a retrofitted Delco 10SI one wire self-exiting alternator which is working properly. (e.g. 14.2V @ 1200 RPM)

The dash light with the battery symbol has always come on for about 20s after turning the ignition on and then fades out, even if the engine isn’t started.

My question is, what is this light supposed to be doing and how does it work, as it has no electrical connection to the alternator?

As I said, the alternator is a single wire type so it doesn’t have an external excitation input which would normally be provided by current flowing through a dash light.

I have the stock alternator in my 87 XJS V12 coupe and it does the same thing, I have posted about this before and everyone tells me this is normal, just another XJS oddity.

That’s what I thought too. Thanks for the input though.

Same here, 1987 XJ-S and the light comes on, give some revs and it goes away.