Expected alternator behaviour?

[kiloran]

With a fresh MOT on Val, it's time to look at a few of the quirks she has, including the idle which has a mind of its own most of the time.
Whilst warming the car this evening I thought I'd do some experimenting, channelling the Gary Sinise character from Apollo 13 by adding load during the power up sequence.

So from a cold start it idled nicely just about 1000rpm for a few seconds and then settled down below that. With each extra electrical load I switched on the idle got lower and lower until it was really struggling sitting at just above 500rpm. Blipping the throttle didn't change the idle - straight back to 500rpm once off the throttle.

Power up sequence was:

Side lights
Dipped beam
Full beam
Heated seats
Stereo
Blower (on full heat)

The straw that almost broke the camel's back was the front and rear fogs going on. That took it down to 500 and a whole lot of shuddering going on.

Each load I removed bumped the idle up a bit.

Is this normal behaviour or a sign that the battery or alternator is struggling a bit?

On other cars I've owned the ECU has bumped up the idle to cope with extra electrical load - e.g. my Golf Mk4 adds 150rpm to the idle if I switch on aircon.

Car is starting fine - first time every time, even at -4C this week - and driving fine (well, other than the aforementioned haunted idle and ABS issues)

I know Luke barely saved my battery when he popped the recon engine in so maybe it's struggling a little?