Alternator Issue?

It's been a warm day in Sydney today (45˚ C). Driving home from the shops my alternator reduced it's output voltage, it varied between 12.1 and 13.6 as shown via the OBD2 plug. I was running the AC at the time and had a drink cooler running off the second battery. DRL's were also running. This is the first time this has happened but it was extremely hot in the engine bay. After I let everything cool down, I started the car and the reading was back to normal. Anyone got any ideas, or is it just a heat issue?

I would guess its heat related - higher heat resulting in greater resistance perhaps? I also think diodes are also temperature affected re performance - maybe a more electrically aware person can comment. I have had a similar thing happen to me, PIA and right on dusk, so headlights dragging the voltage down, but next day, all good. After that I ran a 12V ventilation fan , blowing engine bay air over the back of the alternator, used to ventilate boat engine bays, and never had the issue again. This was when I had the 2.8 TD in the GQ and the ambient temps were above 40 Deg C. No issues yet, fingers crossed with the chev.

the guys running 6.5 TD chevs have plenty of under bonnet heat ,
never heard of any complaining of alternator issues.
but out of ordinary conditions could be the cause @Yendor.
Have you checked or re-newed any earths??

I think I'll replace the battery and earth leads, anyone know what size they are?