If I remember well, according to the user manual the 12V battery should be recharged also when connected to the external power (i.e. when charging the car).
Could very well be, since our current car is petrol only, i wouldn't be able to tell. I can check once they finally deliver our GTE. That said, jonfls message there still convinces me that it's the 12v.
Interesting about the 12v charge. This issue originally happened about six months ago, but after a full charge went away. Only in August did it go wrong permanently.
If it went away after a full charge, and now continues to persist, there's two options from my point of view, and you can test for them (if there's nothing vastly different in a PHEV compared to a petrol in that regard). Take the time, drive your car around, and once you get the dreaded message that the engine can't shut off, drive home, park up and check the battery voltage with a multimeter. This just convinces me more that it's the 12v at fault - either it doesn't take full charge anymore, discharges too fast or there's a big parasitic draw somewhere/for some reason, leading to the same problem of the battery draining too fast.
The only thing i don't quite understand is why you don't get an explanation anywhere. Our car (Ateca, 2019) with MIB2 infotainment does tell us when, in our case, the start stop system is deactivated. Or more precisely, it tells us, at least to some extend, why it's deactivated. In winter, quite often it's because of "high power consumption" when we have heated seats etc going, other times it's the heated windshield being active (it tells us in particular), etc. There
should be something similar to that in a Golf with MIB3, no? With an MIB2 system, it's under "Car" and "Info" (maybe Status, can't even recall). From youtube videos, it's the same - from the Menu go to "Car", then to Status, when the message appears. It's kind of a "have you tried turning it on and off again" suggestion, but might as well give it a check if you haven't done so. Maybe you get a pointer at the very least.
Different car, but exact same display as ours. Car tells us there
why the start stop ain't working. Instead of "No Entries" we get told "heated windscreen active", "high power consumption" etc.