Glad you found the issue @HarmLessSolutions! And thanks for the tip, good idea to replace it after x years. Our is from 2016. Just tested it by putting it in ACC (not in Drive) and turning on the heater. Within 3 minutes the voltage had dropped to 10.4v, and the charging kicked in. According to some other posts that's way too short, it should last for 10-15 minutes. So time to replace it!
A different question, I'm using our Leaf as a backup 220v generator by connecting a 220v inverter to the 12v battery & putting the car in On. Works like a charm, but the one issue is that I have to disconnect the 12v plugs from the car. This is inconvenient, causes small sparks, and resets the car (e.g. trip meter). Also this is not something my wife can do.
Think the best solution is to connect 2 Anderson plugs permanently, but I don't have the gear to make them; premade they are about $60 from Aliexpress.
Is there any other alternative?