I have to say the system intrigues me. I have a 2019 NX 300h - and I have an AA Car Genie fitted, which under normal circumstances monitors the car for faults - and on my previous car, a diesel Nissan X-Trail, it would happily tell me that my 12v battery had good charge for weeks on end. Now that the Genie is in my NX, it regularly a day or so after using the car, tells me that the 12v battery is low on charge and could do with a drive to charge it up. I have on a number of occasions now, ignored this warning and left it another week or so before getting in the car and driving it off quite normally. So now I don't know whether the Genie is misunderstanding the hybrid system and the role of the 12v battery and is a bit hypersensitive about it, or whether actually I should be charging the battery up far more than I am, and that I'm risking not being able to "start" the car...!? I read, on here, I think, that the cars don't have starter motors as such and the 12v has no possible way of providing the power required to start the engine - which begs the questions: is it true that there is no "starter motor" as such, does the 12v battery not matter much then if the charge is low, and is my Genie misunderstanding a hybrid 12v battery's role? Can anyone with hybrid knowledge explain, please? The answers could all be connected to your issue, I think....
Cheers