Swapped my 12 Volt battery yesterday

12.1 is too low, assuming you’ve measured that without disturbing the car. I expect it has increased the charge schedule in an attempt to correct that.
A couple of hours in Utility Mode might help.

 

I just measured 12.1V again. The HV battery is at 70%. I put the car into Utility mode, the voltage on the 12V battery goes to 14.3/14/4. I expected the yellow light to come on, since the HV battery is charging the 12V, but it did not. So it is activated by the software under a different 'charge mode'. It is OK. Will the car leave it in Util mode for a while see what happens.

 

Thanks. No clue about BMS check.

 

Our car has so many options/features/bells and whistles that unless you are a nerd (like us ) a large number of folks just ignore them. Therefore in my humble opinion it should not be necessary for the owner to 'charge/maintain' the 12V battery by putting the car manually into Utility mode. This car has plenty of smarts to avoid this.
The 12.1V I measured across by Aux. battery was shown on my digital meter when its range was set to 200V. My mistake . Setting the range to 20V it measures 12.4/5 volt which is OK.

I will test the behaviour of the battery SOC in Utility mode under a 500W load on my 1500W inverter over time. My forced air gas furnace needs 440-450W to run. I figure this setup, even at 50-60% battery, will give me many hours of keeping my house warm if needed, LED lights plus some other stuff (fridge?). The plan is to increase the load step by step and observe. There many unknowns with the behaviour of the car to rely purely on calculations.