So with the charger connected under the bonnet, I am monitoring the batteries. It only seems to charge the main battery.
When driving I saw both charging at the same time, but can confirm that.

This morning I wanted the FBH to turn on as it was -3C. It didn't I think due to the main battery being low. When I parked the car last night. There were a few trips to the car for the shopping and other bits. This I think dropped the main battery down.

I triggered it another 2 times during the day today after driving. Making sure I just closed and locked the car when I turned off the engine. It worked both times.

The next time to drive the car is Monday morning when it is going to be about -4C. I check the battery level and start the FBH from the phone.

Looking at the monitor I can't see anytime that the secondary battery has been used. SDV8 AB MY 2020
MINI Cooper S Cabrio
Kit: Jaguar XK140 (in progress - the website is link to build)

The last 2 mornings I have used FBH before getting into the car. It drops the main battery voltage by at least 0.5V. If the battery is a bit low at the end of a journey I can see this causing issues.
Again the AUX battery isn't being used. SDV8 AB MY 2020
MINI Cooper S Cabrio
Kit: Jaguar XK140 (in progress - the website is link to build)

The auxiliary battery is only switched into the circuit to provide extra current capacity during starting  Kev

2014 L405 RR Vogue SE 4.4 SDV8 in Corris Grey
2010 L320 RRS HSE 3.0 TDV6 in Stornoway Grey

Ok. It doesn’t seem to have got to a level where it needs to do so. SDV8 AB MY 2020
MINI Cooper S Cabrio
Kit: Jaguar XK140 (in progress - the website is link to build)

The aux. battery should be in circuit during cranking, it's main function is to prevent electrical disturbance during cranking (lights flickering etc.) they were introduced as part of the autostop functionality.

Last night there was a drain on the main Battery. I don't know what it was - maybe the monitor I have plugged in for the soot. After some time, I saw that the Main and Aux connected together to raise the voltage in the main battery.
I expect that when I get in the car there will be a low voltage warning but the car will start without issues. SDV8 AB MY 2020
MINI Cooper S Cabrio
Kit: Jaguar XK140 (in progress - the website is link to build)

The FBH started and then stopped due to the low main battery - that was expected. The AUX (secondary) battery was used to help the main battery start the RR. So it helped jump itself. It was -10C here overnight.

I will check the car doesn't drain over the next few days. When at home and know that I want to use FBH in the morning attach the CTeK overnight. Don't worry about the main battery voltage. SDV8 AB MY 2020
MINI Cooper S Cabrio
Kit: Jaguar XK140 (in progress - the website is link to build)

Changed my battery today as it was struggling in the cold. Only lasted 10 years and 210k miles 😮

Click image to enlarge

Click image to enlarge

Do you need to reset the BMS when replacing the auxiliary battery, or just the main battery? 2018 P400e AB LWB

Each battery has its own BMS control module. It is the chunk of metal on the negative terminal. Chris

2014 FFRR 4.4 SDV8 Autobiography Barolo Black
2012 RRS SDV6 HSE Baltic Blue
and before I saw the light:
2011 Suzuki Grand Vitara SZ5
1998 Ford Mondeo V6 Ghia X
1990 Citroen BX GTi
1973 Triumph GT6

Hi guys my local garage said my battery may need changing but I got no fault codes or warning on dash etc.. car starts first time no issues here are some reads I took with GAP tool first while engine running after 5 mins second engine off after 5 mins. Are they both due a change even though I’m experiencing no issues?

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Click image to enlarge