I have an 2015 RRS (Autobiography SDV6), and got a couple of (small) fault messages (ie airbag yellow warning) intermeditally in the last weeks. As battery is >5 years old I have asked my mechanic (indy garage) to change both (main and auxiliary) battery. He has changed the battery, but said, that there is only ONE (main) battery in my car (checked, he says).
I am a bit confused, as I allways thought that RRS has a main and an auxiliary battery. Is this correct, only ONE battery in this car?

I’ve just changed my big battery and I was fully expecting to change the Aux also. However when I looked I realised my car has a big battery and a capacitor rather than the AUX battery. My car was registered December 2015 but a 2016 model which is when they first started using the capacitor. 2016 Yulong White L494 AB/DYN 3.0SDV6

I could well be wrong but I think you may find with the earlier RRS2 only the 4.4 TDV8 and cars built for cold climate market's had the Aux battery fitted.

Cheers
Col

So, chapter 2.
As, after replacing the main battery not only start/stop does not work, but I also still sometimes have the yellow airbag warning on (which is said to be often battery-related fault?) I have visited my (semi)official LR service point.
They discovered within 1 minute (opening the trunk cover and showing it) that there is wel an aux battery, and after hearing my story he said that the RRS needs a "reprogramming" after battery change, that might be the culprit (if my indie garage haven't done it).
His proposal to take the car to them to check programming battery, measuring batteries - and, IF needed, replacing aux battery (which is quite expensive, he says; but he advises to use OEM battery).
Appointment is over a week, I am curious about their findings; maybe battery replacement is more than just changing the battery in our loved RRSs ('reprogramming')?

Sounds reasonable. My RRS is a 2006 model, so a bit different, but I know that manufacturers have introduced monitors in the ECU for the battery condition, which could be why your dealer is saying that it needs a "reset". The ECU need to know that the battery has changed.
As for the auxiliary battery, sounds best to change it as well.

Stick it on a ctek conditioner mate, my car had no start/stop and an airbag warning light. I was going to replace the battery but got the ctek mxs 5.0 instead and it fixed both the issues