Was it ever determined if the smaller 50 AH/12v aux battery requires coding or not? I'm starting to think that it doesn't because it appears to be under the control of the PCU (power control unit).
And registering the battery is required when it is hooked up to an IBS (intelligent battery sensor...and I noticed in the parts database that there's only one IBS used on the F15...and that's on the larger (main) battery. And when using some of the software/apps for registering the battery...many find nothing for the aux battery or the info is grayed out for the aux battery...which would suggest that it isn't used for that particular battery.
If the car is optioned with the 48V aux battery...perhaps in that set-up...the aux battery requires registration.
Just tossing some thoughts out there in the stream to see where it goes.
Again, the parts database only lists an IBS for the larger main battery (item #1 in the diagram below for the battery & aux battery cable diagram). The diagram for the main battery has comments/notations about registering the battery...but the auxiliary battery diagram doesn't have any comments about registration.