Battery -A-, DISCONNECTING AND CONNECTING, 0K4 AND VH2: Notes

WARNING: This page is about a different car, the 2021 Audi S4 and 2021 Audi A4 Quattro. However, it is still accessible from the selected car via links, so may be relevant.

The 12 V system is supported by the 48 V system via the Voltage Converter, 48 V/12 V -A7-. So the only way for the 12V vehicle electrical system to be de-energized is to de-energize the 48V system beforehand.
For subsequent work on the SCR system, wait until all of the reducing agent has returned after switching off the ignition.