A car with an enormous rack of high capacity batteries able to accelerate an 8000 pound object to 60mph and sustain that for hundreds of miles generally doesn’t depend on the backup battery for literally anything. It has so much excess energy storage in the form of electricity in the primary batteries it generally doesn’t power down the onboard computers at all.

Indeed when you get close to exhausting the main battery rack it starts selectively shutting down everything. I’ve never personally let mine get to 0% ever - but for instance a Tesla is continuously on, and if you use sentry mode it’s not just on but the GPU is constantly doing classification of the environment to determine if someone is prowling your vehicle.