unsure what you mean here. most of the systems go to a sleep state in modern vehicles ev or not. the 12v battery keeps only certain ECU's up - think ECUs that control alarm, lock and unlock state and any communication with the mobile app via LTE... but the rest of the systems are OFF, you don't want an EV battery to hit 0% and 12V to also hit 0% - that would basically make it a brick from what I understand- because EV's have contactors which need to shut for the battery to be 'engaged' the 12V battery controls these contactors.
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.
Every EV depends on the 12v battery for starting up / has the HV battery off when your car is off, that's why if the 12v battery is dead your car won't start.
Low voltage battery death in any EV essentially causes a brick. The only exception is some cars (I think Tesla does this?) keep their contactors closed all the time when the 12v is determined to be failing. It makes the drain at idle much higher, but then at least it can continue moving… as long as you don’t let the HV pack drain…
