Just had this happen to me as well! It's hot this summer in TX, and this car has ~80K miles over the ~2.5 years we've owned it. There was never any pre-warnings leading up to failure. My wife went on a Costco run, came home, plugged the car into our Wall Charger to do a little mid-day charging (we drive a
lot of miles, so in the TX summer heat we end up charging mid-day sometimes!). As best I can reconstruct the timeline, right after the charging session finished (to 60%), the car went into 12v-death-mode. It was a few hours later in the evening when I went out to get in the car, and found the door wouldn't open. Checked my app and it couldn't even wake or talk to the car :/
So I went through the basic bullshit to get basic access to the car: pulled the little wires out the front bumper hole, hooked up a little 12v trickle charger I happened to have in the garage to those to get the frunk popped. I checked the service manual on my phone, then pulled the HEPA filter unit out to make room, and tried jumper-cabling it to an old BMW running in the driveway. That did revive the car enough to open the passenger door and light up the display. Lots and lots of error codes popping. The battery ones were obvious, but also a bunch of seemingly-unrelated ones about various modules around the car and how some things wouldn't work right due to low voltage. HVAC was non-functional, the whole driver side of the car remained inoperable (the left doors/windows, etc), which seems really stupid. Also couldn't unlock the charge cable from the charge port, because that was inoperable too (can't charge the HV battery, either, wouldn't even light up the Tesla logo by the charge port).
It was ~11pm on a Weds night by the time I got to this point, and decided to book a Mobile Service appt through the app. They responded with a first available slot of Monday afternoon
! I took the slot for now anyways and went to bed, doing some googling and youtube watching on various related topics/procedures.
Today I woke up and just drove down to the local Service Center (in a different car, obviously!) and asked if I could just buy the battery and install it myself. They had no problem with that and had one in my hands in a few minutes, same AtlasBX model I had before, $80 + tax. I turned the car off from the Safety menu, popped the rear seats out and disconnected that one battery-related connector in the passenger side corner that some videos reference, Just In Case. Then the battery swap itself was pretty much like any other car (other than the annoyance of it being hidden behind the HEPA unit). Once the battery was back in and the rear seat power connector plugged back in, the car booted up fine and cleared all related messages and we're back to normal.
My main complaint about all this is the total lack of warning. With all the advanced diagnostics on this car, it should've
known that the battery was imminently failing and given me a little warning to replace it before it became a catastrophic failure. Given it's easy to swap and can be picked up for $80, my advice would be: if your 12v hasn't been changed in the past ~50K miles or past ~2 years, just proactively swap in a new one while it's easy and not yet failing!
