When I went to upgrade to the 4.3 update I got an error message saying that the update couldn't be done because the 12v was too low. Tesla service came out and replaced the 12v and everything is fine. Still weird that a 12v battery goes bad on a 4 month old car, but this got me wondering when exactly is the 12v charging? Here are some situations and what I think is happening. Anyone that knows for sure please correct this. 1) driving - 12v is charging via DC to DC converter 2) parked and main battery charging - 12v is charging 3) parked and main battery is not charging but charge cable is plugged in - 12v is slowly draining 4) parked and the charging cable is not plugged in - 12v is slowly draining 4) parked and charge cable is not plugged in but car is "on" - this is a situation when you may be parked and waiting for someone and are sitting in the driver's seat so the car is on and the HVAC and all other systems are on. I'm assuming the 12v is charging here, but I'm not sure. 5) parked and charge cable is not plugged in, but car is in accessory mode - this is the situation I'm most interested in in what is happening. I often when waiting for an extended time for someone will sit in the passenger seat of the car with the car not "on" but with screens and AC on and do some work on my laptop - sometimes for up to a couple of hours. Every 20 minutes or so the car turns off (ie screen go dark and the AC shuts off, but simply touching the screen brings everything back). I don't know what is happening to the 12v in this situation. Am I draining the 12v? Is it OK for me to use the car in this way? This is actually one of the great bonuses of this car - using it as a mobile office. Here is Arizona it is great to be able to run the AC in a car for extended periods without running the engine and releasing CO2 for a couple of hours.