I've got a 11/13 built S85 with the MCU2 upgrade and have finally had the "12V battery low" message pop up after 112K miles. Tesla has preemptively replaced the 12V battery twice before, one when the HV contactors were replaced under warranty and the other time rear window defroster was fixed under warranty. I plugged in my Anker battery monitor into the 12V socket over the last week and saw the voltage fluctuate between 12.7-13.4V. I sent in a request via mobile app for Tesla to remotely reset the battery warning because I'm not convinced it needs to be replaced. Instead of doing that, Tesla set up mobile service to replace the battery for ~$265 ($159 for the battery, $89 labor, plus tax). I've searched threads to learn what I could about the 12V battery and thought instead of having threads about the 12V pop-up every now and again that perhaps there should a be a sticky with how the Tesla Model S uses the 12V battery, how it charges, and how to prolong the life. Personally, I'm surprised Tesla hasn't engineered their own auxiliary battery system that to be better than the traditional system.
BTW, does anyone know the threshold voltage that the Tesla needs to power and start the car? I believe 10.8V was necessary for both my old 2007 Lexus Rx400h and our 2013 Fiat 500e. Our S typically sits for days on end in our garage and with the MCU2 and newer software I've noticed that the pumps run and the contactors audibly open and close at least a few times a day. In hindsight, I regret not placing a 2A trickle charger on the battery posts (behind the nose cone) as that could have prolonged the life of the 12V...it's about 2.4 yrs old at this point. We've gotten more than 6 yrs out of the tiny battery in the 500e, but that car does not have all the connectivity and vampire drain that the Tesla come to know.
BTW, does anyone know the threshold voltage that the Tesla needs to power and start the car? I believe 10.8V was necessary for both my old 2007 Lexus Rx400h and our 2013 Fiat 500e. Our S typically sits for days on end in our garage and with the MCU2 and newer software I've noticed that the pumps run and the contactors audibly open and close at least a few times a day. In hindsight, I regret not placing a 2A trickle charger on the battery posts (behind the nose cone) as that could have prolonged the life of the 12V...it's about 2.4 yrs old at this point. We've gotten more than 6 yrs out of the tiny battery in the 500e, but that car does not have all the connectivity and vampire drain that the Tesla come to know.