Yesterday my S showed a message saying something to the effect of "12V battery needs service. Contact Tesla Service." I called my local service center (Burlingame, CA) and they said they would come out and replace the 12V battery, which they did. No charge of course, my car is just over a year old, has 20K miles on it. The service guy was very nice and took about 20 minutes to change the battery. As has been posted before, it is located below the cabin air filter at the rear of the frunk on the passenger side, actually just below the most forward portion of the dash. It is not easy to get to. The service guy said that the 12V battery is typically replaced every 12 to 16 months. It runs all the microprocessors (as well as 12V motors for the windows, wipers, etc.) and has to deliver a very precise and stable voltage for the computers to run properly. He also said that when the car displays the message about the 12V battery needing service, it is still safe to drive the car for a few days on short trips, but to immediately contact Tesla Service and set up an appointment to have it replaced The battery model that came out of my car, and the new battery, were both C&D Technologies DCS-33IT "Deep Cycle Series" C&D Deep Cycle Battery I searched the web for them to see how much they cost and could not find them listed for sale with a price, but found companies selling what they say is an "equivalent replacement" for about US$53.00, likely less than the DCS-33IT would sell for. I did not realize that the Model S 12V battery would need to be replaced every 12 to 16 months. When the car is out of warranty, that is a higher maintenance expense for 12V battery replacment than any ICE I have ever owned, as ICE 12V batteries typically last 5 years or more if you buy a good one. And the Model S 12V is not a battery I would try to replace myself, it's rather tricky to reach and you have to know exactly how to do it. I am posting this because I do not recall reading before on TMC about the need for the 12V battery to be replaced every 12 to 16 months.