While I was in that painful stage of waiting for delivery of my Model S and reading everything I could on the forums one of the issues I ran across was a rash of 12V battery failures on the Model S that Elon even commented on in an earnings call.

The reports said that Tesla had chosen a poor vendor initially and the early Model S’s were plagued with this problem but that the problem had been resolved and was no longer an issue.

Wrong – my Model S indicated a problem after only 7 months of use (18,000 miles) and several others have reported the same.

12V Battery Purpose

The 12V battery in the Model S performs many of the same duties as the 12V battery in an ICE car. While the manual for the Model S says very little about this battery, it is critical for normal operation of the car. Unlike a normal 12V battery in an ICE car, the connections/access appear to be a bit more complex as is shown by a picture from the TMC forums.

The 12V battery maintains power for critical systems when the main battery is damaged or disabled. It powers hazard lights, airbags, door lock/unlock operation, etc. The 12V battery is also keeping electronics alive to listen to the key FOB lock/unlock and the 3G connection for remote access when the rest of the car is off. Another interesting tidbit is that if the 12v dies it will isolate the main battery pack preventing charging which is part of the Model S safety features and helps protect first responders.

What the warning means

The “12V Battery Needs Service” warning that comes on can indicate a number of different problems as its a general warning message. So the first thing to do is call Tesla Service (they’re available 24×7) and have them pull the logs and let you know the severity of the problem.

Call Tesla Service (open 24×7) when the warning comes on and have it diagnosed remotely.

In some cases they will tell you to stop driving the car, in other cases it just is indicating that the voltage level has dropped below a threshold level but not that the battery has failed and that you can keep driving normally.

Most of the time the problem is the “keep driving it” type. But do pay attention. There are messages on the forums from owners stranded on the side of the road from 12V battery failures. If your 12V battery does die leaving you stranded, you may be able to jump start the Model S. Kman has a good video on getting access to the terminals behind the nosecone to jump start the 12V, but consult please Tesla service before you attempt this.

Depending on what Tesla Service says, you may have 2-3 weeks from when the warning comes on to get the battery replaced.

When I spoke to Tesla Service they said I had 2 to 3 weeks from the warning in which to get the battery replaced as the warning level is very conservative. Many owners manage to get the battery replaced quickly, but in my case a busy SC, our Thanksgiving holiday, and New England weather had me driving with the warning light on for almost 2 weeks (about 1,000 miles) without any issues.

Another interesting point to note is that Tesla may call you before you see the warning to have your 12V battery replaced. Multiple owners have reported this proactive call in advance of a warning so evidently Tesla’s warning threshold is even lower than the one that triggers the indicator on the dash.

Why are the batteries failing?

Some owners are on their 3rd 12V battery already. Many others are reporting failures at the 1 year mark or even short of that like me. In ICE cars you can normally expect about 4 years of life out of your 12V battery. While it could just be “different” with the Model S where it may require a new battery every year, we’re seeing failures short of the 12 month mark which causes inconveniences to owners.

Some of the failures have been in the DC to DC converter that charges the 12V battery from the main battery pack. That converter is sort of the equivalent of your alternator in your old ICE cars. However, this doesn’t seem to be the main cause of the 12V battery failures. Current conjecture is that vampire load and the excessive strain on the 12V are causing the reliability issues. While a better battery (better vendor, improved technology) may help delay the issue, unless Tesla fundamentally changes the way the 12V battery is accessed its going to remain a problem.

Tesla may have a lingering design issue in their use of the 12V battery.

From my reading, I think they upgraded the quality of the battery in the earlier models but we’re still facing the same underlying design issue in the way the 12V is stressed. Can Tesla really fix the way the 12V battery is used with software updates alone? We don’t know, and fortunately they’re paying for all these replacements. But unless they can do some software magic this could be a brewing factory recall.

Tesla should be more upfront about these issues and let owners know what they’re doing to address the underlying problem.

Getting the 12V battery replaced

First, from everything i’ve heard and my own experience, the replacement (labor and parts) is free.

12V battery replacement is free (for now).

Next, Tesla can do the replacement on site (in your driveway etc). They say it only takes 30 minutes but i’ve heard it take up to 90 minutes on site. Given the weather here in New England and my compassion for the poor technicians I opted to have them pick up my Model S and do the replacement at the Service Center and return it. They did that within the same day and the warning has gone away. As usual (for Tesla) they fixed a bunch of other things for free too at the same time (also another reason I opted for the pick-up option).

Summary

The 12V battery is a relatively inexpensive part in the Model S and its fairly simple to replace. While its a bit concerning for these to be failing as quick as they are and its inconvenient when they do, it isn’t a major issue but one we should continue tracking as concerned owners.

I’ve started a poll over on TMC to see what kind of failure rate we’re seeing for the people that answer if you’re interested in monitoring this issue more. So far over 75% of respondents have had their 12V battery replaced. Keep in mind those most owners have had their Model S’s for less than 2 years…