12V Battery Replacement

12V Battery Replacement

I just got a call from Tesla Service. They "identified my car" as needing to replace the 12V battery. I got the car in August and my VIN is 32XXX. I've never noticed any problems.

Anyway, they scheduled a mobile service appointment for Wednesday at my office. So, Great! I guess.

Anybody else?

jordanrichard | 22 oktober 2018

You wouldn't notice any problems and be glad for that. Tesla's computers can identify when there is a potential issue and if possible, will let you or them know in advance. So this 12v swap out is preventative maintenance in the best way.

You may or may not know but for a long while "back in the day", Tesla was having a problem with the 12v's failing unexpectedly on the Model S's. Long story short, with improvements in the software, the car would give owners a heads up that the 12v needs service/replacement. The warning typically was a 3-4 weeks heads up.

This past March, specifically the beginning of March I got the 12v warning but since I had my 4th annual service scheduled already for the end of March, I just kept driving the car.

Never heard of another car being able to proactively tell you that your battery was on its way out.

jon | 22 oktober 2018

Maybe there is a batch of defective 12v batteries that have been recalled and your car has one. Frequently not every unit on a recall list will fail, and some will happen to fail much earlier or much later than the others on the recall. Or maybe it's not a recall and they got some diagnostic code pushed from the car. Given how annoying it seems to be to have the 12v die, I would take it in.

Though I'm puzzled why the 12v dying is such a problem, seems like the car should be setup so that the charger can be used to "jump" the car as well. I know it is much more efficient to power low voltage electronics from 12v than from 375v which makes sense when the car is sitting there not plugged in, but when plugged in it would be nice to have it able to power up the remainder of the systems.

RJMIII | 22 oktober 2018

I didn't mean my post as a complaint. I think it's cool that Tesla is letting me know about a problem I didn't bring to their attention and they are proactively doing something about it.

I was just wondering if it's something specific to my car or if they are recalling a bunch of batteries as potentially problematic.

jordanrichard | 22 oktober 2018

RJMIII, only Tesla knows if this is a wide spread problem. Now, this is pure anecdotal with a degree of agreement from my service center, but it does seem that cars that don't get used very often, tend to have 12v issues. There is no magic number of miles to keep this from happening, but every time someone posted on the Modes S forum about their 12v failing prematurely, it was people with relatively low miles.

As I mentioned my 12v lasted 4 years and I had 107,000 miles on my car by that point.

Jon, one can not compare past experiences and thus expectations with the 12v in a Tesla vs an ICE. It is used/charged in a completely different manner than in say a Chevy. The 12v in a Tesla is being used to run all of the computers, including the BMS and the dash. In a Chevy, with the car off, there is essentially nothing putting a demand on the 12v.

pweady | 22 oktober 2018

The same thing happened to me after about 2 weeks of ownership. (August delivery, 77XXX) I thought it was pretty cool of them to proactively take care of it.

RJMIII | 22 oktober 2018

@ jordanrichard - I drive about 90-100 miles daily. I've put 5650 miles on since August 13 delivery. So, I don't think that is the problem.

I know it's difficult to know if it is a widespread problem, thus my post asking if others had the same experience.

tucsonsims | 27 januar 2019

-Accidentally posted this reply on the Model S forum. Take two...

Well it looks like the Tesla Model 3 12V battery management system may still be less than optimum...

I was working out in the garage for a few hours the other day where my Model 3 is parked, within earshot of the car. Over the course of 3 - 4 hours I noticed the car closed the main contactors (familiar double clunk-clunk sound) several times, presumably to energize the DC/DC converter to support the 12V loads. The contactors stayed closed for about 5 minutes or so each time and then would open with a slightly quieter single 'clunk'.
Curious, I popped open the front trunk access panel and attached the Fluke DMM to the 12V battery. I observed voltage levels of 14.90 14.95V when the DC/DC converter was on and settling down to around12.5V before the cycle would repeat.

These thresholds, especially the upper limit, seem a bit too high to me. I know that lead-acid batteries, even AGMs, don't like voltage levels over 2.4V per cell and at almost 2.5V per cell, as I observerd, I don't see this battery lasting very long...

Anybody else take any measurements on their 12V system?

TM3Q | 28 januar 2019

Did you gents look at what kind of battery it was? Was there any temperature sensor connected to the battery?

As for battery recharging like AGM type it’s ok to have 14,8V and could be higher if the battery temperature is lower. As long as you don’t have a high voltage for a long time it should be fine.

Battery chargers with temperature compensation will vary the bulk and float voltage in order to recharge properly your battery and maintain it

tucsonsims | 28 januar 2019

@ TM3Q Good point with regards to the 12V battery temperature and a temperature based charging profile. No temp sensor on the battery as far as I could tell however perhaps the charging algorithm uses the ambient temp sensor for the car. I will have to repeat this measurement in the summer to verify. I still think 14.95V is quite excessive and will eventually cook the battery. Time will tell...

The 12V battery is a 45Ah unit by AtlasBX, part No. 85B24LS. I assume it is an AGM. I would be very surprised/disgusted if it were a flooded type... | 28 januar 2019

Likely more than anyone wants to know, but I wrote this extensive article on Tesla 12v batteries (S/X/3) last month:

The Model 3 12v battery is a deep discharge AGM type.

TM3Q | 28 januar 2019

Thanks ok so it's a AGM type battery wich makes sense.

bobr829 | 25 marts 2020

2 days ago my 2018 Model 3 died while being parked in my garage, no warning. It looks like it is the 12 volt battery. Problem is finding a battery replacement, even Tesla service didn't have one. Just called again and they are closed - I think because of the virus. Does anyone have any thoughts about a place to buy one? Thanks

jhbeak | 25 marts 2020

Schedule mobile service on your TESLA app... | 25 marts 2020

@bobr829 - While possible the 12v failed, it could be something more such as the DC-DC converter. As it's still in warranty, I'd schedule service. I think they are open but limited operations in some areas for critical repairs. This sounds critical to me.

fazman | 25 marts 2020

At least it broke down at home