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Dead 12v Questions

Commish

Member
Aug 16, 2018
41
37
San Jose, CA
fseir- This is pretty much the same scenario that happened to me. If you wait an hour or so, try opening the door again and see if the Tesla turns on. Mine seemed to click back to the big battery after a few hours sometimes. At least it gives you enough to drive the car into the service center.

I was given a lot of excuses through my service guy and the service center contact I had. They both told me a ton of 12v replacements were happening now. Some of it is just the nature of the length of time these batteries last in the 3, some of it has to do with reduced use and charging during Covid stay-in-place, blah blah blah. I personally think its bad design to even need a 12v, so who knows.

If you can't open the doors BTW (from the outside), you can actually open the frunk with a 9v battery- check out YouTube for how to do that.

Took me 3 days of waiting to get a new 12v installed after my "urgent" Roadside Assistance Request. Tesla Service is terrible and needs to be fixed.
 
Guess another question, is why all of these 12V batteries are failing so soon? The 12V in my leaf lasted 6 years.

I was lucky that my model 3 bricked inside of my garage, especially since two of the windows are part way down and can’t be raised.
We've been seeing a lot of 12v failures this year. I believe that it's due to an algorithm change or bug that Tesla had in their software that controls how the 12v battery is charged. AGM batteries can be damaged or their capacity reduced from both under- and overcharging. If Tesla was tweaking the way the batteries are charged to, for example, reduce phantom drain it could have caused issues. It could have also been a bug. Anybody that's had a Tesla for a while knows that sometimes bugs are introduced and later fixed. As a computer engineer, I know that it's impossible to have software that is guaranteed to always work as specified without bugs.
 
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Guess another question, is why all of these 12V batteries are failing so soon? The 12V in my leaf lasted 6 years.

I was lucky that my model 3 bricked inside of my garage, especially since two of the windows are part way down and can’t be raised.

How many miles did your Leaf have after 6 years? My Model 3 is less than 2 years old but I had 80k miles when my 12V died. I consider that acceptable.
 

smatthew

Active Member
Jun 9, 2018
1,300
2,250
CA Bay Area
I swear reading this thread jinxed me, lol!

Walked into my garage last night and noticed a strong smell of sulfur. Thought it was some butane camping stove canisters leaking, so I moved them outside, but the odor didn’t clear, and after a little while, figured out it was coming from the car.

Tried to open the driver door, and it was dead, but oddly, the window was down slightly and the mirror was folded out. The rear driver’s side door was also unresponsive, so I started worrying about how I was going to get into the car. Fortunately, the passenger door opened, but the window actuation was slow, and it was acting like it had very low power. The foot well ambient light on the passenger side was flickering like a horror movie, but the light on the driver’s foot well was off completely. No other signs of life in the car.

Since the driver’s window was down a little, I was able to use the emergency handle to open the driver’s door without damaging the window.

Next, I tried to remove the charging cable, but there was no power, so I couldn’t. I tried to open the trunk, but it wouldn’t open. I could hear slight sounds of electrical motor actuation when I pressed the trunk lid button, so I kept trying, and eventually, the lid opened and I was able to pull the charging cable release cord. Interestingly, the passenger side light in the trunk was also flickering on and off, but the driver’s side light was totally off. It was like the passenger side of the car had some power, but the driver side had none.

I contacted roadside assistance and they sent a flatbed truck out (a contracted towing company). First they tried hooking up a battery to the 12V, but it did absolutely nothing. If anything, it seemed to kill it completely, as the right sided flickering stopped and all electrical motor sounds ceased. Then the guy started trying to figure out how to get the car onto dollies, and it was obvious he didn’t really know what he was doing, and he wasn’t being very careful. Since it was getting late, and I didn’t want him to damage the car, I told him to just leave it, and I would contact Tesla and try to get mobile service out as soon as possible.

So now that it’s the weekend, I have to wait until Monday to call, and hopefully they can send someone out to swap out the battery. And I really hope the weird 1/2 of the car not having any power, doesn’t mean anything more ominous about the electrical system. Haven’t heard anyone report that before.

I’ll post an update if it turns out to be more complicated than a battery swap.
That smell of sulfur means at least one cell has shorted out, and at that point hooking up a 12V battery will not help. Tesla says you can jump the car from another car, you just can't jump another car from your Tesla. But Tow drivers don't want to fry an expensive car.
Tow drivers know how to use dollies. They use them on a regular basis. I doubt any tow truck driver tows Teslas on a regular basis.
 

electrongeek

Metrology Fanboy
Nov 1, 2019
69
75
Maine
This was very helpful, thanks. Right now I have the option of renting a car for a week at $150/day or putting in this battery... so battery wins.



Thank you! Again, very helpful... and a funny story!


Just an FYI - not every AGM battery is a deep cycle battery. If you don't replace the TM3 12V battery with a deep cycle battery, it will not last nearly as long. I don't know myself if the original Tesla 12V battery is a deep cycle battery, but I can't imagine that it isn't.
 
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My leaf had close to 90k. My model 3 had 12,000.

Proof positive that YMMV. 12k falls under warranty so that’s good. I’m not trying to say some Model 3s aren’t seeing premature 12V failure, obviously some are (yours included). But I also know this occurs from time to time in non-Tesla vehicles. And what we don’t know is the rate of failure and how that compares to the rate of failure in the overall auto fleet. I have seen way too many comments that essentially say “there are so many people on this forum replacing 12V batteries early that there must be a problem with the battery or with Model 3.” We simply don’t have the data to reasonably draw that conclusion. As I pointed out earlier in this thread, no one starts a thread titled “50k miles on my car and I still haven’t replaced my 12V.” Yet many people are in that exact situation. That’s why I point out that mine went 80k miles on the factory 12V.
 

Siciliano

Member
Apr 6, 2016
382
333
New York
It seems kind of sadly ironic that you could brick an EV that is literally sitting on a bed of batteries, all because of a measly 12v. :confused:

This displays my limited knowledge about these cars...but I'm wondering why (unless it's not possible?) they didn't design a back-up system that temporarily draws power from the main batteries in the event the 12v dies...
 

smatthew

Active Member
Jun 9, 2018
1,300
2,250
CA Bay Area
It seems kind of sadly ironic that you could brick an EV that is literally sitting on a bed of batteries, all because of a measly 12v. :confused:

This displays my limited knowledge about these cars...but I'm wondering why (unless it's not possible?) they didn't design a back-up system that temporarily draws power from the main batteries in the event the 12v dies...
But how do you power on that backup system if 12V is dead? Hand crank?
 

RayK

Active Member
Apr 5, 2016
2,308
2,319
San Jose, CA
One could assume that if a constant 12V tap or DC-to-DC converter could be engineered into the main battery pack, that could serve as a "backup" source of 12V. It wouldn't be needed as long as the 12V battery is good and it would replace the stand-alone 12V battery if/when it dies (basically an SPDT relay) but of course it would be entirely dependent upon the main pack not being drained down to 0. The tap would have to be made independent of the high voltage connectors so that might be a safety issue. And there would have to be a manual way to disconnect the main pack's 12V supply when the car needs to be serviced.
 

smatthew

Active Member
Jun 9, 2018
1,300
2,250
CA Bay Area
The problem is the only way to get "automotive 12V" out of the main pack is to use a DC/DC converter, or tap 4 bricks. If you use a DC/DC converter, that's fine, but how do you get it to stop working when there's a problem? Does the car have to get another pyro disconnect?

And if you tap 4 bricks, you'll get 12-16V, which is consistent with automotive 12V, but you'll cause an imbalance in the battery pack, unless you design some complicated mechanism to change which 4 bricks you're connected to in an automated manner.

I'm sure all the Model 3's with failing batteries are causing someone at HQ to wonder what would be better. If it was cheaper, I would expect Tesla to start putting LiFePO4 12v batteries in the cars.

But the truth is, 12V needs to go. Sure, there's lots of stuff that works with it already. But moving to 24V or 48V would allow smaller conductors and/or reduced power losses. It would require Tesla to source automotive equipment that worked on 24/48V. But that's the future.
 

camalaio

Active Member
May 28, 2019
1,483
2,198
Vernon, BC, Canada
It seems kind of sadly ironic that you could brick an EV that is literally sitting on a bed of batteries, all because of a measly 12v. :confused:

This displays my limited knowledge about these cars...but I'm wondering why (unless it's not possible?) they didn't design a back-up system that temporarily draws power from the main batteries in the event the 12v dies...

There are many more concerns in the opposite direction, i.e. what do you do when the high-voltage primary battery dies? It can be shut off for many more reasons than simply dying, many such reasons being safety-related. There basically aren't any equivalent reasons to shut off the 12V: it doesn't have enough energy contained within it to be a deadly show of sparks and fire if something goes terribly wrong. But the main pack does, so there are many situations in which it can be electrically cut off, either passively or actively.

You still need 12V available for many reasons when the main pack is dead or otherwise disconnected:
  • Safety-related indicators ("hazards")
  • Access to the vehicle (convenience or maybe safety, but there are some mechanical backups)
  • To enable "Tow Mode" - that rear motor isn't budging when unpowered normally
The last point matters not just because you need to enable tow mode via the screen, but my guess for how it actually works is by using the 12V battery to provide high voltage to the drive units. This would go with the documents (can't remember if it's the owner's manual or info for first responders) stating that tow mode cannot be depended on to roll the vehicle very far.

The problem is the only way to get "automotive 12V" out of the main pack is to use a DC/DC converter, or tap 4 bricks. If you use a DC/DC converter, that's fine, but how do you get it to stop working when there's a problem? Does the car have to get another pyro disconnect?

And if you tap 4 bricks, you'll get 12-16V, which is consistent with automotive 12V, but you'll cause an imbalance in the battery pack, unless you design some complicated mechanism to change which 4 bricks you're connected to in an automated manner.

I'm sure all the Model 3's with failing batteries are causing someone at HQ to wonder what would be better. If it was cheaper, I would expect Tesla to start putting LiFePO4 12v batteries in the cars.

But the truth is, 12V needs to go. Sure, there's lots of stuff that works with it already. But moving to 24V or 48V would allow smaller conductors and/or reduced power losses. It would require Tesla to source automotive equipment that worked on 24/48V. But that's the future.

Fun fact, and I hope I'm not wrong, but the first Tesla Roadsters did exactly that: tap from a certain group of cells for 12V power.

They stopped doing this, and later roadsters included a separate 12V battery. Something was obviously learned, but I do not know what. Could be any combination of discussed points here, or something else.

If nothing else, the HV battery in the Model 3 is made simpler by not additionally having tapped 12V. The compartment with the actual cells is quite a bit more organised and homogeneous than their previous designs. Simpler compartmentalized designs -- at least I find -- have benefits on the human level when adapting or using built-up systems.
 
Just a $500 upgrade to LiPo4 with a heater, then problem solved
The Ohmmu and EarthX batteries have a BMS, which could possibly include a "heater". There's probably resistors in there to balance cells, and using them would provide thermal heat from the resistance as well as cause the cells to heat up from being used. Heating may not be necessary at all though, since LiFePO4 batteries retain a higher percentage of their energy at cold temps than lead acid batteries. And EVs don't need a huge amount of amps to crank over an engine.

For example, the EarthX battery used by the Mountain Pass Performance 12v replacement kit is rated to operate down to -30°C, and at that temp to have 88% of it's capacity. A lead acid battery loses 50% of it's capacity at -20°C. As long as the lead acid battery is charged to 100%, it's electrolyte won't freeze until -70°C. A discharged lead acid at 0°C will have it's electrolyte frozen.
 
There are many more concerns in the opposite direction, i.e. what do you do when the high-voltage primary battery dies? It can be shut off for many more reasons than simply dying, many such reasons being safety-related. There basically aren't any equivalent reasons to shut off the 12V: it doesn't have enough energy contained within it to be a deadly show of sparks and fire if something goes terribly wrong. But the main pack does, so there are many situations in which it can be electrically cut off, either passively or actively.

You still need 12V available for many reasons when the main pack is dead or otherwise disconnected:
  • Safety-related indicators ("hazards")
  • Access to the vehicle (convenience or maybe safety, but there are some mechanical backups)
  • To enable "Tow Mode" - that rear motor isn't budging when unpowered normally
The last point matters not just because you need to enable tow mode via the screen, but my guess for how it actually works is by using the 12V battery to provide high voltage to the drive units. This would go with the documents (can't remember if it's the owner's manual or info for first responders) stating that tow mode cannot be depended on to roll the vehicle very far.



Fun fact, and I hope I'm not wrong, but the first Tesla Roadsters did exactly that: tap from a certain group of cells for 12V power.

They stopped doing this, and later roadsters included a separate 12V battery. Something was obviously learned, but I do not know what. Could be any combination of discussed points here, or something else.

If nothing else, the HV battery in the Model 3 is made simpler by not additionally having tapped 12V. The compartment with the actual cells is quite a bit more organised and homogeneous than their previous designs. Simpler compartmentalized designs -- at least I find -- have benefits on the human level when adapting or using built-up systems.
I agree with you. Just wanted to clarify a few things.
Tow Mode just releases the parking brakes on the rear wheels. It uses 12v to turn a small motor at the rear calipers to unclamp the brake pads. In this mode, it's like the car is in neutral gear and will roll easily.
Taking 12v from a small subset of the cells in the HV battery pack will cause those cells to become unbalanced and cause issues. As mentioned above by smatthew, the original Roadster early versions did this. Tesla switched to a dedicated 12v battery for it after they realized that earlier method would negatively affect the HV battery and possible even brick it.
 

fseir

Member
May 1, 2016
678
537
Honolulu
Mobile service swapped out my 12V today, so after 5 days with a bricked car, it’s nice to have it back.

I think I could live with having to replace the 12V every 2 years or so, if I could count on a warning that the battery is about to die, and it’s time to make an appointment. Based on this experience, though, and those of others who also got zero warning, I feel like it’s a ticking time bomb.

I really hope Tesla addresses this with a software update.
 
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AZAV8R

Member
Aug 29, 2020
144
117
Arizona
Guess another question, is why all of these 12V batteries are failing so soon? The 12V in my leaf lasted 6 years.

I was lucky that my model 3 bricked inside of my garage, especially since two of the windows are part way down and can’t be raised.

The answer I got to this question is that Sentry and other accessories run off the 12v battery, causing it to cycle much more than one in an ICE, where it’s sole mission is pretty much to start the engine. This make sense to me as the batteries in our Teslas are deep cycle units.

While the above sounds plausible, it sure would be nice if Tesla would somehow verify it.

:cool:
 
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Went to go out on a drive on Saturday, Got in my car and it was silent, the screen wouldn't come on. First I thought, try a reboot. Nothing. Then I noticed a bit of a motor whine and it seemed to be coming from the seat. The Windows rolled down really slowly. Ok, 12v is dead. Contacted roadside assistance. A flatbed was at my house in about 30 minutes. Hooked up his cable to the under-bumper wires. She's alive. The tow truck driver was able to drive it up on to the flatbed. I contacted service via the SMS text they sent me that I may still need to get around this weekend and they provided me with a code for 3 Uber rides Which expired Sunday night. Turns out I didn't need them though.

This morning (Monday) I was contacted by Dania SC that the 12v battery was dead. They replaced it. The car is a 2019 LR AWD with about 13k miles on it. Got it in March 2019. I still don't have confirmation yet that it was fully covered under warranty. I haven't seen a bill yet. I contacted them via SMS Text they sent me that the Uber credits I didn't use expired and asked for a new one to get me to the SC that's 26 miles away. New code provided. I'll update after I pick up the car and find out if there's a bill I have to pay.
 
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