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12v battery issue, Tesla horrific response

Discussion in 'Model 3' started by Tjhappel, Sep 19, 2020.

  1. ggnykk

    ggnykk Active Member

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    I never understand why Tesla couldn't engineer the big 75 kwh to charge up the 12v battery when low voltage is detected. Or just drop the 12v battery altogether, and only use the 75 kwh battery. Is it really that hard?
     
  2. brkaus

    brkaus Well-Known Member

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    Yes. The 12v is used for all the safety systems and allows the car to disconnect the HV pack if it detects problems. Without the 12v there would be nothing to activate the air bags, help with the power brakes, etc.

    There is probably a way they could use part of the HV pack, but then it’s impossible to fully disconnect it in an emergency.
     
  3. brkaus

    brkaus Well-Known Member

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    In the old days, all lead acid batteries had vents and we’re supposed to check the water. Then they made the water capacity a bit larger and covered the vents with a sticker. Instant maintenance free battery.
     
  4. neurocutie

    neurocutie Member

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    yeah, the good ol' days, when every gas station had a pitcher/spout of water for you to top off your battery water level -- supposedly distilled water, but prolly tap for many stationed that skimped...
     
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  5. MarsOrBust

    MarsOrBust Member

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    Reading thru the thread just had some questions about the 12V battery.
    1. What normally charges this battery?
    (They main battery pack apparently can not so I have to assume it only charges when car is plugged in.)

    2. I assume this is a sealed lead acid battery the does not require maintenance?

    3. How long is the OEM battery expected to last?
    (I have read that some have failed within the first 2 years.)

    4. Does switching battery to an OHMMU lithium void any type of warranty?

    5. Why did a software change create issues with the OHMMU battery and what would prevent that from happening again?

    6. What is the best way to monitor the health of your 12v?
     
  6. brkaus

    brkaus Well-Known Member

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    regarding the model 3

    The main battery pack charges the 12v. But not constantly. While the car is being operated the DC to DC converter is supplying 12v. When the car is off it will occasionally wake up to charge the 12v.

    No maintenance required. It’s still not clear if it’s SLA or AGM.

    Mine lasted 23 months.

    Not unless it caused the problem. So if the charging system failed there might be an argument.

    Nothing would prevent it from changing again. My guess is that Tesla is trying to get better at estimating battery failure and the ohmu has a different charge/discharge profile.

    hopefully Tesla will improve this in software.


    The good news is the battery is cheap (more evidence it’s SLA and not AGM) and easy to replace. I’ll probably buy a spare in 18 months.
     
  7. MarsOrBust

    MarsOrBust Member

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    #267 MarsOrBust, Oct 17, 2020
    Last edited: Oct 17, 2020
    thanks brkaus !
    Seems like the Lithium would be better but if the Tesla software is expecting an OEM type battery I think I rather just use that instead.
    As you said if the charging system failed they might have an argument against the Lithium battery and even if it did nothing I rather not have to deal with that.

    Just one more, did you get any kind of early warning when the 12v is going bad or does it just die on you?
     
  8. brkaus

    brkaus Well-Known Member

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    We had no warning, just a stranded wife.

    The positive in my case was that roadside assistance was responsive, they called for a tow, the tow arrive quickly, the guy knew exactly what he was doing, and the car was ready by noon the next day.

    Of course it sucked being stranded, late to picking up kid at after school activity, finding a ride home that night and a ride to Tesla the next day.

    This kind of thing happens to all my cars. I typically just get a lift to the nearest auto parts place, replace it, and move on.

    hopefully the landscape changes in the next 18 months-

    - Tesla gets better at managing the battery for longer life & better failure warning.

    - battery more readily available at auto parts stores

    - more experience with ohmu battery
     
  9. neurocutie

    neurocutie Member

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    well, compared with ICE, the difference is that one rarely gets stranded... in 40yrs I've never been really stranded. At most, I get a jump within 15mins or so, often faster if a friend is around. I then drive home or whatever. ICE batteries rarely are truly dead, dead and invariably I can limp along for another week or two, sometimes bringing along a jump battery just in case, so I can replace the battery at my convenience. I have never not been able to drive the ICE cars because of a dead battery.

    It would be good for Tesla to figure out how to minimize stranding its customers, keeping the car driveable in the face of a "dead" 12V battery.
     
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  10. brkaus

    brkaus Well-Known Member

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    may car was drivable once it was jumped.

    I think the problem is the battery gets in far worse shape without noticing. An ice car will sound different when starting. An EV will just turn on the dc2dc and cover it up.
     
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  11. iwannam3

    iwannam3 Member

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    So they gave you credit based on the battery warranty even though you had it changed out before any problem?
     
  12. FalconFour

    FalconFour Supporting Member

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    #272 FalconFour, Oct 17, 2020
    Last edited: Oct 17, 2020
    Oh my god, there's so much misinformation about the 12v in Tesla, I'm almost legitimately crying while I read this thread, thinking people are going about their lives thinking these things about the car... not your post in particular - just quoting it as it's asking all the right questions - but just... so many things. 14 pages, and so much has gone on :eek: (I wish we could use emoji here)

    OK, let me present information about the 12v system, in the form of answering each of your excellent and to-the-point questions:

    1. What normally charges the battery?
      The PCS - Power Conversion System - located in the "penthouse" of the main HV battery (under the rear seat, passenger side, in the 3). The PCS is also known as a "DC-DC converter" which takes the 300-400v of the HV battery and converts it down to 10~15v depending on the state of the system (average 13v at most times). The PCS is connected to the 12v bus through a solid-state switch (a MOSFET or similar; an "E-fuse" or a software-defined fuse) and this is what drives the entire 12v bus any time the car is not in deep-sleep (or just sleep). That is to say - the devices that run on 12v power do not "run on" the 12v battery when the car is awake, driving, or otherwise interacted-with. The 12v battery is just along for the ride.

      In fact, the car's software actually charges the 12v battery at a controlled 10-amp rate by modulating the 12v bus voltage while monitoring how much is going into the 12v battery. It's a really clever system, but you'll often observe the voltage coming out of your 12v accessory socket as being 12.5... 13.2... 14.2... varying like that, which is how it manipulates the charging process of the 12v battery. Because 12v systems have a relatively wide operating range (usually anywhere from 11v to 15v), the car takes advantage of that to avoid needing a secondary charge controller on the battery.

      The 12v battery merely exists on this 12v bus - it is the one thing that never moves, but exactly what device powers the bus can shift, due to the nature of electricity. Thus, for the overwhelming majority of the car's active existence, it's running on the HV battery, not the 12v battery. The 12v battery is just standing-by, ready to power things in case of an accident, a HV systems failure, or when the car goes completely to sleep.

      See also: Controlling Tesla's Sleep Mode ~-or-~ Lucid Dreaming for Robotaxis : teslamotors regarding sleep.
    2. I assume this is a sealed lead acid battery the does not require maintenance?
      You assume correctly. As best I can tell, it's just a bog standard 12v automotive grade battery. There's no maintenance you can do on it - no caps, no water. For what it's worth, my Model 3 has lasted 2-1/2 years on its original battery so far - and in my testing, it's performing just like new. I am honestly dumbfounded by how 12v batteries in Teslas fail, because of how lightly they're used. It's also worth noting that the behavior of 12v battery usage changed noticeably around FW version 2020.28 when lithium aftermarket batteries started being falsely flagged for "needing replacement".
    3. How long is the OEM battery expected to last?
      See above. But honestly, I'd expect them to last forever with how they're treated. I suspect that failure cases are caused by firmware bugs, e.g. a system failing to go to sleep and bleeding the battery dry, not due to routine use cases. As explained with #1, normal operation of the car doesn't touch the 12v battery at all.
    4. Does switching battery to an OHMMU lithium void any type of warranty?
      See: Magnuson-Moss Warranty Act - which boils down to "warranty is modular, and they can't deny a warranty claim on the drive train just because you changed the 12v battery". It would have to be proven that your replacement of the battery with an aftermarket part could reasonably cause the incident you're trying to cover under warranty. And OHMMU is a really good battery with far more self- and vehicle-protection than the OEM battery even comes with, so long story short: no.
    5. Why did a software change create issues with the OHMMU battery and what would prevent that from happening again?
      Tesla controls everything, and a whole lot of the car is software-defined. Since they can manipulate the 12v system voltage with just an algorithm tweak, my guess is that they're trying to solve the mystery of the 12v battery failures, and warn people sooner or gather data on field batteries. Unfortunately the lithium batteries have slightly different (but benign) differences in the way they charge and discharge, so that difference seems to be flagged in software while the car is doing its testing. It doesn't line up with a "healthy" 12v battery, so it thinks it's a bad lead 12v battery - even though the battery is performing all its duties perfectly. What you can do is contact service if you get that "12v battery must be replaced soon" message, and tell them to look up the issue for lithium-based 12v batteries causing false alerts. That's what I did. The more people report the issue, the more likely it is to be addressed. But honestly, I just hope they have passive flagging going on - like they do for "suspected 3rd party eMMC" - and can gather the statistics to show how many people are having this issue themselves. Or just fix it by letting the user say to ignore the fault / say it's a lithium battery, or something. :(
    6. What is the best way to monitor the health of your 12v?
      Honestly, it's hard to tell for sure. For me, coming to the conclusion that my original battery performs like-new, was when I installed my lithium replacement battery and took the original out to my bench and used it with my iCharger 208B to charge some other batteries. It still held its original capacity. A bit of knowledge of lead batteries and their charge/discharge characteristics is in order, but honestly, I kinda feel like you can just take it to an AutoZone for a quick free test on their elaborate machinery. Whatever you do, DON'T try testing it while it's still in the car. Any time the car is awake (which will include you standing in front of it with the hood open and all your doors closed, even with the MCU screen off), it'll be trying to keep that battery charged. You put a tester on it and you'll be testing the car... not the battery. So it has to be tested while disconnected from the car completely. Not terribly easy to pull off...
    7. Bonus round: when does the car charge the 12v battery?
      Any time it's not fully sleeping, it's charging (or has already finished charging, most of the time) the 12v battery from the HV battery, which is also powering all the systems in the car - including headlights, brakes, the cooling fan, stereo, MCU, seat heaters, fan, door/trunk/frunk latch motors, windows... anything else you can think of, it all runs on the PCS which is powered by the HV battery.

      Aside from removing it from the car completely, I really, honestly don't know how better to test it.
    8. Bonus round: What happened to OP, then?
      Honestly, it seems like the PCS tripped and shut down. I've had that happen several times during my tinkering. It causes the car to be undriveable, but the display and everything remains totally awake and seemingly nothing happened. But in reality, a massive thing happened - the car entirely shifted to running on 12v battery power. Obviously not good, and the car will not drive in this case. A possible solution may be to simply unplug the 12v battery negative terminal under the hood (10mm wrench) - and if the car completely powers-off when you do that, then that was indeed exactly what happened (otherwise, the PCS will continue powering the car - which means the PCS wasn't faulted). You should then wait 30 seconds (trust me, it matters), then reconnect it. The car should reboot, and if you hear a "clunk-clunk" shortly after connecting the terminal - that's the HV battery turning on and the PCS coming online - then you're good to go. The PCS is well-protected and tiny surges can trip it - it's designed into the sealed battery pack and is basically an indestructible fortress (that is: it's hard to break, BECAUSE it's impossible to get into the battery to repair/replace it). Usually, a reboot will clear the issue, but if it recurs, it's definitely time to call Tesla.
    Hope this mind-dump helps explain a few things. I'm happy to back this stuff up with evidence from Scan My Tesla and all the neat internal information it provides. Also, I have fun with batteries. Some may say, too much fun.
     
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  13. Daniel in SD

    Daniel in SD Well-Known Member

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    Teslas put more cycles on the 12V than any other car. The power used while “sleeping” is significant. It is true that if you were to drive the car 24/7 the battery would probably last a decade.
     
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  14. EnrgyNDpndnce

    EnrgyNDpndnce Member

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    Hate to repeat myself, but after jumping back into this thread it seems worth it. There is no reason a dead 12V should strand a Tesla owner either. It’s really no different than an ICE. When mine died (after 80k miles) I jumped in my wife’s Volvo and headed to O’Reilly. After educating the kid who worked there (had never heard of Tesla) I bought the battery needed and returned home. Installed new battery and BAZINGA, not stranded. Entire process took maybe 90 minutes soup to nuts. This is really a non-issue unless you are unwilling or unable to perform a simple maintenance item on the vehicle. Some people are and that’s fine, to each his own. But those people probably wouldn’t change the 12V on an ICE either so again, no difference.
     
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  15. Austindude

    Austindude Member

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    No idea if I was given credit or not. All I know is what I paid after examining the paperwork that I keep in my glove compartment. I can't speak for others on this thread but I have also replaced the 12V battery in all of my ice cars, usually after about 2 & 1/2 years.
     
  16. SageBrush

    SageBrush 2020: Drain the Sewer

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    I am waiting for my purchase to arrive of a 12v battery tester, I think this one

    I'd like to hear your opinion
     
  17. Daniel in SD

    Daniel in SD Well-Known Member

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    The difference is AAA won't replace them and while auto parts stores do sell batteries that fit (Group 51R) if you call and ask for a Tesla battery they'll say they don't sell them. I agree that someone reading through this 14 page thread should have no problem finding all the information needed to change their battery. :p
     
  18. EnrgyNDpndnce

    EnrgyNDpndnce Member

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    Anyone who is reasonably good at problem solving should have zero difficulty finding a proper replacement and making the switch. Most of us know better than to trust the opinion of some kid making $10 an hour with anything important.
     
  19. lUtriaNt

    lUtriaNt Member

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    Low cost way to monitor your 12V Battery

    what would you like to know?

    ive used this little guy for a year on my old car i had before i got model 3 this year.

    its simple - it works. its reliable. its also accurate. the battery i use is an ohmmu.

    20200927_203740.jpg
     
  20. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    We're trying to come to some conclusions over in this thread about what the "correct" behavior of the 12V should look like.

    I monitored the 12v battery voltage during an OTA update

    My belief is that a battery with high internal resistance or high self-leakage should look substantially different when viewing the behavior of the PCS/DC-DC and 12V battery interaction. It probably does, since Tesla can figure it out remotely. (Though they have access to more than just battery voltage info.)

    Most of what you say is very informative... But these statements, as has been pointed out, are somewhat inconsistent. A well-behaving car is sleeping for the vast majority of its life. So that's the mode that is the most common. So the 12V battery receives a LOT of use.

    So, for the majority of the car's active existence, it's running on the 12V battery, and the contactors are open. From what I understand it's usually just drawing a few watts (less than 10W?) from the 12V, but there are transient increases in load just prior to contactor engagement, presumably (like when you open a door and turn on the headlights and ambient lights prior to the contactors closing).

    That constant use is why they fail pretty quickly, IMO. As with many batteries (I suppose some batteries do benefit from use, but they're not that common), if you're constantly flogging it (yeah 7W counts as flogging it), even if you keep it at the most optimal of voltages and strictly adhere to the best charging profiles, it's going to wear out faster than if you did not use it and kept it at the optimal voltage. Don't you think?

    A voltage monitor should be low impact. I'm hopeful that we'll be able to see some before and after replacement pictures soon, so we have some signatures to look out for.

    My feeling is that currently my 12V is hot garbage, since I feel that the voltage monitor is not showing anything close to normal behavior (see the other thread). No failure yet, though!

    The end goal here would be to be able to detect a proper failure warning signature before Tesla does, so that you don't get stuck with a non-functional car. Not sure whether it is possible or not.

    This is not a pretty picture. Unfortunately I am currently unable to perform any sort of service request through the app for whatever reason. I have an existing appointment I never scheduled which I cannot view details on.
    IMG_8144.PNG
     
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