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Near annual replacement of 12V battery is typical according to Tesla Service Tech

Discussion in 'Model S: Battery & Charging' started by ecarfan, Jan 15, 2015.

  1. SteveS0353

    SteveS0353 Member

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    The main purpose of the HV contactors is to isolate the high-voltage inside the battery. Firing up the OBC would yield ~400V DC at various points in the car, even if the main contactors remain open. I believe that would be contrary to the safety design of the Model S.
     
  2. qwk

    qwk Model S P2681

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    Nobody smart is going to be servicing the car's HV while the car is plugged in anyway. Just like nobody smart services the HV with the 12V, and safety harness connected.
     
  3. JRP3

    JRP3 Hyperactive Member

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    A few points:

    Raising the voltage from 12 to 24 or 48 does not solve any problems.

    Using the NCA chemistry with it's own temp management is not practical, and would only add to the vampire load. LiFePO4 or even better lithium titanate, which is pretty much immune to temperature extremes, would be a better choice. A small resistance heating pad might be all that is needed for extremely cold temps, (maybe -40C.)

    Since lithium has a higher round trip charge/discharge efficiency than lead acid that change alone would somewhat lower the vampire drain.

    I really don't understand why Tesla is not using a LTO battery, other than cost, but when you figure in all the replacement labor and customer annoyance I think it would be money well spent. Maybe someone will offer one in the aftermarket someday, like right after a large number of S's start coming out of warranty. If the stock unit is about 30 amp hours the LTO version would only need to be about half that capacity.
     
  4. Oba

    Oba Member

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    #164 Oba, Jul 24, 2015
    Last edited: Jul 24, 2015
    It's hard to tell if this is just a tongue-in-cheek, or not.

    If not, you are making assumptions... big ones, that I didn't post.

    1) Yes, the cells for the 12 volt can be in the main pack housing.

    2) That has absolutely nothing to do with HV, parking brakes, or the rest.

    3) Coming out of the pack (and all the climate controlled cells) would be two HV cables to the inverter, both on a relay internal to the pack. That relay is powered by 12 volts, and is normally open (NO). That is what exists today.

    4) In addition, two non-switched LV - 12 volt cables would ALSO come out of the pack. That is different than today.

    5) There would be no external 12 volt battery, no lead acid, and no changes to the operation of the car.

    6) This 12 volt would likely last as long as the main traction battery cells.

    7) If the pack cells are getting some Temperture Management System (TMS) love, then so will the 12 volt portion of the pack. The amount of extra energy required for TMS duties of the additional 16-40 cells would be difficult to measure... that's how small it would be extra in relation to the 7104 cells already in the pack. I calculate less than 0.5% additional energy in exchange for a lifelong 12 volt battery.
     
  5. qwk

    qwk Model S P2681

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    It's all about cost. Some of the issues that Tesla has had would have been cheaper in the long run to just completely redesign a part instead of just "patch engineering", but they continue to do it over and over. Tesla is really good at jumping over dollars to pick up nickels.
     
  6. spottyq

    spottyq Member

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    It seems I had posted a little bit too quickly… I did not think about putting the 12V inside the main battery.

    Then I agree with all your points except the last two.

    For (6) I'm going to assume we have separate cells (of the same chemistry) inside the main pack wired to make 12V.

    In that case, the 12V cells will have a very different usage compared to the HV cells. When the car is off, the 12V cells will be cycled a few times a day, while the HV do not get used (just to top off the 12V.) When the car is running or charging, the 12V cells are maintained at their ideal SOC while the HV cells get cycled. So the more you use the car, the less the 12V cells gets used.

    I can't say if it they will last more or less than the HV pack, but I don't believe it will have the same life. Either way, if it get a good TMS, I agree it will probably last longer than the actual lead-acid.


    Regarding (7), it is my understanding that the batteries Tesla uses have different acceptable temperature range depending on whether they are not used, charging or discharging. If I understand it correctly, they have a very large temperature range when not used, a somewhat smaller temperature range when discharging and an even more restricted range when charging. Lead-acid accept a much wider range of temperatures, and it is not dependent on the whether or not the battery is in use or not. The temperature just changes the voltage setpoints for the battery charging.

    In most situations, when off and not charging, the MS does not actually run the TMS because the cells are not used and thus are in the wide range of acceptable temperature. (That's why the vampire drain is almost the same for everyone. It does not depend on the external temperature.)

    If Tesla were to replace the lead-acid by a battery using the same chemistry as the actual HV pack, the TMS would have to run more often to keep these few cells in the acceptable temperature range for charging/discharging. In that case, the difference would not be 0.5%, it would be 0 versus whatever is required to keep the temperature of these cells acceptable.
     
  7. Tree95

    Tree95 Member

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    Telsa 12V adapater fuse 1.1040.jpg

    Telsa 12V adapater fuse 2.1040.jpg
     
    • Informative x 3
  8. FlatSix911

    FlatSix911 Porsche 918 Hybrid

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    Good post. I am going add the jumper to keep the 12V accessory plug always on.
    Then I will use my CTEK charger to keep the 12V battery topped off in the garage.

    Here is a photo of the 12V battery location on a P85D above the front electric motor.
    Note that the article states it is not an easy task to replace the deep cycle battery...
    http://insideevs.com/wp-content/uploads/2015/01/Want-to-know-what-it-really-looks-like-under-the-hood-Imgur.jpg

    Want-to-know-what-it-really-looks-like-under-the-hood-Imgur-750x562.jpg
     
  9. Ingineer

    Ingineer Electrical Engineer

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    It's WAY easier to replace the 12v battery at the new location than at the original location below the air intake. I helped someone replace one on an older car and it took well over an hour. (interspersed with lots of expletives!) I guess Tesla assumed that you'd not be doing this as often as it turned out to need!

    I sure hope Tesla ditches the AGM and starts putting in a small lithium pack when they get the Gigafactory swinging and their cost per cell goes down.
     
    • Informative x 1
  10. FlatSix911

    FlatSix911 Porsche 918 Hybrid

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    I added the jumper to the relay location and the 12V accessory plug is now always on.
    The only issue is that my CTEK is in a constant state of charging the 12V battery - is this expected?

     
  11. Ingineer

    Ingineer Electrical Engineer

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    Yes, the battery will likely never enter the Float (final) phase because there is a constant load applied from the myriad systems in the car. MCU, Cellular Modem, Body ECU (looking for fobs), Door controllers, Charger, and many others. There's probably at least 15 microcontrollers or so, always on, and that's not including the ones in the battery pack!
     
  12. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Most smart, lead-acid chargers go through charge cycles that are something like this:

    • Bulk — Charging at maximum current/power to get to the Acceptance Voltage.
    • Acceptance — Charge with enough current to keep the battery Voltage at the Acceptance Voltage. The current will slowly decline. Stay at this phase until the current drops low enough or enough time has passed to declare the battery charged.
    • Float — After the battery has been declared charged, let the Voltage drop to a Float Voltage to maintain the battery at full charge and avoid over charging.

    At room temperature (25˚C, 77˚F), the typical lead-acid Acceptance Voltage is 14.2 Volts, and the typical Float Voltage is 13.2 Volts. There is a negative temperature coefficient, so that these Voltages go up as the temperature drops and go down as the temperature increases; that is why it is important to have a temperature compensated charger; without temperature compensation, the battery is undercharged at low temps and overcharged at high temps.

    Most smart chargers will transition from Acceptance (14.2 Volts) to Float Voltage (13.2 Volts) after a couple of hours or 1.5 Amps per 100 Amp-hr if the battery capacity is known. With the Vampire drain in a Tesla, the Float current will stay in the 2-5 Amp region, even after the charger is in Float mode, to feed the thirsty Vampire, and the lead-acid battery will not be discharged.
     
  13. scottm

    scottm Version 9 software sufferer

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    OK, thanks for explaining that!

    Please recommend a suitable smart charger (few links as examples would be nice).

    I want to go out and get one.

    These things are going on sale about now.. seems to be the season.
     
  14. chucks

    chucks Member

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    I am at 23.6K miles and 19 months, with no indication from Tesla or the car that my battery needs replacement. Did they start installing better 12v batteries or link them to a generator later in the production cycle?
     
  15. santana338

    santana338 Member

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    I just had my car in for service after 30 months. I never saw a notice about the 12V system, but my service advisor said they had been getting notices about the 12V system for a while. If I were you I would check with your service center and see if they are getting any notifications. If so, schedule a service visit for the battery to be replaced.
     
  16. pedriscoll

    pedriscoll Active Member

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    At 5 months, 8900 miles, I had the message "12 volt battery needs service, call Tesla service" show up. They told me that upon checking the vehicle logs that the the battery was slightly below threshold. They came out and replaced it 3 days later.
     
  17. JRP3

    JRP3 Hyperactive Member

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    Ridiculous that this is still an ongoing issue at this point. Obviously a fundamental redesign is in order, or a more robust battery chemistry, or both.
     
  18. California Roll

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    I will be taking delivery of my 90D next week. It would be interesting to track the performance of the 12V battery in this new late 2015 model.

    Is replacing the 12V battery an easy DIY operation? Wondering just in case...
     
  19. Cyclone

    Cyclone Cyclonic Member ((.oO))

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    DIY, yes. Easy, that depends. Newer cars like yours moved the battery to a more easily accessible location, but it is not as easy as changing the battery in my Toyota 4Runner, which I can do in about 3 minutes.
     
  20. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Why bother? Tesla does it under warranty.
     

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