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Why the 12 volt battery?

Discussion in 'Model S: Battery & Charging' started by Todd Burch, Dec 19, 2012.

  1. Todd Burch

    Todd Burch Electron Pilot

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    #1 Todd Burch, Dec 19, 2012
    Last edited: Dec 19, 2012
    OK, I'm not an EE so forgive me, but it appears as though the 12v battery is a weak link in the reliability chain of the Model S/Roadster drivetrain. Failure of this battery (which has happened to several owners already, admittedly in most cases due to a software bug) renders the car completely unusable.

    So why is the 12v battery necessary? Couldn't a 12v DC feed pulled off the main battery be used instead? I know you'd need to step down the voltage obviously, but this would be one less subsystem to maintain/monitor. Sorry if this question's been asked before--a search showed a bunch of unrelated 12v battery threads.
     
  2. Babylonfive

    Babylonfive Power12

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    IMHO the battery provides a fall-back to the emergency equipment even if the main battery is completely discharged. Looks like a case of redundancy for safety.

    Also, the staqndard 12-volt battery smooths the load requirement to recharging it from the pack - the 'step-down' converter you mention can actually be sized well below the peak load requirement for all existing and possible customer equipment, and the battery provides the peak current when needed.
     
  3. Doug_G

    Doug_G Lead Moderator

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    They had a tap from the ESS in early versions of the Roadster, and it caused more problems than it solved. So they moved to a small 12V motorcycle battery. The Model S is an improvement on that little battery.

    There were also problems running 12V accessories in the Roadster; power inverters aren't allowed because they can damage the electrical system. So a large 12V battery fixes that too.
     
  4. dflye

    dflye S Sig Perf 414, VIN 814

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    Be nice if they'd stick a 15A grounded outlet in the center console next to the 12V outlet.

    Not like there is a lack of battery capacity to power something like a laptop for long road trips (as mine doesn't always work well with 300W inverters.)

    Or for a blender for tailgating! :biggrin: We have a 1500W inverter that I used with my old car for such purposes, but that requires direct connection to the 12V car battery, which looks to be a pain to access on the S.
     
  5. Todd Burch

    Todd Burch Electron Pilot

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    This must not have anything to do with it. If the main battery is completely discharged, you have some major issues and you're not going anywhere anyway. What "emergency equipment" are you referring to? Hazard flashers? If the main battery's discharged so much that it can't run the hazard flashers, then how on earth did you get to the side of the road?!? :) The car would've reached 0 mi range and shut off long before.

    Not sure I understand you here. Without a 12-volt battery, there's no need to recharge it, so that can't be listed as a reason for having the 12-volt battery :).

    Edit: On second reading, it sounds like you're saying the peak load of the step-down transformer would be less, therefore cheaper/smaller/lighter weight? I wonder if that would make up for the additional battery systems needed. The 12V accessory load is peanuts compared to load required for moving the car, of course.

    What types of problems? Just curious since that seems like an ideal route. I have no doubt that the Model S system is an improvement from the Roadster--and I'm sure that once the software glitches are worked out this will be much less of an issue.

    An inverter isn't required (I don't think) because the main pack is already DC, as is the 12V battery. Seems like all you'd need is a step-down transformer to drop from pack voltage to 12V. (Again, perhaps clearly I'm not an EE).
     
  6. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    It can also address odd feedback loops, for example, you can see some strange effects when the contactors for charging are powered by the very battery that's being charged. Ever try to plug a UPS into itself? :)
     
  7. qwk

    qwk Model S P2681

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    The 1.5's used one of the ESS battery sheets to power the accessories. I think one of the main reasons that this didn't work out very well is problems with balancing that particular sheet.
     
  8. Todd Burch

    Todd Burch Electron Pilot

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    Step-down transformer from the main feed would avoid that issue as well.

    I'm really not trying to challenge Tesla's expertise on the subject...just trying to find a reason to grasp onto for why I'll need to have a motorcycle battery changed in my electric car every few years :). Just trying to understand the technology, yo!
     
  9. qwk

    qwk Model S P2681

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    Yes, there are a number of ways to deal with this, and there are pros and cons for each option. The 12V battery option must have the least headaches associated with it, as all other EV's on the market use one too.
     
  10. Todd Burch

    Todd Burch Electron Pilot

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    <Triple-Rainbow Guy Voice>But Whyyyyyy?!?!?!?</Triple-Rainbow Guy Voice>
     
  11. ElSupreme

    ElSupreme Model S 03182

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    Think of it as a accumulator of power. Tesla puts in a small charger it runs off the main pack, not large enough to power the maximum instantaneous load. But the Pb Acid battery is big enough to supply any instant load you would need. But this keeps Tesla's costs down because they only need a 'trickle' charger to supply the AVERAGE power consumption. The battery is sized to provide any situation where the stars allign and for a short period you need a lot of 12V power.

    An example. I work in a plant that produces about 500 gallons a minute of 'treated water'. The plant if it ran all its pumps at once could use 3000 gallons of treated water a minute. Instead of a huge treatment system, they store a buffer in some tanks. It is cheaper and more reliable to do it this way. For many reasons.
     
  12. Todd Burch

    Todd Burch Electron Pilot

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    Yes I understand that, but I still haven't heard why the 12v battery may be better to have in the first place. (At least not any detailed reasons why). After all, the main pack could power 85 hair dryers for an hour :), so instantaneous power consumption is not the concern with the main pack--or doesn't seem to be at least.
     
  13. ElSupreme

    ElSupreme Model S 03182

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    The main pack isn't at a usable voltage. They have to step it down. Doing that DC->DC is tough and expensive. If you are using all the 12V accessories (windows, sunroof, AC, fans, coolant loop, headlamps, brights, foglamps, hazard lamps, all your computers are pegged) you will pull a large amount of power. Probably more power than the stepdown can handle. In stead of building a bigger one, Tesla decided to install a 12V Pb-Acid battery. Charge it at an acceptable rate and use its capacity to handle high demand.

    I assume you get all that.

    It is most likely cheaper to add the battery and have a small stepdown to 12V, than it is to have a larger (enough to handle the worst case stars aligned scenario for their 12V system) stepdown to handle everything without a buffer/accumulator. To have a water system to provide 3000 gallons per minute the plant would spend many tens of milions of dollars. A few tanks only cost hundreds of thousands of dollars.

    I also alluded to other benefits using a accumulator for water. You can run your supply system at a constant set rate, this is very nice for a step down voltage system. You don't fluctuate current wildly that affects performance and requires the stepdown to be more robust. It runs at a constant rate when the 12V battery is past a threshold.

    You get a balanced voltage from the battery. Your battery will smooth out any voltage spikes or dips, this is a very nice feature.

    You are not required to have your 12V stepdown from your main pack available 100% of the time. You can use the seperate battery for temporary power. This may come in useful when high demand is placed on the main pack, and you have a voltage dip. You can also have a full shutoff of the traction pack (to do some diagnostics when parked) and still have power for your keyless entry, door handles, and all your diagnostic systems.

    The bad part about accumulation is you have to make sure your accumulated power is good quality (which is what is going wrong) and maintains charge properly. The stored water has similar problems in storage, you have to keep things from growing, and maintain a temperature range.

    I do know that having a DC->DC stepdown voltage system to be the size Tesla needs, the quality of voltage, and the varying current demands would be overly expensive, and complex. A 12V Pb-Acid battery is an elegant, cheap, reliable solution to the problem. Now Tesla just has to make sure they charge it, and discharge it properly. I beleive this is a communication problem between the 12V system, and the traction battery system. And there are gaps where the 12V system will discharge the battery, without the main high voltage system getting a bit to charge the 12V battery.



    EDIT: In summary a 12V Pb Acid battery system is more robust, more tolerant, and less expensive than any alternative. Until we get accessories that run on a main pack voltage system there will be a separate accessory battery system. Why they don't have a 12V Li-Ion battery is probably only due to cost, and existing electronics designed to run on Pb Acid batteries.
     
  14. dsm363

    dsm363 Roadster + Sig Model S

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    You mean frunkgating=) They should have an outlet for a drop in liquid cooled frunk liner for frunkgating at football games.

    - - - Updated - - -

    Thanks for explaining this, very helpful. Other than the cost you mentioned, does the Li-Ion 12V pack not provide the correct kind of power for these accessories? You said existing electronics were designed to run on Pb Acid batteries but thought if the power is 12V from a Li Ion 12V battery, that should work too. Thanks.
     
  15. arg

    arg Member

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    My guess is that the reasoning behind this goes:

    - There has to be a 12V subsystem to run various small loads that would be inconvenient to drive with the full pack voltage. Obviously 12V rather than any other voltage to take advantage of off-the-shelf automotive stuff that's been 12V for years.

    - That 12V subsystem could be powered entirely from a DC/DC converter from the main pack voltage, or with a 12V battery charged via a (similar but maybe smaller) DC/DC converter. The savings on the converter
    size offset against the cost of the battery so that there isn't a huge cost difference to the two solutions.

    - There are two main reasons for favour the 12V battery approach:

    1) The need to protect the main battery. Whenever power is going in or out of the main battery, it must be carefully monitored, temperature maintained etc. So powering it up to drive any kind of load
    is a big deal. When the car is 'off' and only needs to run tiny loads like the fob receiver, the 12V battery allows you to run those loads without needing to power up the main battery and all its
    protective systems - which would add significantly to the 'vampire load'.

    2) There is a chicken-and-egg situation when the main battery is down to minimum. Getting the main battery charged requires quite a lot of power - operating the charge port door/cable locks,
    sufficient compute power to talk to the charger and negotiate power on (easy for J1772 AC, harder for supercharger with its power-line-carrier protocol), and engage contactors inside the car
    to connect up the relevant charge circuits. If the main battery is at minimum, you can't risk drawing current from it to run these other things, as you risk damage - even if you think you are
    about to charge and so can afford it, maybe the charger doesn't work and the user sits there plugging it in and out, using up battery yet never charging. The 12V battery can be used to run
    this stuff instead, and if you don't ultimately succeed in charging, worst case you have a flat 12V battery that can be recovered with a jump start or traditional charger, rather than a damaged
    main battery.


    So, it seems like a reasonable compromise to me - albeit that some details of the implementation are a bit tricky to get right.

    Oh, and the lead-acid vs. Li-ion for the 12V? The 12V battery doesn't need to have much capacity as it can always be topped up from the main battery. However, it wants to have good instantaneous current capability, and it needs to be robust since there's not going to be a monitoring system for it. Those requirements play more to the strengths of lead-acid than lithium.
     
  16. Todd Burch

    Todd Burch Electron Pilot

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    Thanks ElSupreme/arg, that's the explanation I was grasping for. Makes sense. (Sounds like Babylonfive was alluding to this as well). Although arg, I'm under the impression that the battery monitoring is 24/7, so it never "powers down". Maybe not?

    So it primarily comes down to:

    1. The cost/complexity of a step-down transformer that can handle the instantaneous loads of auxiliary systems,
    2. Smoother/Less extreme 12v charging rate,
    3. More constant battery voltage,
    4. Chicken and egg.

    These all make sense. Had no idea that stepping down voltage from a few hundred volts to 12 volts was that complex or potentially costly.
     
  17. ElSupreme

    ElSupreme Model S 03182

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    Well there is this:
    And Pb Acid batteries also have a large voltage band (it goes lower voltage when it has less charge), which indicates state of charge pretty easily. Li-Ions requires more state of charge management and watching to get accurate readings. But they just behave slightly differently, and if you stick with Pb Acid you won't likely uncover any weird problems.
     
  18. dsm363

    dsm363 Roadster + Sig Model S

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    The battery monitoring (at least of the big battery pack) is 24 hours. That's why when it is 'asleep' even with the 4.0 update you still drain about 2 miles of range a day (plus I'm sure other systems add to that drain as well).

    - - - Updated - - -

    Thanks for your help. Makes sense and always learn something on here.
     
  19. ElSupreme

    ElSupreme Model S 03182

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    With DC it is. With AC it is simple.

    This is the main reason why AC 'won' the current wars for long distance distribution. They could step up the voltage very high to avoid line losses across large distances (less current to provide the same power) and then step it down in your neighborhood for the 'last mile'.
     
  20. Babylonfive

    Babylonfive Power12

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    I think I'd like to install my own fused 12V outlet as well as an inverter in my frunk.

    One thing to worry about is a 'battery cutoff' so that anything drawing power can't reduce the 12V battery to below its safe minimum capacity (which damages batteries) - such modules use a relay/contactor to separate the load from the battery if the voltage (when used with temperature as a gauge to current capacity level) is too low.

    Otherwise, it's a cool thing to add. For FrunkGating. tm, dsm
     

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