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P85D Power draw numbers do not add up...

Discussion in 'Model S: Battery & Charging' started by wk057, Oct 12, 2014.

  1. wk057

    wk057 Senior Tinkerer

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    So... 691HP. That's 515,278.6 watts assuming 100% efficiency.

    Assuming a fully charged pack, and no voltage sag (impossible!), that would be roughly 1300A. Realistically, this would be closer to 1500A.

    Either way, the main pack fuse on the 85kWh pack is 630A.

    2014-08-27%2018.14.25-1280.jpg

    Looking up this part number, this is a fast acting fuse. So, I would think that much over 630A for multiple seconds would pop this fuse.

    630A at nominal voltage of 355V is about 225kW, even less than the current P85 draw.

    Originally spec'd P85 was 416HP. So, 310kW. That's already 85kW over this fuse's rating.

    The P85D, that's 290kW over... more than double even under ideal conditions.

    Now, in my testing for my off-grid solar project using the Tesla pack (see thread on this forum for details), I've run this fuse at 410A. It gets *hot* quick. FLIR cam registered it at over 250 degrees fahrenheit. Not enough to pop it, but I'm pretty certain that over 630A would do it.


    So, this doesn't add up, for the P85 or the P85D especially. Reaching for explanations...

    One might be that the extra juice comes from the precharge capacitors. From my pack tear down, I know there is a precharge resistor, which means there are capacitors somewhere in the inverter/drive unit. Would these be enough to make up for the full power acceleration? Math time...

    I've done a top speed run in my P85....... shhhh.... and a manual count was at least 7 seconds of the bar around 320kW. That means 2240000 watt seconds, or about 622Wh for that period. Assuming the pack is delivering the max fuse rating, or around 225kW, that leaves 185Wh that is needed to make up the difference.

    OK, so maybe there are 185Wh worth of capacitors somewhere? That's 666,000 joules. That would be about 8.33 farads at 400V... 3333 coulombs.

    The best ultracapacitors I could find setup for this would weigh over 100 lbs, and cost somewhere in the $10,000 range... so, this is doubtful.



    So... how are they pulling this power out of the pack without popping the main pack fuse? :cursing:
     
  2. TEG

    TEG TMC Moderator

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    That fuse shows 630A @ ~700V. Model S pack it about half that voltage, so does the fuse allow higher amps at lower volts?
     
  3. miimura

    miimura Active Member

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    I thought you previously said that the fuse has an accessible cover. If that is the case, it is possible that S85, P85, P85D use different fuses, even if the cells and wiring inside are the same.
     
  4. mitch672

    mitch672 Active Member

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    This. And, they could come up with another Rev of the packs, so far we've seen A, B, C and D revisions.
    whose to say they haven't designed a pack with larger internal wiring & fusing? no one has seen one in the wild up close yet...
     
  5. Bighorn

    Bighorn Member

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    Do we know that there is a time when both motors are creating peak HP simultaneously? Is it even possible? Is 691 HP a marketing tactic rather than a real world figure? Do the three modes that Elon mentioned that included "Insane" relate to how the motors can draw their power, similar to a launch mode that is only allowed a certain number of times? I'm not sure we'll have solid answers until the December builds start getting tested.
     
  6. wk057

    wk057 Senior Tinkerer

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    It is accessible from the top of the pack, so, only while not installed. Guess that makes sense at production perhaps, but, would make battery swaps complicated...
     
  7. Sad

    Sad Member

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    Is this 691 bhp figure mentioned anywhere else than the Design Studio? I find it weird that neither Elon or the blog mentions this number, which would kind of be something to brag about. I'm leaning towards marketing tactic! I desperately hope I am wrong.
     
  8. pgiralt

    pgiralt Active Member

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    Only explanation I ca think of is if there were two of these fuses in parallel somewhere, but you would have found them in your breakdown of the pack.
     
  9. wk057

    wk057 Senior Tinkerer

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    Nope, definitely only the one. Broke the pack down to the aluminum casing...
     
  10. tom66

    tom66 Member

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    #10 tom66, Oct 13, 2014
    Last edited: Oct 13, 2014
    No, the fuse doesn't "care" what the voltage of the pack is. The 700V rating is a safe interrupt rating limit. For example at 1000V fault the fuse could arc over and not be a fuse any more - which would be bad.

    The 630A rating for that fuse is nominal, and likely the continuous operating rating (or up to 30 minutes.) We know the P85 limits to 160kW after a lot of excursion, which would put it within that fuse rating.

    P85D (and all other -D models) will likely use a different battery pack, maybe an -E variant, to get the additional power. And they will have to get the power to the front motor. They could use the routing system currently used for the HVAC/DC-DC (which is something like 15kW and so has "thin" cables), but that would have to be increased in size considerably, and may not fit through the current channel requiring bodywork changes (which is always expensive -- additional crash testing may be necessary.) So maybe they now have a front battery connector (which internally is wired to the same battery, but cabling length is reduced.)
     
  11. yobigd20

    yobigd20 Well-Known Member

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    nobody here yet knows if they made battery pack changes to achieve the required power draw. what battery pack "version" is that fuse from? A/B? you know they are up to D packs and nobody yet knows if the new dual motor battery packs have changes in them (I wouldn't be surprised if they do) and they are now on something like 'E' or 'F' packs....
     
  12. Todd Burch

    Todd Burch Electron Pilot

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    The D likely uses a slightly different pack.

    As for the numbers being a marketing ploy: Guys, we're talking about accelerating a nearly 5,000 lb sedan to 60 mph in 3.2 seconds, faster than a 700+ hp Hellcat. Do you really think that 691 hp number is far from reality?
     
  13. wk057

    wk057 Senior Tinkerer

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    The fuse was from a D pack.
     
  14. JRP3

    JRP3 Hyperactive Member

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    Don't know if this would explain it but acceleration requires torque, to produce torque the motor requires current, and until the motor approaches base speed the current from the inverter to the motor should be higher than the current from the pack, while the voltage to the motor from the inverter is lower than the pack.
     
  15. Johan

    Johan Took a TSLA bear test. Came back negative.

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    I agree with tom66 in that likely both the 60D, 85D and P85D will have two high voltage connectors to the battery: one aft and one rear so to speak. If the horsepower rating was just a marketing ploy, as in "the combined rated horsepower of the two motors is 691 but they can never output that much in reality in unison since there is not enough current available", then there's no way the P85D could do 0-100 in 3.2 seconds as opposed to about 4.0 for the P85. That kind of performance boost could never come from just having torque on both axes and different gearing but not higher total output. Unless the P85 could already do it but was that much grip limited, which it is likely not.
     
  16. Sad

    Sad Member

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    As a matter of fact, 691 bhp is far from necessary to achieve that, as traction is a big factor, see GT-R to see how a relatively heavy car manages 3.0 secs with 500 bhp. For the P85D, there's the added advantage that maximum torque can be applied from the start, more torque in total can be produced compared to the P85 (thanks to the two engines) and more torque can effectively be supported by the tires thanks to the AWD. And let's keep in mind that for the P85, peak power (so ~400 bhp) is not achieved before 40 mph.
     
  17. scaesare

    scaesare Active Member

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    Well, the GT-R weighs ~3,800lbs, as opposed to The D's ~5,000lbs.

    According to one calculator you need all of that HP to get that 3.2 sec 0-60 time... (I used "dual clutch" as the trans type, as that seemed to be the closest thing to no shifts at all):

    HP.PNG

    And as for the original question... I agree that the pack is likely a revised model...I suspect there are a few places where the need to accomodate ~50% more power draw has necessitated changes...
     
  18. tom66

    tom66 Member

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    #18 tom66, Oct 13, 2014
    Last edited: Oct 13, 2014
    Secondary school physics:-

    Acceleration = 3.2s / 60mph = 8.382 ms^2
    Force = mass * acceleration = 2270 kg * 8.382ms^2 = 19.02 kN
    Displacement = 0.5vt (initial velocity zero) = 42.92 metres
    Power = Fd/T ((force * displacement) / time) = 255kW



    So MINIMUM required power (assuming 100% could be put on the road) = 342 hp

    This ignores air resistance and tyre slippage.
     
  19. Saghost

    Saghost Active Member

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    It also assumes you can apply the same amount of power during the entire acceleration event - which is quite impossible since that would result in infinite acceleration/torque during the first moment of movement.

    Actually, the Model S will do most of the 0-60 run in a constant torque situation with power increasing linearly throughout (In the existing single motor cars, the S60 and P85 went power limited (first achieved peak power) at about 43 mph, while the S85 was about 55 mph.)

    If that's the case, the peak power would be twice the average power - 510 kW/684 HP. It actually probably is slightly less than that, because it likely hits peak power 2/3 the way through the run and then the torque drops off - but you need integrals to model that correctly instead of simple math. :)

    I'm operating under the assumption (as I stated in another thread) that Tesla has made a revised high power draw pack version (call it an 85 kWh "E" pack until we know better) specifically for the P85D. It'll be interesting to see if P85s get it too - available information suggests that they are currently limited by pack dynamics rather than motor/inverter ones.
    Walter
     
  20. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    A couple of small corrections:

    1. You have calculated the average power needed assuming constant acceleration with no power limit. In fact, power is the product of force and speed. To maintain the same acceleration, the power goes up with speed. 60 mph is 26.8 m/s, so the power needed at 60 mph is 19.2 kN*26.8ms = 515 kW or 690 hp. Very interesting, is the power of the P85D just enough to maintain constant acceleration to 60 mph?
    2. Other losses include DC-DC conversion from the battery to the motor and resistive losses in the motor, etc, especially at low speed.
     

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