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What technically would need to happen to get the P85D the 691 hp?

Discussion in 'Model S: Driving Dynamics' started by rns-e, Nov 1, 2015.

  1. rns-e

    rns-e Member

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    I thought it was time to start a thread about what it takes for a P85D to get the coveted 691 hp from a technical angle. Instead of discussing whether Tesla said one or the other, it would be interesting if we could gather our technical curiosity and look at how P85D can get an energy source that can provide enough energy to the motors so they can deliver 691 hp at the shaft.


    Is it even possible? If so, within what time frame?

    Is a larger battery enough or is the internal resistance to big a hurdle? Should it be a separate battery for the front motor? Should it be a boost system like the one we know from F1?

    Any idea is welcome and please add as much technical argument as possible
     
  2. LargeHamCollider

    LargeHamCollider Battery cells != scalable

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    Put 200lb of high C-rate LFP batteries in it. Solved.
     
  3. kennybobby

    kennybobby Member

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    Quick and dirty, back of the envelope answer: You would need a current draw of 1500 to 1700 amps from the 7104-cell pack at ~6000rpm, 51mph.

    Show your work on how to get 691hp at the motor shaft from an awd tesla.

    What do we know about the motors?

    from post by Dennis
    Max hp on the rear engine is at 5950 rpm and 6100 rpm for the smaller front motor on the p85d

    from the tesla website for the performance model (latest data P90D?)
    front 193 kW (259hp)max at 6100 rpm, max speed 18000 rpm, max torque 330 mN
    rear 350 kW (469hp)max at 5950 rpm max speed 16000 rpm, max torque 600 mN

    motor rating 320V
    overall final drive ratio: 9.73:1


    from R&T article for P85D

    front 221 hp 165 kW, 244 ft-lbs
    rear 470 hp 351 kW, 443 ft-lbs
    total 691 hp 516 kW, 687 ft-lbs

    Assume the front and rear motor power peaks are at the same rpm for the 85 and 90 based upon post by sillydriver about the numbers:
    "Next, the manual says the max ‘net power’ for the ‘performance’ rear motor is 350Kw @ 5950RPM (divide by .7457 to get 469HP), and the front ‘performance’ motor is 193Kw @ 6100RPM (259HP). This disagrees with the 503HP for the rear now on the website, but DOES agree with the 259HP for the front. Discrepancy unexplained. Note that the ~6000RPM for peak powers in each = 51MPH. It sounds like the MS, with its single gear, begins to fall out of its power band over 51MPH, but how this interacts with max battery power is unclear."


    So use 6000 rpm for both front and rear to get the 691 hp (516kW)

    for 516 kW output, and 95% efficiency of inverter => 543 kW input power

    543kW/360V = 1509 Amps (using 10% sag on full pack voltage)
    543kW/320V = 1697 Amps (using motor rating voltage)

    Extra Credit:
    Use cell test data graph from wk057 to account for cell internal resistance.
     
  4. sillydriver

    sillydriver Member

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    #4 sillydriver, Nov 1, 2015
    Last edited: Nov 1, 2015
    Flux capacitors!

    But seriously folks, about 100 pounds of high-quality supercapacitors. If we want 762 HP but got 463 HP that's a 300 HP diff or 223 Kw, which we want for about 10s, so thats 2230 Kw-seconds (kilojoules) or 619 watt-hours. Wikipedia says commercial supercapacitors store up to 15 watt-hours/Kg. So 619/15 = 41 Kg = about 100 pounds. We can use the frunk.

    I should add that supercapacitors fit the bill because they have a very high POWER density, although a lower energy density than batteries. They can discharge over 10 Kw/Kg, so those 41 Kg of capacitors can blast out at least 400 Kw = 540 HP on top of what the battery provides.

    Corrected some first pass mistakes -- too big a cocktail.
     
  5. wk057

    wk057 Senior Tinkerer

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    Fun idea.

    Unfortunately the 85 kWh pack (and evidently the 90 kWh pack) is just not capable of supplying the power needed to even get 691 HP worth of power on the *input* side. Internal resistance of the cells would require nearly a 2000A draw at 100% SoC to get the required ~515kW, and even more to get 515kW output at the motor shaft assuming the inverters can handle it. Unfortunately that's 27A per cell, and the cell level fuses pop at ~25A. So, that's out. (Oh, and this current is nearly short circuit current for these cells... and they um, tend to explode after being shorted for a little bit...)

    Supercapacitors could potentially work for 1/4 mile drags and such I guess. The problem would be safely discharging them and charging them in parallel with the current pack without blowing things up. Would require some beefy power electronics, since once they're drained they'll accept as much power as you can give, which would basically short the main pack without smart systems in between.

    In short, I think the best bet would just be more batteries or different batteries with lower internal resistance.
     
  6. sillydriver

    sillydriver Member

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    To continue on this subject, the best supercapacitor I could find was the Skeleton SCHE3500 and when I looked further I found it discussed on this forum in a thread that you, @kennybobby, participated in

    How long until supercapacitors overtake batteries in EVs?

    Small world.

    - - - Updated - - -

    A ten second jolt is all we need, since the car would be doing 130+ at the end of it. Oh and also, any longer and smoke would be pouring out of the undercooled motors.
     
  7. stopcrazypp

    stopcrazypp Well-Known Member

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  8. Saghost

    Saghost Active Member

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    Hmm. At 50C, 515 kW would need to come from a ~11 kWh or larger pack. Just how much worse is the energy density?

    One obvious option would be something similar to the Enginer approach for making PHEVs out of hybrids - put the motors onto a high voltage circuit connected to the new 10-15 kWh "power" battery, then flow power into it from the larger "energy" pack using a 30-40 kW DC-DC converter (like the one between the batteries and motors on all Prius type hybrids except the very first generation.) Depending on the inverter design limits, you might be able to get significantly more power than the promised 762, because the motor side could run at substantially higher voltages if it was helpful and the inverters can take it (the Prius uses 650V motors with a 220V battery pack...)

    The easiest way to integrate this into the car would be a replacement main pack that's a hybrid with both sets and the converter built in to it. If the volume is adequate, you could have take a "70D" style main pack of 14 modules, and use the double height space at the front for the "power" battery pack and DC converters. You'd probably have a total of about 80 kWh onboard, but you'd get full performance until right at the very end (and introduce a new limitation for sustained power usages beyond the converter capacity for extended periods.)

    Another simple option would be a much more expensive, lower capacity pack made up purely of "power" cells. I haven't looked at numbers to try to figure out how much capacity you'd have, or how much more expensive it'd be.
    Walter
     
  9. stopcrazypp

    stopcrazypp Well-Known Member

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    Looking at the official spec sheet, the 50C is probably an instantaneous peak. The actual 10 second peak seems to be 35.7C (500A from a 14Ah cell). Energy density is 90.6Wh/kg.
    Accounting for the sag by using the power spec instead of C-rate, this cell has a 1.4kW 10 second discharge, which means 368 cells for 515kW. 368*3.3V*14Ah= 17kWh. The cells alone would weigh 187.68 kg / 414 lbs.
    https://www.buya123products.com/uploads/vipcase/95d458c5c49e552539acb37507b49b09.pdf
     
  10. wk057

    wk057 Senior Tinkerer

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    Why do you assume these "50C" batteries have no voltage drop? ;)
     
  11. stopcrazypp

    stopcrazypp Well-Known Member

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    If you are talking about my comment, see my latest one. I use the power spec of the cell (which is given in watts) instead of the C rating in order to eliminate the voltage sag issue.

    Long story short, I got 368 cells for 515kW and a 17kWh pack with cell weight of 188kg / 414lbs.
     
  12. EarlyAdopter

    EarlyAdopter Active Member

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    Time, you say?

    bttf-005a.jpg
     
  13. rns-e

    rns-e Member

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    That would just be to funny, having 'hotspots' like Mariocart where you can get an extra boost. May just be to easy for the police to cite you for speeding
     
  14. Saghost

    Saghost Active Member

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    #14 Saghost, Nov 2, 2015
    Last edited: Nov 2, 2015
    Ugh.

    Between the new prismatic shape not fitting the packaging well, needing a new cooling solution, and already being a third of the pack weight, the hybrid isn't looking too promising.

    I think the high power cell option is doable, though.

    Using Samsung 25R cells (2500mAh, rated for 25 A continuous) 18650 cells instead of the 3.1/3.4Ah cells they use now would yield a pack of ~65 kWh capacity - but capable of 1850A continuous discharge - probably somewhere near 600 kW/800 horsepower with voltage drop.

    Somewhere in the middle, there's the newer 30Q cell - 3 Ah, with 20 A continuous, about 80 kWh and 1500 A continuous.

    Presumably they could incorporate those cells (or others of similar chemistry) into the existing production process with no changes (except possibly heavier cell fuses.)

    I'm not positive, but I think faster Supercharging would be a side benefit...
    Walter
     
  15. freeewilly

    freeewilly Member

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    Just a comparison, I pulled these specs from official Formula E website

    • Maximum power: 200kw, equivalent to 270bhp
    • Race mode (power-saving): 150kw, equivalent to 202.5bhp
    • FanBoost (race-only): Temporarily increases max power from 150kw to 180kw. (Increase of 30kw / 40.5bhp)
    • Acceleration: 0 - 100 km/h (0-62mph) in 3 secs
    • Maximum speed: 225 km/h (140mph)
     
  16. Rashomon

    Rashomon Member

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    #16 Rashomon, Nov 2, 2015
    Last edited: Nov 2, 2015
    If someone wanted to build a Tesla Model S that would run in the 9 second quarter mile range or quicker, you could build a pretty light pack from A123's F1 cells, which have been used in race vehicles from the Killacycle drag bike to Formula 1 KERS systems to the latest Porsche hybrid Le Mans winner. See: A123 AHR18700 F1 KERS Cells | eBaracus. These cells are capable of 200C when warmed to operating temperature. If you gutted a S to take weight out and went with a full custom pack, you could probably lighten the vehicle by more than 1000 to 1500 pounds while increasing power. Fun for drag strip use only, and a project that probably someone will do when used S's get cheaper.
     
  17. sillydriver

    sillydriver Member

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    #17 sillydriver, Nov 2, 2015
    Last edited: Nov 3, 2015
    I will make only this one attempt at beating a dead horse, but I still think supercapacitors have good characteristics as a performance booster pack. The idea is a special high performance model, priced several 10s of thousands higher. Rather than compromise using special batteries with a higher power density but lower energy density, which compromises range, there would be division of labor between components specialized for energy density (batteries) and power density (capacitors), retaining long range and adding high intermittent power without too much weight. Yes, the interplay of two power sources requires a lot of power electronics intelligence, but so does the car now, with two motors, alternators, a wide operating range, regen, etc. The combination would not only offer drag strip blasts, but also vastly stronger regenerative braking, probably up to 1 g (which would need to be integrated with the pedal, which it is not now). Yes, energy would need to be moved back and forth between the capacitors and battery pack intelligently, but at a slower rate (power level) reducing thermal load on the pack and its long-run degradation.

    The objective would not just be occasional acceleration runs, but track day performance with the capacitors supporting braking into, and acceleration out of corners. A further advantage of capacitors here is that they are good for an extremely large number of charge-discharge cycles relative to batteries. Using batteries to store and release energy in each corner might work for a KERS system lasting a race or two, but not for the life of the car. With this system, the 'Ring may become possible without reversion to limp-home mode, although enhanced motor cooling would probably be required.

    This idea may have been born from a large martini last night (see earlier post) but it's really growing on me.

    Editing: I originally said capacitors in the frunk but on reflection, that would compromise the front crumple zone and crash safety. They should probably go under the rear floor where the jump seats would fold up.
     
  18. kennybobby

    kennybobby Member

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    @rns-e, hoping to answer your question...

    using a linear extrapolation of some of the data from the TM website compared to a simple physics model i estimate that your 5000 lb model S with a true combined max 691 motor shaft power *battery limited could do a 0-60 mph run at no quicker than 2.5 to 2.6 seconds. When Moron Trend tests it they will report 2.3 to 2.4 with the 1ft rollout.

    The assumption is that the losses due to friction, aero, energy conversion, etc will follow a linear loss trend from the P85D(463hp) and Ludi(532hp) listed in the specs. Those losses will not be linear and the run time will actually be slower than my estimate, but nevertheless, it would still make your nose bleed.
     
  19. lolachampcar

    lolachampcar Active Member

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    SuperCaps are neat but not applicable to this situation.

    Think of the battery and SCs as separate glasses of water.
    In the case of the battery, full is the top of the 8 oz glass of water and empty is the 6 oz mark.... This equates roughly to voltage and you have 2 oz of water available to you. (I know, I am interchanging voltage/potential and energy but I need this approach to make the visual work).
    In the case of the SC, the glass is much thinner with the top of your glass being 2 oz and the bottom is 0.

    You can now suck water from the both glasses at the same time but you have to stop pulling water when you get to the "battery" glass 6 oz point as this is battery empty. This leaves way too much energy in the SCs.

    Put differently, the battery works from roughly 400 vdc down to 300 vdc (or something like this) while the SCs full capacity is only realized when you go from 400 vdc to zero. You would need a lot more SC capacity to accomplish the goal; likely four or five times more.

    I ran into this problem with a friend that wanted to use the Tesla drive unit in a project using SCs to provide a very low impedance source for 1/4 mile runs.
     
  20. sillydriver

    sillydriver Member

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    #20 sillydriver, Nov 5, 2015
    Last edited: Nov 5, 2015
    Yes, this issue occurred to me! I figured it might need 2x capacitance so the voltage only falls by half discharging the required energy, and it might need a DC to DC converter to keep the voltage where it's useful. You certainly raise a good point.
     

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