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Firmware feature request - Option for Regen > 60 kW?

Discussion in 'Model S: Driving Dynamics' started by scaesare, Jul 25, 2013.

  1. scaesare

    scaesare Well-Known Member

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    I'm not including the Prius route of "Brakes + Regen using the brake pedal" in my suggestion at the top of this thread for several reasons, some of which have been pointed out:

    1) This would likely take additional R&D and hardware not already present on the Model S

    2) Energy lost to friction is energy not recovered in the pack

    3) I'd rather not incur brake pad wear that's just wasteful

    4) This requires a two-pedal driving effort and foot transition I'm trying to avoid in the non-panic stop scenarios I mentioned at the outset.

    As for the physical limits, I doubt that safely charging the battery at this level is one of them... we already have 90KW Supercharging today, and we know 12KW is coming.

    You might be able to make the case that the rear wheels have less braking potential than the fronts (which is typically the case as inertia pitches a car forward during braking), and you don't want to push the limits of tire adhesion... but with the weight of the car, including battery pack and motor/inverter over the rear axle, it doesn't feel like we are really anywhere close to that with the current 60KW setting...
     
  2. J in MN

    J in MN S60 P12635

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    We've known for more than a year now that Model S does exactly this: A constant deceleration of 0.15 g with an upper limit of 60 kW.

    The upper limit of 60 kW of course implies that above a certain speed and mass combination you will get less than 0.15 g deceleration.
     
  3. Larry Hutchinson

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    If we are voting, put me down for an option for higher regen.
     
  4. scaesare

    scaesare Well-Known Member

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    I actually was going to make this a poll... and then I got busy and when I found a few minutes to post this subject, I forgot to do so. :(
     
  5. dirkhh

    dirkhh Middle-aged Member

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    I wonder if the 60kW is a hardware limit of the inverter/generator.
     
  6. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Doesn't it actually go up to 90 with 60 just being the last printed number?
     
  7. scaesare

    scaesare Well-Known Member

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    I've seen it alleged here that while going down hill you can put the car in reverse and "press the accelerator" in order to get greater regen effect.... if that's true it doesn't seem to be a hard limit. Perhaps not a safe one if there are other engineering limits being exceeded... but I'm not sure wha tthose would be.

    - - - Updated - - -

    You know, I noticed those additional hash marks today, but I'm not sure I've ever seen the "needle" swing to the limit... now I need to go test that.

    If they do, then it's evidence that what I'd really like is something more like 120KW of regen potential...
     
  8. stevezzzz

    stevezzzz R;SigS;P85D;SigX;S90D;XP100D;3LR

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    It sure doesn't feel like a constant 0.15g as the speed decays and the power meter starts to decline from the limit of 60kW of regen. To me it feels like the deceleration increases even as the regen power decreases, down to about 20 mph or so. Have you seen some data that I missed?
     
  9. Vger

    Vger Active Member

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    During my first test drive of Model S at GET AMPED Fremont a year ago, I asked the copilot why the regen was not stronger. At the time it felt less than my Roadster (I am now agreed that it is about the same).

    In addition to all the factors mentioned above, he added that they were trading off efficiency with wear on the rear tires. I had not considered that, but of course it makes sense. All the braking we do without the friction brakes all tugs at only the rear tires. Since they are also the only driving wheels, the rear tires literally get it coming and going.

    I realize that some might consider that a small price to pay, and I agree. I am just reporting that additional design input.
     
  10. PaceyWhitter

    PaceyWhitter Member

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    So it follows that with AWD, you could increase the regen and make it front wheel biased, helping save the rear tires, interesting?
     
  11. scaesare

    scaesare Well-Known Member

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    Sooo..., I tried to get regen up above 60 on my way in this morning, but I didn't have too many opportunities to get up to much of a high speed where I could then just dump the accelerator and see what happens.. .to omany folks behind me. There's a downhill exit ramp I'll see if I can try on later.

    But... I did take a closer look at the regen "gauge" on the speedo. The layout is non-linear, with designations at 15, 30, and 60KW. Interestingly, there are two has marks in between each value, and they are located quite far "off center", at about 80% of the distance to the next value. There's a medium density (same as in between values on the Speedometer side) and a very light one. Here's a pic where you can compare the hash marks on both sides:

    IMG_20130729_082004_866.jpg

    So, it appears that the regen gauge actually ends right about where you'd expect the next numerical designation, which had been doubling up to that point... so would that be 120KW?

    Has anybody ever seen regen above 60 and in to that region?
     
  12. bhzmark

    bhzmark Supporting Member

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    The prius and other toyota hybrids that regen from the brakes is superior for a number of reasons:
    1. It results in much less brake pad wear. Prius owners routinely go more than 100k with no brake serivce at all because most braking is done bythe regen. Only very strong braking actually employs the old fashion brake pads converting motionto heat and brake dust.

    2. You can capture more excess energy as regen instead of wasting more of it as heat and brake dust. This would also prolong the range if more braking enrgy could be captured as regen elec.

    Using the brakes for regen is much better -- if the front wheels are driven.

    The main reason it isnt used in the tesla is because the front wheels are where most of the braking needs to be done but the model s is rear wheel drive with no drive generator unit on the front wheels. Hopefully the model x will use braking regen on the front driven wheels.
     
  13. scaesare

    scaesare Well-Known Member

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    This makes no sense to me. Using regen with NO application of the brakes whatsoever (what the Tesla does currently up to a point, and I am suggesting "more of") can't cause more wear than combining brake action with the regen.

    Now, if you are saying that hitting the brake pedal on a Prius doesn't really apply the brakes initially and uses regen instead, then you are simplt chaging what driver input initiates the regen: lifting off the accelerator or pressing the brake pedal.
     
  14. Ben W

    Ben W P85 #61, Roadster #108

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    I suspect that the ultimate reason for the 60kW limit is because it allows a perfectly consistent experience across the "normal" (10% - 90%) battery range. Already, above 90%, regen tapers off; below 10%, max acceleration tapers off. If regen went up to 120kW, the strength probably would have to start tapering above 70% or 80% charge, which would impart an inconsistent driving experience for "typical" non-techie drivers.

    That said, I would still like the option for stronger regen, even if it's only available at lower charge levels.
     
  15. bhzmark

    bhzmark Supporting Member

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    The latter is correct. Applying any brake pedal but a very hard brake pedal in the prius simply engages the regen to absorb energy to slow the car re charge the battery. Only slamming on the brake pedal hard engages the old fashioned brake pads and calipers. Note the lack of brake dust on the front wheels of any prius.

    A more natural drive input would allow lifting off the accelerator to simply coast with no drag inducing and wasteful regen. Coasting is always more efficient than regen which loses at least 30 percent of the energy. If you previuosly over accelerated had to slow down, getting regen from the brake pedal application and getting old brake pad from hard applications would be most energy efficient.

    Prius also engages a light regen instead true coasting when lifting off the accel which is why many prius drivers switch into neutral or feather the accel to find the neutral coasting level for max mileage. And then can trigger harder regen from brake pedal when really needed.
     
  16. WarpedOne

    WarpedOne Supreme Premier

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    #36 WarpedOne, Jul 30, 2013
    Last edited by a moderator: Mar 13, 2016
    Yes, regen is above 60kW already, exactly where is debatable:)

    Here you can see regen above 60kW:


    I'd say it is about 70kW max.
     
  17. scaesare

    scaesare Well-Known Member

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    #37 scaesare, Jul 30, 2013
    Last edited by a moderator: Mar 13, 2016
    Then you previous points don't make sense to me in supporting the notion that the Prius's method of engaging regen from the brake pedal is "superior for a number of reasons"

    All of your points regarding using regen instead of brake pads hold for the model S as well, it's just triggered as soon as you lift off the accelerator.


    - - - Updated - - -

    Cool vid, thanks for that.... I tried today on a downhill section of freeway merge ramp.. I was able to get up to about 80MPH and then dump the accelerator, and I might have barely gotten a blip above 60KW, but not much if so.

    So while it seems that the car can "spike" above 60, it appears that the car is calibrated to attempt to limit it at 60 during deceleration. In your vid, while it took less than 13 seconds to go from 0-100MPH... it took about 15 seconds to bleed of about 60MPH of speed (down from about 112 to 52 or so)... at which point it appears you re-engaged the accelerator.

    I'd happily take some additional "regen juice". :)

    Thanks for posting that.
     
  18. bhzmark

    bhzmark Supporting Member

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    The prius gives a weak regen on lifting the accel also. But it also offers additional stronger regen when applying the brakes (i.e., when applying the brake pedal to get stronger regen). more braking equals more regen. Only for the strongest braking does it ever use the brake pads.

    The model s doesnt offer any variable regen and what it offers is very weak. Hence the subject of this thread.

    With a frontwheel driveor 4wd tesla hopefullythey will capture that braking energy as regen instead converting it to heat and brake dust.
     
  19. WarpedOne

    WarpedOne Supreme Premier

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    You should postpone your comments until you have driven the Model S.

    There IS variable regen and it IS fairly strong - 60kW is plenty. It is only that some drivers wish for even more power. Some turn it off.
     
  20. brianman

    brianman Burrito Founder

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    It never looks above 63 kW in that video to me.
     

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