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yet another regen thread

Discussion in 'Roadster: Technical' started by AnOutsider, Oct 26, 2011.

  1. AnOutsider

    AnOutsider S532 # XS27

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    This may have been answered elsewhere, but how much does regenerative braking really add back into the battery?
     
  2. Doug_G

    Doug_G Lead Moderator

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    In my typical usage it is on the order of 20%, according to the VDS.
     
  3. AnOutsider

    AnOutsider S532 # XS27

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    so, just to understand, let's say you got 200 miles on a charge (just saying). Do you really get 160 miles and the regenned miles (20% = 40) make up the rest? Or do you get 200 and then an extra 40 on top from regen?
     
  4. dsm363

    dsm363 Roadster + Sig Model S

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    I believe it's already figured into your estimated range. It has a bigger effect in the city of course than the highway.
     
  5. NielsChr

    NielsChr Member

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    #5 NielsChr, Nov 2, 2011
    Last edited: Nov 2, 2011
    It really depent about how you are breaking....if you can forsee the breaking you can awoid using the break pedal and only relay on the motor break (regen).
    But if you are forced to break fast you need to use the break pedal (energy lost due to heat).

    If you teoretically can drive 200 miles (without breaking at all) - then lets prettend you are driving and some times need to slow down or stop, then you may only gain a range of 160 miles (without any form of regen)
    (my own ICE goes down from 12 km/l at highway to 10 km/l in city - this is a reduction by aprox 15% due to more offen breaking in the city)

    Tesla is regenerating kinnetic energy using the motor break (you are lifting the gas pedal, while the motor is runing in reverse as a generator) . I asume that it would be posible to regen up to 70% of the energy if ONLY using the motor breake (the last 30% will still be lost as heat).
    if you are in need of breaking faster than the motor break is cable -you will need to use the break pedal together with the motor break - then maybe only 10% of the kennetic energy can be regenerated, wasting 90% of the break energy into heat (mostly in the break discs instead)
    Doug_G have mention in above post that a typically regen value of 20% is posible in daiyly use.

    I your math this will be:
    160 miles + regenerated energy (20% of the 40 miles loss) = 168 miles total range.

    How much your energy regen rate will be, really depent about how fast you need to break. If you can forsee the trafic in good time, and having the time to only use the motor break, you might be able to regen more than 20%. if you are waiting until last chance to break (using the break pedal) you may loss more and regen less than 20%. - what the practically max value is for regen, I really can not say, since I do not have a Tesla my self - mayby Doug_G knows ?

    fast/offen breaking = less range
    slow breaking = more regen, less loss
    not breaking at all = max range


    BTW: Tesla Model S 160, 240 and 300 miles range has already pre calcuated som breaking in the range - since this is messured as a standard driving pattern - making it posible to compare cars, Model S and roadster are using same pattern as ordenary ICE's cars
     
  6. AnOutsider

    AnOutsider S532 # XS27

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    Ok, I'm actually a bit more confused after that than I was before. Put simply, the ideal range quoted, is that WITH or WITHOUT regenerative braking? It sounds like it's WITHOUT, so if I do a lot of regen braking, then I can theoretically get MORE miles.

    If so, that's great, as I'm a 1-pedal driver when the conditions permit.
     
  7. William13

    William13 Member

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    The ideal range is supposed to be using the EPA standard course which does involve coming to a stop many times but I don't know if the breaking is at a proscribed rate. The acceleration is proscribed. Tesla historically has been accurate to conservative versus others(Leaf and Volt).
     
  8. dmckinstry

    dmckinstry Model S - U.S. P - #1649

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    More than you get by using the brakes, but not as much as if you don't slow except for your final stop. That is, never letting the brake pads touch until the end of your trip.
     
  9. speedy99

    speedy99 2.5 Roadster

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    I use my Roadster as a daily commuter, mostly freeway. My VDS shows 10% regen. As an aside, in my experience driving with a light foot (and at/under freeway speed limits) will get more range than aggressive regen attempts, though I rarely have to brake.
     
  10. Doug_G

    Doug_G Lead Moderator

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    I don't really know; obviously you could fudge a really high number by accelerating and decelerating continuously. I suspect my regen ratio is fairly high, as I don't drive on highways during my daily commute.

    The point of regen is that it recovers most of the energy when you're forced to stop or slow the car. It doesn't extend your range so much as recover range that you would have otherwise burned off by braking. If you could drive continuously at a constant speed, without ever braking, then you would go farther.
     
  11. AnOutsider

    AnOutsider S532 # XS27

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    Is the energy lost by braking, or by the acceleration after the braking?
     
  12. Doug_G

    Doug_G Lead Moderator

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    Let's assume we're starting at a fixed speed, slowing to a stop, and accelerating back up to the original speed again. Let's look at how much energy is lost compared to not stopping, and I mean "lost" in the thermodynamic sense: how much useful energy is converted to unusable waste heat. For the purposes of this discussion I am arbitrarily assuming the drive train is 90% efficient both in acceleration and deceleration.

    If you have to use the friction brakes, then 100% of the kinetic energy is lost while braking. But of course you have to replace that energy when you accelerate back up to speed again, and with 90% efficiency you lose another 10% while accelerating. So you lose 110% of the original kinetic energy of the vehicle.

    If you use regen, then some of the energy is "spent" decelerating, and some of the energy is "spent" re-accelerating back up to speed again. You lose 10% of the original kinetic energy slowing down, and another 10% of the original kinetic energy accelerating back up to speed again. So the total loss is on the order of 20%.

    In this example, regenerative braking is 550% more efficient than using friction brakes. Energy is lost both during acceleration and deceleration, but in different amounts in the two scenarios.
     
  13. vfx

    vfx Well-Known Member

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    Don't forget the wear on tire from regen.
     
  14. Doug_G

    Doug_G Lead Moderator

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    Sure, but I'm pretty sure my tire wear is predominately due to pressing the pedal down, rather than lifting it up.
     
  15. vfx

    vfx Well-Known Member

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    Yah... I'm not so sure...
     
  16. Tesla 940

    Tesla 940 Member

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    Further Regen question

    When I let off that little pedal on the right I can watched my total kwh used decline. However, on several occasions on long decents I have witnessed a total kwh used decrease in excess of 500 wh (over 2 Ideal Miles) yet the Ideal Miles indicator does not increase. Any of you experience this and why doesn't the Ideal Miles increase?
     
  17. Doug_G

    Doug_G Lead Moderator

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    I suspect you're into the "error bars" for the battery capacity measurement. I've "lost" two km just backing out of the garage.
     
  18. Mycroft

    Mycroft Life happens

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    I don't know if it's been mentioned before, but I like the safety feature of regen braking. In an "emergency" braking situation, your car has already begun braking before your foot even has a chance to hit the brake pedal. The heavier the regen, the better!
     
  19. Tesla 940

    Tesla 940 Member

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    I don't know what you mean by "error bars" please explain. The two or three times I've done this decent I had already driven ~75 miles so the battery had a little more than half the charge remaining.

    Thanks
     
  20. Doug_G

    Doug_G Lead Moderator

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    In science, any time you make a measurement you also need to record the estimated errors. When you plot the data you usually include "error bars" for each data point on the graph, showing the likely range of values including the measurement error.

    attachment.php?attachmentid=3377&d=1320876821.png

    Basically I'm saying that 2 miles is probably on the order of noise in the measurement.
     

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