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Question on Regenerative braking

Discussion in 'Model S' started by xrayvsn, Jul 10, 2017.

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  1. xrayvsn

    xrayvsn Member

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    First off, I am a big fan of regenerative braking, absolutely loving "one pedal driving."

    I was wondering however if during periods of non-braking, does it actually create more electricity requirement? If I let go of the accelerator in an ICE vehicle, there only rolling resistance of the tire to the ground which allows one to coast and thus increase fuel economy. In the same setting, if the go pedal is not minimally pressed, there is a large resistance from the motors which are in regenerative braking mode (which I take to mean that there is a large resistance that requires some use of battery to overcome).

    In a long distance highway trip with little braking I would think the benefits of regenerative braking would pale in comparison to the loss in ability to coast.

    Curious if others have a better explanation.
     
  2. Acho

    Acho Member

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    I might be wrong but how I understand it actuality generates energy.

    Some one with more knowledge may correct me or explain it better
     
  3. croman

    croman Active Member

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    the inverter is very efficient. While there is a loss of energy anytime you do a conversation, coasting with an ICE engine isn't the same. The ICE still runs (idles) and some energy is converted using an inefficient system (alternator) to charge and condition the 12V. Regen is super efficient and with an EV you could coast and get a bit more but often, especially on a highway, you don't really take your foot off the go pedal so much as feather it (lessen energy input rather than remove it and then press it again).
     
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  4. xrayvsn

    xrayvsn Member

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    I agree, during braking it definitely puts energy back into the battery, my question is mainly does it use more energy when no braking but just cruising along highway on long road trip (i.e. are you using more electricity constantly to overcome the regenerative effect of the motor when you do not want to slow down but maintain same speed).
     
  5. derekmw

    derekmw Member

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    If you completely let go of the 'throttle' pedal, yes, you will be slowing down and doing regen. But that is not an apples to apples comparison to an ICE. When you're 'coasting' in an ICE, it's usually because you're probably either going down a slight incline and you're trying to maintain a given speed, or you're wanting to gradually slow down in anticipation of something ahead. If you're trying to maintain speed, then you would probably feather the 'throttle' to where you're right at the 0 point of not generating or consuming battery (You can see this on the power graph on the instrument cluster.). For the latter, you would just wait slightly longer before completely letting off the 'throttle' to use regen braking to slow down as needed.
     
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  6. John5396

    John5396 Member

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    there is not a separate regenerative braking system to draw power when not actively braking. Regenerative braking is accomplished by reversing the operation of the electric drive motors to use them as generators.

    So in answer to yopur question, No there is no additional losses from regenerative braking when coasting or accelerating.
     
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  7. D.E.

    D.E. Member

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    I must be missing something here. If you don't release the accelerator pedal enough to actively slow the car, you are essentially coasting. It regenerates when you use the motors to slow the car. You'd normally use the brakes so would lose that kinetic energy as heat. It is not an all or nothing control. If you are traveling at say 45 mph, you only need add sufficient energy to keep you at that speed. If you are going down hill, you won't need to add energy so you are essentially coasting.

    If you are talking about accelerating to say 60 then allowing the car to coast to a slower speed then repeating, then you are not saving energy. Air resistance goes up at the square of the speed so you'll use more energy when traveling at any speed above the final average. You are better off constantly maintaining the average speed. The energy you save in the "coast down "is less than that spent in building the excess speed to allow the coast down.

    The assumption that there is only rolling resistance as you coast is not accurate, there is wind resistance. At highway speed the wind resistance is the major factor.

    You can allow wind resistance and rolling resistance to slow the car. Just don't lift the right foot enough to start regenerative braking. Basically you only use regeneration when you'd otherwise be using the brakes, so it's all good.

    If I haven't addressed exactly what you meant to ask, please have another go at your question and I'll take another shot at answering. And for anyone else, if I ever post anything that is inaccurate or if there is a better way to explain, please jump right in.
     
  8. BrettS

    BrettS Member

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    I think this is the flaw in your logic. Regenerative breaking means this the car is, in fact, regenerating power. Essentially, instead of providing power to the motors to create motion it reverses the system and uses the motors as generators to recharge the battery. The braking effect comes from the fact that the motors are not easy to turn as they generate power, so they slow the car down.

    As someone said earlier, the system is pretty inefficient due to losses to heat and friction and such, so you you slow down from say, 60 mph to 0 mph you would generate about 1/3 of the electricity necessary to get you from 0 mph back up to 60 mph, so it’s much more efficient to drive at a steady speed rather than continually stop and start. However, when you have to stop anyway regenerative braking does allow you to recover some of the power that would have otherwise been lost.
     
  9. SabrToothSqrl

    SabrToothSqrl Active Member

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    The "most efficient" way to stop is to put the car in N, and coast until it stops.

    That ensures that 100% of the available energy is used to keep the car moving.

    However, in the real world, we have stop signs, and cars in our way.

    The heavy and aerodynamic S loves to coast... but all the energy savings in the world isn't worth crashing into something.

    You can also do with with just your foot, by trying to keep the energy meter/graph thing showing zero kW. At that point you're essentially coasting.
     
  10. MarcusMaximus

    MarcusMaximus Member

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    I think others explained the answer to your main question well enough, but I'd note that the part about stepping off the accelerator in an ICE car is incorrect. If you step off the accelerator in an ICE car, you'll get engine braking, with the internal resistance of the engine in gear slowing the car. In order to coast, the car must be put in neutral(or, for manual shift, you have to step on the clutch) to disengage the engine from the drive axle(s).

    Similarly, if you REALLY want to coast in the MS for some reason, you can always shift to neutral(I believe that's long press on the park button).
     
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  11. BrettS

    BrettS Member

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    Nooooo... don’t do that while you’re driving. A long press of the park button engages the emergency brake:) To get into neutral just gently push up or down on the gear shift lever.
     
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  12. David29

    David29 Member

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    The other loss of energy when you have your foot off the accelerator in an ICE car is the transmission, especially if it is an automatic. Automatic transmissions consume some of the power that would otherwise be transmitted between engine and wheels. How much depends upon the type of transmission and what gear you are in.

    In the old days, the automatics were called "slush" drives for a reason....!
     
  13. MarcusMaximus

    MarcusMaximus Member

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    Ah, that's the one. Mix those up all the time ;)
     
  14. mach.89

    mach.89 Member

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    Not to hijack the OP thread, but been always wanting to pose this question and this seems like a fairly appropriate place to bring it up.

    With regenerative braking in my Model S, I've always wondered when coming to a stop from say 30 mph if it's more efficient (for the regen system) to time it so I let off the accelerator completely for max regen to slow me to 5mph (where regen cuts out), or feather it to have an almost coasting effect/longer time in the green energy graph?

    Basically, I'm asking: is there any difference to having a short duration of max regen or a longer duration of say half regen, all other things being equal.
     
  15. animorph

    animorph Member

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    As far as coasting, watch your energy meter. On flat highways I usually have a choice of more or less power, but I'm always using energy unless I'm trying to stop. On a good downhill stretch you can get some good coasting or regenerative breaking. In that case I'm simply adjusting the accelerator to maintain my desired speed, so I'm not really trying to coast, it just happens sometimes. Since regeneration is well controlled with the accelerator (not just on or off), a little bit of regeneration when you're trying to coast is not too inefficient. The amount of power involved is pretty small in that case.

    From some of the casual numbers reported on TMC it has seemed like regenerative braking recovers around 60% of the available energy. So yes, coasting to a stop will be better, because it uses less energy than staying on the accelerator and recovering only 60% of that extra energy used over coasting.
     
  16. Tam

    Tam Active Member

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    When I go downhill in Tejon pass in California, the battery gauge would stop going down even the car is still driving on autopilot at 65 MPH.

    After a while, the battery gauge would suddenly add incrementally 3 rated miles once if I autopilot at 65 MPH and twice (for a total of 6 more rated miles) if I autopilot slower at 55 MPH.



    So not using physical brake does generate more electricity than the car could consume in this case.
     
  17. ggr

    ggr Roadster R80 537, SigS P85 29

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    I don't know where you get that 1/3rd from. It's much more efficient than that. 80-90% for the round trip from wheels to battery and back to wheels is a commonly quoted figure. (Edit: just to be technical, don't forget that the car actually made progress both while slowing down and speeding up again. Some people just talk about the gross energy used without taking into account this forward progress)

    To @xrayvsn, the meter on the dashboard shows you how much current is being drawn; if you're just driving at a constant speed on the flat, it will be a bit orange, but if you start a gentle downhill and back off the go pedal a bit it will show less orange, and maybe even go a bit green (which means the energy is being put back into the battery). At least that's how my old Model S shows it... I confess I haven't paid attention to more modern ones.
     
  18. ig_epower

    ig_epower Member

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    I thought about this topic on a recent long-distance drive. What about changing the regen setting to the lighter mode when highway cruising? The feathering effect going from acceleration to coast to deceleration is more tolerant and perhaps that may result in slightly more efficiency. I have no idea but coasting as speed has to be better than consuming and generating energy back and forth?
     
  19. BrettS

    BrettS Member

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    I think this really depends on your driving style. If you use autopilot or cruise control while you’re on the highway then it won’t matter as that setting won’t affect the way autopilot/TACC drives. If you are driving manually and leave your foot on the accellerator to maintain a reasonably constant speed then it probably won’t matter either as you will likely rarely get to the point where you lift your foot up enough to engage regen (except in cases where you really do want to slow down and then it will be helpful). If you really are going back and forth between accelerating and decelerating and not maintaining a constant speed because you keep lifting your foot and allowing regen to engage, then yes, I think it would help to change that setting.
     
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  20. TIppy

    TIppy Active Member

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    #20 TIppy, Jul 11, 2017
    Last edited: Jul 11, 2017
    [​IMG]
    This is the power required by a tesla roadster vs speed. If you start at 70 mph and coast to 5 mph in neutral, you will coast 0.983 miles in 106.6 seconds and dissipate 681.3 KJ of energy as heat to the atmosphere. Your average speed over that distance will then be 33.2 mph. On the other hand If you quickly regen down to 33.2 mph from 70, and cover that same distance at 33.2 mph, and then regen down to 5mph, it will take the same amount of time. The energy dissipated will only be 555.9 KJ. About 80 percent of the difference, 100.3 KJ, ends up in the battery. Using regen less aggressively than this will save you less energy, but it will save you some.
     

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