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Regenerative brake settings and lead feet

Discussion in 'Model S: Driving Dynamics' started by Lucent, May 3, 2016.

  1. Lucent

    Lucent Member

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    I test drove (and ordered) a 90D and commented on how little the car would coast with my foot off the gas. The salesperson directed me to the regenerative brake settings and I changed it to low. She said with this setting, the brakes wear out much faster (believable) and total mileage is negatively affected (questionable for my driving style).

    For people who are always pressing either the gas or brake, I can see keeping regenerative on high helping a lot, but for those of us who understand the nonlinear dynamics of traffic and the benefit of rolling up to traffic lights and leaving a gap in stop-and-go traffic, I can't imagine how this setting could help. Surely someone whose goal is to maximize the amount of time spent driving with neither the gas nor brake pressed would do better never expending the energy rather than inefficiently recapturing it.

    Any theoretical insight or real world observations about range between the two settings?
     
  2. Tree95

    Tree95 Member

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    Try it for a while. For those of us who learn how to modulate the pedal, normal regen is awesome for having a high degree of speed control and braking control with just one pedal. If you want to coast into a light, don't lift your foot all the way off the pedal.

    When I switch to my gas car, I'm amazed by how often I have to move my foot to the brake pedal as compared to my S. And, I'd have to do the same pedal dance in my S if it were set to low regen.
     
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  3. Camera-Cruiser

    Camera-Cruiser Fully Charged

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    Tree is correct. Give it a week or two and then go back to ice. You will hate the lack of control.

    I love driving barefoot. The big toe knows all!
     
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  4. newtman

    newtman Member

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    I thought the same for the first hour behind the wheel of a Tesla, then quickly got used to just using the power pedal 90% of the time. Much more efficient and less tiring.
     
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  5. brianman

    brianman Burrito Founder

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    Use TACC.
     
  6. Lucent

    Lucent Member

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    I tried it for a while and what convinced me to keep it on was that it seems intelligent with respect to how quickly I let go of the accelerator. If I let go at a slow rate, it seems to coast a lot longer on its own than if I let go quickly. For that reason, I'm keeping it on high.
     
  7. chillaban

    chillaban Member

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    There's definitely a learning curve on standard regen but I think it's a better approach like most others. I generally make a separate driver profile with low regen for when I offer someone else a test drive... But within the first 2 days with my Model S, I was doing pretty well at modulating the regen. It's great in fact for aggressive driving habits and lets you close gaps quicker knowing that regen can burn off a lot of the overshoot.

    Another nice perk is I'm finding that aggressive vs calm driving habits are having less of an impact for my efficiency: I can do maybe 290-300Wh/mi in calm city driving and 320-340Wh/mi for aggressive city driving. That's a ton better than the wild fuel economy swings in a gas car driving a similar way because of constantly using the brakes rather than relying on coasting.
     
  8. jdw

    jdw Member

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    You'll get used to regen and modulating the go pedal within a few days and then you'll hate driving cars without it. A Tesla doesn't have an engine or a transmission, so if you are in Drive, the motor is connected to the wheels. The only real need to use low regen is on a slippery or icy road to make the car easier to control.
     
  9. chillaban

    chillaban Member

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    Right, and definitely don't get all OCD about hitting exactly zero for coasting. Being slightly above or below 0kW is pretty much the same as at 0 in terms of energy efficiency. It's easy to get overwhelmed and turn OCD about the data, but it's important to recognize that often the hype is in your head.
     
  10. tennis

    tennis Member

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    I don't think anyone answered the OP's question (no offense). The OP isn't having an issue with getting used to one pedal driving -- rather he/she is questioning the efficiency of high vs low regen for those of us that tried to be as efficient as possible with our automatic transmissions by coasting as much as possible and avoiding stopping as much as possible.

    The question is: Is it more efficient to try and coast with low regen or to let standard regen do its thing?
     
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  11. dgpcolorado

    dgpcolorado Member

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    Some of that depends on what sort of terrain one drives. I descend steep hills and need maximum regen to control speed without using friction brakes.

    That sort of situation aside, the best efficiency is to coast (to a stoplight or whatever) in max regen setting by modulating the go pedal and use regen when more slowing is needed. In most situations (save when approaching terminal velocity due to losses from drag) it is more efficient to coast than to use regen. A good hypermiler can make the low regen setting fairly efficient by anticipating stops and coasting as much as practical to reduce inefficient braking. But the same thing can be done in a high regen setting with a little practice and that has the benefit of allowing more regen when more slowing is needed; it also slows the car more quickly in panic stop situations, a safety consideration.

    Short answer: it is possible to coast just fine in high regen setting so there is no advantage to the low regen setting.

    The misconception here is that one has to be on the go pedal or off it and on the brake pedal. With single pedal driving one modulates just the go pedal and doesn't use the brake pedal at all, save for holding the car still at a stoplight. A single pedal driver uses just the amount of acceleration or deceleration needed for the situation and it is very easy to modulate that control. Coasting up to a stoplight is more efficient with single pedal driving than just backing off the pedal in low regen or in an automatic transmission car. If slightly more slowing is needed than true coasting, adding a slight bit of regen, by backing off the go pedal a bit more, is easy and precise.

    A true "lead foot" driver is inherently inefficient so trying to ask how to make that sort of on or off pedal driving — "to maximize the amount of time spent driving with neither the gas nor brake pressed" — more efficient seems to miss the point, to me.
     
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  12. AWDtsla

    AWDtsla Active Member

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    The pedals are not on-off switches, in any car.

    People usually detest the feeling you get from automatic transmissions, that the car has completely disconnected from the wheels.
     
  13. mikeash

    mikeash Active Member

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    Why would that be your goal? Are you trying to keep your feet comfortable or something?

    If your goal is efficiency, then you want to do a pure coast. Neither regen setting will achieve that. Low still activates regen when you're off the pedal, it's just, well, low. If you want to coast, then find the pedal position that's neutral. You can do this easily by pulling up the Energy screen on the instrument cluster and putting the pedal at a point which shows neither energy usage nor energy generation.
     
  14. Stiction

    Stiction Member

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    Bingo.
     
  15. TIppy

    TIppy Member

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    Why is coasting the most efficient in general? If you coast to zero from 60 mph, all the energy has been lost to drag. The power the car is losing to drag goes up as the speed cubed. If you coast at 1/2 the speed you are dissipating 1/8 the energy per unit time. There's probably some optimum combination of regenerating to a lower speed and then coasting. If the car was 100% efficient, you wouldn't want to coast at all. Any time spent above zero miles per hour would just be wasting energy to drag.
     
  16. Snerruc

    Snerruc Member

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    Not using regen is a mistake. I regularly travel down a twisty 6 mile stretch of road with a 2000ft drop. If you coast, you use no energy and heavily use your brakes. If you use regen, you never touch the brakes and GAIN 1.5 to 2 kw. This means you can travel 4-6 more miles than the person coasting. Also the brakes last much longer.
     
  17. dgpcolorado

    dgpcolorado Member

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    It is a bit more subtle than that. Clearly if you need to control speed on a downgrade regen is better (something I know well because I descend a very steep road with hairpin turns every time I leave my home in the mountains). But at the bottom of a hill, or when approaching a stop well in the distance, coasting might be more efficient.

    The problem with regen is that it is inherently inefficient: not all of the kinetic energy is recaptured and put back into the battery, not even close. The problem with coasting is drag and at terminal velocity all of the potential energy from descending a hill is lost to drag, so coasting to terminal velocity* is very wasteful. Somewhere in between all regen and all coasting is the most efficient mix, much of it dependent on speed and terrain (hills).

    In general, if you can coast down a mild grade and maintain the desired speed, that is probably the best. Using regen in that situation would slow too much and then require energy to be used to get back up to the desired speed. The same is true for coasting up to a stoplight. If the speed can be reduced gradually over a long distance that is probably more efficient than slamming on the regen then adding power to maintain enough speed to make it to the light. I trust that this seems obvious.

    For experienced hypermilers these decisions become fairly easy after practice and are a game to play (why not have fun while driving?). But the energy savings in hypermiling, from adjusting coasting and regen in an EV, are small so if you can't be bothered, don't worry about it.

    However, the general hypermiling strategy involving gentle acceleration, anticipating stops so that slowing can be gradual, and the like, is quite beneficial to energy savings and useful to know. We have all seen those lead-footed drivers who sprint away from a stoplight and speed up to the next red light only to slam on the brakes at the last minute; it is very common behavior. This is much less efficient than gradual acceleration and gradual slowing, regardless of whether one is using coasting, regen, or both. And that makes a bigger difference in energy use than the precise mix of coasting or regen.

    My 2¢.


    * [Terminal velocity is when the deceleration due to drag equals the acceleration due to gravity, so velocity remains constant. With cars, the terminal velocity speed depends on the slope of the hill being descended and, of course, the inherent drag (Cd•A) characteristics of the car.]
     
  18. Stiction

    Stiction Member

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    Not using regen is a mistake, mostly but not always.

    A very slight slope downhill that results in a coast at the speed limit or below is perfect.

    A bigger downhill: yes, gotta regen. Even if air drag is not a factor (but I agree with Tippy) the police eventually will be.
     
  19. Stiction

    Stiction Member

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    Our posts crossed. Should have just waited for your post 8)
     
  20. TIppy

    TIppy Member

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    I haven't seen any numbers from tesla, but rimac says regen efficiency can be as high as 96%.
     

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