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Thread: Regenerative braking

  1. #1
    mod squad TEG's Avatar
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    Regenerative braking

    Many hybrids and BEVs have a feature called "Regenerative Braking". This was Recently discussed on the Tesla Motors Blog.

    It appears that the Roadster may activate regen when you let up on the accelerator pedal, but may not add any extra regen based on brake pedal pressure.
    (In other words as soon as you start to put pressure on the brake pedal you start to engage the friction brakes).

    Apparently many other BEVs did tie the regen to the brake pedal, so it would be interesting to understand why Tesla chose not to do this.

    Also, people have been asking if Tesla could add variable regen controls to their cars... Apparently the regen profile changes depending on 1st gear or 2nd gear selection, but otherwise is not intended to be driver adjustable.

    Here are some examples of places that mention brake pedal tied regen on other BEVs:

    In an account of driving an EV1, John wrote:
    "..time to employ my usual high miles per charge techniques. I switched off the coast-down (we EVers call this off-throttle regen) features, so that I could let the EV1 glide along on slight down hill runs without the car trying to slow down putting a small bit of juice back into the batteries (I'd save this feature for later, as when exiting the freeway and for congested traffic stop and go driving)..."

    If you go to the EV1 FAQ, and go down to the bottom there is a section called "What is regenerative braking". They mention " 'Regen' is the process of making the EV1's motor work like a generator when the brake pedal is pushed, or when the vehicle is coasting with the 'coast regen' on."

    First Peek Inside the (AC Propulsion) eBox:
    "When I applied the brake pedal, the regenerative braking was smooth. When I pushed the pedal further, the transition from regenerative to mechanical braking was seamless."


    Discuss...

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  2. #2
    mod squad TEG's Avatar
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    Re: Regenerative braking

    Tesla tidbits from the web:

    http://mn.eaaev.org/resources/Tesla%...on%20Notes.pdf
    "• Regen is controled by removing your foot from the accelerator pedal.
    • Regen has 2 levels: Max in low gear, and reduced in high gear (although, it is fully
    proportional).
    • Maximum limits on regen are similar to agressively downshifting an ICE sportscar.
    • Note that regen is inverse of the drive wheels (i.e. By using the motor, and the car is
    rear-wheel-drive). Therefore, because the rear wheels don't provide majority braking, a
    maximum of about 80% of the regen opprotunity can be realized."

    http://jalopnik.com/xml/comments/188590
    "
    The Tesla Roadster has regenerative braking but it works differently than the typical hybrid. Hybrids apply regen braking when the driver steps on the brake pedal. The Tesla Roadster applies variable regen braking based on accelerator position. As the driver eases off the accelerator the regen braking is applied. Regen braking is also a function of the gear selected (2 speed manual gearbox with electric shifing and no clutch). First gear has higher regen braking than second. In driving, it feels very much like a standard transmission gasoline powered car. You can actually come to a complete stop without touching the brake pedal. Also, for those who wonder, the brake lights are computer controlled and based on measured deceleration rather than the standard switch on the brake pedal."

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    Re: Regenerative braking

    Yeah...I wish there were better descriptions of this too. Maybe you just have to experience it.

    Does the accelerator essentially function as the stop and go pedal in circumstances other than those requiring heavy braking? How sensitive is it to small variations in foot position?

    If the stop light ahead is red, and I want to preserve momentum while the light goes to green, what behavior do I expect from the car as I ease off the accelerator? Does it begin to slow more rapidly the less pressure applied? If so, would this behavior be similar to the variable regen some people are hoping the car will implement?

    And, of course, there's the much-discussed brake light question...

  4. #4

    Re: Regenerative braking

    Quote Originally Posted by Brent
    Yeah...I wish there were better descriptions of this too. Maybe you just have to experience it.

    Does the accelerator essentially function as the stop and go pedal in circumstances other than those requiring heavy braking? How sensitive is it to small variations in foot position?
    I haven't driven it yet, but. . .* * I thought the Jalopnik description seemed pretty apt -- like driving a manual transmission car and using engine braking.* If the Tesla simulates that behavior, I'll be happy.

    Although, at first I might miss the "burble" of the V8 when I back off the throttle. That sounds so cool.

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    Re: Regenerative braking

    Is it the case that the most efficient driving in stop-and-go traffic would use the least amount of regen?

    I have the sense that any amount of electricity regen might make is not enough to offset the cost of acceleration. To be sure, if you have to stop, you might as well take advantage of it. But slowing well in advance of a red light in hopes it goes green might still provide the better usage than driving to it quickly and then braking.

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    Re: Regenerative braking

    Quote Originally Posted by Brent

    Is it the case that the most efficient driving in stop-and-go traffic would use the least amount of regen?

    I have the sense that any amount of electricity regen might make is not enough to offset the cost of acceleration. To be sure, if you have to stop, you might as well take advantage of it. But slowing well in advance of a red light in hopes it goes green might still provide the better usage than driving to it quickly and then braking.
    I think you are probably correct.*

    Since regen recovers only 80% of the energy of deceleration, not having to decelerate more than that owed to rolling resistance keeps more juice in the battery compared to yo-yo accel/decel.

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    Re: Regenerative braking

    Since regen recovers only 80% of the energy of deceleration
    This is only half the story. When those 80% of recovered energy is used again it goes through another conversion with 80% efficiency giving you total 64% percent of original energy. More than a third of original energy is lost during deceleration/acceleration. It pays to plan ahead, regen is there for unplanned situations.
    I believe in Tesla as a concept, a brand, and a suite of products. No matter who conceived, started, runs, or maintains, I hope it ultimately prevails and lasts.
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  8. #8

    Re: Regenerative braking

    I went back and reread the blog entry on regen. . . It was well written, well explained.

    Then I went to read the comments, and it seems like several people still didn't understand. There may be some who will never "get" this until they experience it.

    I like the idea of having regen controlled solely by the throttle and mechanical brakes controlled solely by the brake pedal. That's simple, that's predictable. You always know exactly what you are telling the car to do. And again, it shouldn't be much of a leap for anybody who has experience driving stick.

    Yet, there are always a few who insist on putting a computer between the driver and the car, and letting the computer decide what kind of braking to use. Tsk. . . That may be okay in a Prius, which is a pretty sedate people-mover, but for a sports car it's just not right.

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    Re: Regenerative braking

    Quote Originally Posted by WarpedOne

    This is only half the story. When those 80% of recovered energy is used again it goes through another conversion with 80% efficiency giving you total 64% percent of original energy.
    Nah, for purposes of comparison it's pretty much the whole story.

    I know that the conversion process is only 80% efficient in either direction, and both directions equal .80 times .80 = .64

    What I was saying is that putting only 80% of the recovered kinetic energy back into the battery means that only 80% of deceleration is on the same footing as the original battery charge from the AC mains.* Both the original charge and the recovered charge pass through the* same 80% loss to accelerate the vehicle whether from recovery or coal fired power plant.

    OK?

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    Re: Regenerative braking

    When Tesla mentioned "because the rear wheels don't provide majority braking, a maximum of about 80% of the regen opportunity can be realized."

    I don't think they were talking about 80% efficiency of the regen system, but 80% efficiency of using rear wheel regen only (since more braking force typically is needed on the front wheels when slowing down).

    So we have 80% * 80% = 64% efficiency when turning rear wheel slow down regen into battery power, then 80% of that back to the wheels for acceleration, so I think we are down to about 51% efficiency now.

    In practice when comparing Toyota vehicles which have Hybrid and non-Hybrid variants, we found something like 30% improvement in city mileage (when using a comparable ICE with and without Hybrid assist). That is in part because the driving does involve some use of friction brakes which don't recapture any of the energy.

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