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Regenerative Braking

Does anyone know if the regenerative brake will have an adjustable setting for more or less braking?
I am assuming that more braking increases the amount of energy recovered. City mode highway mode?

No, we don't know if it will be adjustable or not.

Roadster owners mostly don't want it to be adjustable. I'll be leaving mine on the maximum setting if it is.

Agreed ggr. Personally, I would not make it adjustable. You very quickly get used to the regen braking. Adjustability might confuse people even more.

There seem to be a misunderstanding (not saying that any of you have it) that regen is either on or off. That's not the case, it goes smoothly just like acceleration based on pedal position. That means that it really doesn't matter much how hard regen you get, you can control it with your right foot.

I would prefer strong regen over weak one (up to the point where tires start to slip), because it would allow me to drive more without touching the brake pedal. One foot driving.

You get used to the regen within about 15 mins of driving, i found when i test drove the roadster. It was a really GREAT feature!!!

I don't want to be able to adjust it per se, but I would like the option for it to only engage when the brake pedal is pushed.

As it stands now, you can't coast. If you could coast, and if you drive in a more conscious manner, you could improve range. Applying the brakes on a car always wastes fuel/battery, regen brakes or not.

Yes you can coast. Coast is between acceleration and regen.

This issue has been extensively discussed in some older threads, e.g.,
http://www.teslamotors.com/forum/forums/coasting-retarded-it-roadster

Maybe worth reading, before all arguments are repeated here.

Timo, thanks for expressing it so clearly. Olanmills, I bet I can get better range by engaging the cruise control to a fixed speed on the Roadster, or even just using my foot to keep constant speed, than you could by manually switching to neutral (to achieve coasting). I'll even loan my roadster to the experiment, if you can come to San Diego.

You are right about keeping constant speed when you are just moving, but what about when you see a stop sign ahead?

You can drive the same speed and then apply the brakes very close to the stop sign and decelarate rapidly, like most people do. That's fine; that's a very normal way to drive.

Or you could coast and slow down more gradually and then apply the brakes with less force because you'll be going slower when you get closer to the stop sign. You'll save fuel this way.

Of course there is some balance. It doesn't make sense to coast at very low speeds for long distances, especially in an ICE car where you'll waste fuel.

Whether it's regen or physical brakes, it's braking. Braking always wastes energy* because you are trying to remove energy that you already put in the car to make it go forward.

*technically, it's wasting energy when you accelerate, it's too late to do anything about it by the time you need to brake. Basically, one way to look at braking is that when you're applying the brake, it's because you put too much energy into the car in the first place. You only wanted to go X distance, but you put enough energy in your car to go X+Y distance, and so you have to apply the brakes and get rid of the excess kinetic energy from your car.

I bet you can't estimate the distance to stop sign so accurately with coasting that you don't need to apply some brake before you are at the stop sign. If you apply brake even a little, then you lose to regen by comparison.

I repeat my earlier statement: Coast is between acceleration and regen.

As for option to regen with brake pedal, I believe that can't be done by law. Law in many places force brake pedal to actually do braking.

Timo is not just being theoretical; it's the actual pedal position after easing off accel and before the regen kicks in. So it's even better than your guesstimating of the distance, etc. Some have suggested making a kind of "haptic" notch so that you can feel the gap between go&slow, but it doesn't seem to be functionally necessary.

As for option to regen with brake pedal, I believe that can't be done by law. Law in many places force brake pedal to actually do braking. (Timo)

I think that's precisely the way regen is implemented in the Prius (or was it yet another car?). There is no problem with the law, because if you press the pedal far enough, it'll apply regular brakes. It's just the first inch or so that decelerates the car by way of regen only.

With this specific car (whether it was the Prius or not) users reported that the experience was not as smooth and natural as it should be. In particular, regen is disengaged when actual brakes are applied. If the pedal is in a subtle spot between regen and regular brakes, it can actually happen that you press just a little harder, and the braking effect counter-intuitively decreases for a moment, instead of increasing linearly along the pedal travel.

However, this seems implementation specific and it occurs to me that if Tesla implemented regen in the brake pedal, the experience would probably be smooth, seamless and predictable. Still, with this approach, you cannot do the praised Tesla-style one-pedal-driving.

BMW seem to put quite a bit of thought into the regen implementation in the upcoming i3. They make a difference of "easing up on the pedal" and "easing off the pedal":

[...] The electric drive also allows for deceleration by means of the accelerator pedal. After the driver eases up on the accelerator, the electric motor acts as a generator, converting the kinetic energy into electricity which is then fed back into the battery. Energy recuperation generates a braking effect which makes a significant contribution to vehicle deceleration.

A coasting mode makes this unique "single-pedal control" of acceleration and braking using only the accelerator even more user-friendly. When the driver eases off the pedal, the electric motor's zero torque control keeps the drive train disconnected as long as the pedal is in this position. The vehicle now coasts without consuming power, driven by its own kinetic energy.
http://www.bmw-i-usa.com/en_us/bmw-i3/
(close to the bottom of the page)

And finally, there has already been an authoritative answer to the original question of this thread:

By luck, one of the first I introduce myself to is Drew Baglino, who happens to be directly involved in setting up the Model S regen braking system. [...] When I pleaded for driver-adjustable regen on the Model S, he said this was under consideration. (Two days after the event, while writing up this report, on a whim I e-mailed Tesla boss Elon Musk and repeated my plea for driver-adjustable regen. Three hours later, I got this reply: "I totally agree that regen should be driver-adjustable and it will be on Model S." The message was Cced to JB Straubel, Tesla CTO.)
http://www.thetruthaboutcars.com/2011/10/tesla-model-s-customer-blog-bet...

@Volker.Berlin
Regarding the BMW quote...

1. Suppose you're travelling on a flat smooth road at 50mph at say 50% pressure on the accelerator.
2. You "ease up" on the accelerator to 25% pressure. I would expect the speed to start decaying...so you slow 45mph, 40mph, 35mph, ...
3. You "ease off" the accelerator (0% pressure). Because of the "drive train disconnection" the decay should be less pronounced: 35mph, 34mph, 33mph, ...

That's my interpretation of the quote. If I'm interpreting that correctly, then the behavior from 1% pressure to 0% pressure on the accelerator is incredibly disjoint and likely unsafe for an untrained driver.

What am I missing?

That's my interpretation of the quote. If I'm interpreting that correctly, then the behavior from 1% pressure to 0% pressure on the accelerator is incredibly disjoint and likely unsafe for an untrained driver. (brianman)

I agree, I had the same thoughts. It is not explicit in the quote, but after thinking about it my understanding is that easing up slowly will engage regen, and when you finally let go of the pedal, regen will remain at maximum level, even though you are in fact "off" the pedal. Only if you "ease off" the pedal in a single rapid move, the drivetrain will be disconnected and the car will coast.

Now, of course, you could argue that there is a discontinuity when you slightly press the accelerator again, because that would instantly engage max regen... Not necessarily. It's all in the software, don't forget that there is no physical connection between the pedal and its effect on the drivetrain. If implemented cleverly, smoothly pressing the pedal down could smoothly accelerate from coasting (if coasting before) or could smoothly decrease regen (if coming from max regen), all in the same pedal.

Similarly, for the complete effect, when pressing the brake pedal while coasting, smooth deceleration should be achieved by regen, and brakes should only be applied if you either heavily stomp on the pedal, or smoothly continue to press it further down beyond the point where regen is already maxed out.

Bottom line: The are many more ways to implement this than most of us can think of at first sight. I have been in the software business long enough to know that sometimes, simple implementations result in complex and counter-intuitive user interfaces, and simple and natural user interfaces sometimes require very, very complex implementational details. Just because the implementation sounds (or is) complicated doesn't mean the user (driver) must or should give any explicit thought about using it.

It's the ultimate goal to create the most efficient mix of acceleration, regeneration, coasting (and energy-wasting regular braking), without the driver knowing or thinking about it. It's only us techie early adopters who really want to know what regen is and when it engages. When have you last thought about which gear is currently engaged while driving an automatic transmission? If it works, you don't care one bit.

Elaborating on previous post...

My understanding for the Roadster is that it's something like this bucketing for accelerator pressure:
1. 100%: power spent, high acceleration (+5)
2. 95%: power spent, mid acceleration (+2)
3. 90%: power spent, low acceleration (+1)
4. 85%: power spent, steady speed ( 0)
5. 80%: coast (-0.1)
6. 50%: low power regen, with slight deceleration (-0.2)
7. 25%: mid power regen, with light deceleration (-0.5)
8. 0%: high power regen, with low deceleration (-1)
9. 0%, and brake: no regen, mid to high deceleration (-2 to -5)

From the BMW quote, it sounds like:
1. 100%: power spent, high acceleration (+5)
2. 95%: power spent, mid acceleration (+2)
3. 90%: power spent, low acceleration (+1)
4. 85%: power spent, steady speed (+0)
5. 80%: coast (-0.1)
6. 50%: low power regen, with slight deceleration (-0.2)
7. 25%: mid power regen, with light deceleration (-0.5)
8. 0%: no deceleration ( 0)
9. 0%, and brake: no regen, mid to high deceleration (-2 to -5)

Bucket 8 on the BMW strikes me as troubling.

Transitioning through buckets 7-9...
Roadster: -0.5 to -1 to -2
BMW: -0.5 to 0 to -2

Disengaging the accelerator in phase 8 speeds you up?

Correction:
"speeds you up" should read "decreases your deceleration rate"

Disengaging the accelerator from bucket 7 to 8 decreases your deceleration rate.

See my above post. You assume a fixed mapping between pedal travel and acceleration/regeneration. That's one way to implement it, and it has some appeal merely because it is so simple, but it's not necessarily the way it should be done.

@Volker.Berlin
If you mean the "simple linearish numbers", that was just to simplify the description...or at least attempt to. I don't expect it to be linear; in fact, I expect the location of phase 4 to be speed sensitive and phase 5 to be sensitive to both speed and friction/gravity/etc.

Anyway it sounds like we're seeing it about the same.

IMO, either of these sounds predictable and understandable...
A. Accelerator does zero power (0% pressure) TO full power (100 pressure), no regenerative. Decelerator does zero regen (0% pressure) TO 100% regen (N% pressure) TO 100% regen with braking (100% pressure).
B. Accelerator does M% regen (0% pressure) TO 0% regen (N% pressure) TO 100% power. Decelerator does M% regen (0% pressure) TO 100% regen (L% pressure) to 100% regen with braking (100% pressure).

Where...
1. L, M and N are all in the range (0%, 100%), but not necessarily the equal to each other.
2. There are no discountinuities along the "TO" transitions.

I was under the impression that the Roadster does B; I don't know whether the Model S does A or B. BMW sounds like it's doing some scary C hybrid.

For all we know, we can safely assume that the Model S behaves as the Roadster, with the addition of a user-adjustable setting. It has been invented (well, implemented) by Tesla, Roadster drivers are consistently very happy with it, why should Tesla do anything different in the Model S?

I'm fine with either A or B. The difference between them being that the coast point is a "passive driver state" for A whereas B places that at some non-0% pressure location of the accelerator.

B is likely statistically safer (no accelerator acts as a parking brake via regen), but I prefer A when driving long distances (somewhat like cruise control).

"why should Tesla do anything different in the Model S?"
I wasn't meaning to suggest they should, just that I haven't heard whether they've decided to tweak it from the Roadster or leave it alone.

I wonder if "a coasting mode" is something initialized first, and then the described behavior occurs; otherwise, normal regen.

Brian H, if regen is adjustable in the Model S, then setting the level to zero is effectively "initializing coasting mode" as you describe it.

I just realized that Tesla implementation is not as intuitive as it sounds in engineering point of view. When you start to drive even slight pressure on the accelerator pedal produces torque, so in order to regen at all this pedal position to applied torque has to be variable and not constant.

IOW if you play this in other way around with fixed setting then you gain coasting with foot off the pedal and anything before that just slows down the deceleration caused by other losses if torque is not enough to maintain steady speed.

Quite a feat to make that feel natural. Electric engine does not follow quite same rules as ICE so "engine braking" has to be done artificially.

Timo, that's right. It's much more complex than it seems on the surface, but the nice thing is, that it does not *feel* complex. Discussing this kind of things is almost futile, because it is incredibly hard to imagine a simple behavior based on a complex description of the implementation. You'd need to actually test drive the implementations discussed here to be able to evaluate them. And that's what Tesla has done (lot's of user testing, I am sure) and what has resulted in the regen implementation known from the Roadster.

Is it so difficult?

  • If you push the pedal down at an increasing angle X, the car applies torque, increasing till the pedal hits the metal and you're at max torque
  • decrease the pressure so the pedal rests at X(previous max angle)-N degrees (N between 1 and 5?) and you coast
  • let off more (N > 5) and you go into regen which increases until X is zero, at which point you're at maximum regen
  • increase X slightly and you're in coast mode
  • increase X more and you're back to applying torgue
  • braking is simply applying the brakes - independent pedal
  • Doesn't that cover it?

Difficult thing is that that point two in your list has to be done manually and it can't be constant. If you use constant model then you don't gain regen ever, you coast when your foot is off the pedal.

"done manually" -> "done artificially". Distraction.

The magic of software. Only a very few things in the Model S are not allowed to be programmed. Like the brakes.


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