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Regenerative Brakes

Regenerative Brakes

I recently test drove the Model S at the Dallas event. I loved it. I have a question about the brakes. In my Lexus RX400H, when you applied the brakes, it was in regenerative mode until the car was about to stop or if there was a sudden hard application of the brakes. In the model S, it seems that any time the accelerator is not depressed, the brakes are in regenerative mode, but whenever the brake pedal is applied, the actual brake pads are applied. I wonder if there is an option to have a similar mode to the Lexus, where all but severe braking was handled by regenerative mode. This would give "more" of regenerative mode when the brake pedal was applied, compared to just somewhat when the accelerator is let up. This would increase the life of the brake pads, and also somewhat increase the range.

Michael23 | 23. august 2012

No option. You already brake less because of regen which increases the life of brake pads. I tried it and It works pretty well, but regen could be higher for sure, the brembos are nice and brake fast so you don't have to ride them.

Volker.Berlin | 23. august 2012

The assumption is that regen in the Model S is already as strong as it can be. Remember that regen only applies to the driven (rear) wheels, not to the front wheels, and that regen generates extremely high currents that need to be dealt with. It is not exactly clear what the limiting factor is -- the power electronics or the grip of the rear wheels or something else -- but if there is still room for even stronger regen I think we can assume that Tesla will make it available as another option in an upcoming software update. There have been many requests for stronger regen, so if they can do it I don't see why they wouldn't (although it may not be on the top of their priority list right now).

Also keep in mind that hitting the brake pedal does not disable regen (as far as I know, but unfortunately I did not yet have an opportunity to check it first hand). It is my understanding that the friction brakes are applied in addition to regenerative braking, thus the wear on the brake pads is still minimized even if you have to come to a stop quicker than regenerative braking alone would allow.

In no event I see Tesla putting any regenerative effect on the brake pedal. Owner of the Prius (that does it similar to your Lexus) report that there is an unnatural and uneven feel to the brake pedal, and actually in some situations the slow-down can be less when hitting the brake pedal harder -- when you hit the point where the car shifts from regenerative braking to friction brakes. That's not the kind of brake pedal behavior that you want to see in a sports premium sedan. What's your experience in this regard? Roadster owners have been extremely pleased with regen on the accelerator pedal and the brake pedal plain and simply working the friction brakes. I don't think Tesla will move away from that paradigm anytime soon.

There have been a couple of interesting threads around this issue, that may be worth reading if you are interested:

http://www.teslamotors.com/forum/forums/coasting-retarded-it-roadster

http://www.teslamotors.com/forum/forums/regenerative-braking

Theresa | 23. august 2012

As a Roadster owner I will state that Volker has it correct. The brake pedal only applies friction brakes. The regen on the Roadster feels stronger than on the S but I believe that is because the S is heavier. My belief is that the regen is limited by either the electronics (I doubt that) or the ability of the battery to accept the high levels of charge that more regen would have.

I am really pleased by how the Roadster has engineered the regen and would not think Tesla has any plans to change the basic principle of operation.

archibaldcrane | 23. august 2012

Just remember, when we use the term "regenerative braking" - it has nothing to do with the brakes of the car. It's a type of "engine breaking" (like if you downshift in a manual), spinning the electric motor backwards to recharge the battery.

The brakes are a completely separate system from the energy regeneration - and I think Tesla likes it that way for better, more consistent braking performance.

Volker.Berlin | 23. august 2012

Theresa, thank you for chiming in! Could you also clarify:

- Is regen affected when you hit the brake pedal, or does the regenerative braking continue alongside the friction braking?

- When you sharply hit the brake pedal but then let go again, is there any difference in regenerative braking before and after?

- When you start downhill, from standstill, and do not touch the brake or the accelerator pedal -- does the car immediately start to regenerate? Or is it freewheeling downhill?

(That's all assuming that the battery is about half full. If it is close to full, regen is moderated or even entirely disabled to avoid overcharging of the battery.)

Theresa | 23. august 2012

Volker,

These sound simple and straight forward but may not be quite that clear. I will attempt to answer so it makes things clear.

1) Depending upon what you define as affecting regen it does not affect regen as such. It does lower the regen only because your speed is falling faster and it is the same regen whether you are using the brakes or not at each speed.

2) Refer to answer 1. It is dependent upon speed and accelerator placement relative to your speed. Assuming it is driven as it should be (foot off accelerator when braking) there is no difference.

3) There is a freewheeling effect below ~ 3 mph. Above that the regen begins. Very similar to a automatic transmission ICE.

And these are all assuming, as you stated, the battery is not fully charged (below ~ 95%).

Brian H | 23. august 2012

Teresa;
'Scuse my ignorance, but it was my understanding that under no circumstances can rolling (momentum, downhill, or pushed) in an automatic apply rotational force to the engine. The auto-shifting mechanisms just don't permit it. False? Otherwise, it would be possible to push-start an automatic, and AFAIK that is simply impossible.

Theresa | 23. august 2012

Brian, Apparently I didn't state that well. If an ICE is rolling downhill while running and the transmission is in gear it will tend to hold back the car to some degree. At least that has been my experience in most automatics. If it is not running I am not sure what happens as I have never tried that.

nhurst | 23. august 2012

archibaldcrane - Just a clarification for those who do not understand regenerative braking. Regenerative braking does not spin the motor backward. The motor continues at the same speed but slows down as it resists forward motion (generating electricity for the battery) rather than propelling the car forward.

Brian H | 23. august 2012

Running the motor backward is what you do to go in Reverse in an EV. It's just as easy for the motor, it doesn't really care!

Volker.Berlin | 23. august 2012

Theresa,

These sound simple and straight forward but may not be quite that clear. I will attempt to answer so it makes things clear.

It's perfectly clear the way you explained it. Thank you very much for your careful observation, and for sharing the same!

joesontesla | 23. august 2012

Regen on model S are simple effortless and intuitive unlike toyota systems. I want it like is it on demo cars (perfect) and i want the option to disable it or set it less agressive when i drive on icy roads only.

Theresa | 23. august 2012

Joe, FYI I think that you would be surprised at how good the regen actually is on icy roads. Because only the rear wheels are braking the car slows and holds the road very well. You can easily correct for the back end slide if it gets that bad by steering. Unlike front wheel drive cars you are always under control.

Brian H | 23. august 2012

Teresa;
Doesn't Traction Control manage most of that? It wouldn't permit regen to make the wheels slip.

Theresa | 23. august 2012

Actually the regen doesn't appear to have traction control. At least not on my 1.5 Roadster. The wheels will slide on icy enough conditions.

jerry3 | 23. august 2012

Brian,

Traction control, abs, and vehicle stability control rely on the friction brakes. In the Prius any time one of those safety devices engages, the regen is disabled. I would suspect that it's the same for the Telsa because the individual brakes have to be managed very precisely in order for those systems to work. Of course, traction control is only for acceleration so regeneration doesn't enter the picture.

Theresa | 23. august 2012

I am not sure but I think in electric cars brakes aren't part of the traction control as the power can be instantaneously lowered unlike an ICE where there is a significant amount of lag between sensing tire spin and drop in power applied. I can go on glare ice with the Roadster and not spin a tire even when I floor it. The car may only accelerate at about 1 mph every several hundred feet but I never feel like the brakes are ever applied and the tires never break loose until I let off the accelerator at which point the regen is what makes them slip (if they do slip).

Timo | 23. august 2012

@Theresa, that's frightening. Rear wheels slipping when you just slow down at a icy curve would result a very rapid spin. I really hope that this is not the case with Model S.

Butch | 24. august 2012

I have driven 3 Prii, a Lexus Hybrid SUV, and a Roadster. The Prius, the Lexus Hybrid, and the Tesla Roadster have regenerative braking. I far prefer the Roadster.

Toyota wanted the Prius, like all of its hybrids to just feel like a normal ICE car with an automatic transmission. To do this, they put in a very light regen when you take your foot off the accelerator, and continue applying more in the first stages of brake pedal application. The problem is in the harder application of braking, the transition from regen to friction is always a little funny (better in later models). Also, if you want to come to a stop on regen only, you have to figure out how to apply just enough pressure to the brake.

In the Roadster, Tesla made the model to be a sports car with a manual transmission always in first gear. Take your foot off the accelerator at speed and you get a lot of regen that feels like engine braking in an ICE powered car in first gear. Applying the brake, simply adds the friction braking power to the regen. Simple and nice.

In the Roadster, the regen is constant torque, starting with the point where it is at the maximum power that the PEM and battery want to deal with down to a few MPH. That max power is about 80 Amps at 50 MPH or so. 80 Amps in the Roadster is about 40 HP, which good stopping power, and some find extreme. I like it! For most, reasonable, around-town driving, I use the friction brakes less than 10% of the time, and of course for the last few MPH before a full stop.

It sounds like the Model S is taking the same regen model as the Roadster, but its more like a manual transmission in 2nd or 3rd gear for the stopping feel. I would prefer more agressive regen, but this will work for now. Hopefully, there will be a "Roadster" level of regen as an option in a future firmware load.

Theresa | 24. august 2012

Butch, I think the reason the regen feels less in the S is because of the weight difference. I was told the regen is the same as the Roadster so it is probably due to the power limits that the battery can handle.

Timo, It is not nearly as frightening as you think. If you are going into a curve that is slippery enough to cause loss of traction with the regen you are most likely already going too fast for conditions as the front wheels will be only a little behind the rears in breaking loose. The situation I was describing was in a controlled situation where I shouldn't have even been driving but I wanted to see how the car handled it. I was in a semicircle road with no traffic (they were smart enough to not be out there!) and was trying several scenarios so I would never be surprised while doing "normal" driving.

Volker.Berlin | 24. august 2012

Butch, thank you, that explains it very well!

Brian H | 24. august 2012

Butch +1
Theresa +1

Butch + Theresa +2

:)

Theresa | 24. august 2012

I didn't get to this post before I posted on the age thread. So Brian got my name right after all.

Brian H | 24. august 2012

Nyuk, nyuk!
>:\

Timo | 25. august 2012

@Theresa, if you brake rear wheels without any TC at full power of regen it is rather high braking power. Free-wheeling front wheels are nowhere close to lose grip at that point. This means that your rear-wheels slip and front doesn't and this causes a very rapid spin. Pretty much same case as using hand brake in middle of the turn in slippery surface.

If you didn't get a spin then I believe there is some sort of TC applied with regen. I find that quite probable in fact (this is something so obvious and so easy to control with electric motor that Tesla engineers should have thought it).

cliffordbaynon | 12. februar 2015

I am shopping my 1st Model S now, and have a
REGENERATIVE BRAKING QUESTION:

Isn't coasting a natural part of driving in mild traffic conditions?
To be only ACCELERATING or BRAKING seems like you have lost something in the natural flow of how most of us drive.
Also the constant brake light going on for the poor soul following you every time you don't want to slow down, but simply no longer want to accelerate seems extreme.

I am aware that the regenerative braking can be turned off, but any insight on this issue appreciated.

-Future Model S owner

Bighorn | 12. februar 2015

Clifford
You modulate the pedal--it's not all or none situation with regen. Also, you cannot turn off regen. You can use low regen or some people engage neutral in order to coast.

Haggy | 12. februar 2015

It takes getting used to. If you are the type of driver who zooms up on people's bumpers and then applies the brakes, it will seem less strange than if you are the type who sees traffic slow down ahead, takes his foot off the pedal, and lets the car coast. In the latter case, your car will slow down gradually and the brake lights will never come on. With the MS, your car would slow down more rapidly, and brake lights would come on, and then you'd need to accelerate again. The trick is to simply ease up on the accelerator rather than taking your foot off. Pressing down speeds up, taking your foot off slows down, and easing up lowers your speed. You simply don't take your foot off completely in many situations where you might do so in other cars.

logicalthinker | 13. februar 2015

Haggy & Bighorn are correct.

PBEndo | 13. februar 2015

FWIW - I did not like regen during my first test drive before purchase. I was accustomed to completely releasing the accelerator to coast and in the Model S, this resulted in too much sudden braking making the ride uncomfortable, especially for passengers who weren't anticipating the "back and forth" driving I was doing.
Once purchased, my delivery specialist suggested that I leave standard regen on for 3 days until I train myself to "feather" the pedal to achieve whatever level of power or braking that I needed. I was skeptical.
He was right. Within the first 30 minutes of driving, I was already hooked. Now, I hate driving ICE cars that require me to use the brake pedal so often. I truly wish the max regen could be stronger.
I do find that in congested, but moving, highway traffic, it takes some attention to minimize your brake lights flashing on and off too frequently, but this is a minor issue.

sule | 13. februar 2015

Haggy, Bighorn, logicalthinker and PBEndo are all correct :)

It is awesome, you will learn to appreciate it and expect it and miss it when not there.

DTsea | 13. februar 2015

35000 miles in heavy congested city traffic. No issues of any kind witj brake light flashing.

garygid | 13. februar 2015

Since the brake lights only come on when you accomplish
some "significant" deacceleration with the regen, you can
get the lights to flash, but it takes some work to do so.

More often, with light, smooth, gentle braking (like when
trying to anticipate stopping, to save energy), you will not
have the brake lights come on at all.

In my opinion, the brake lights should come on much more,
to indicate "slowing traffic ahead" to those behind.

PBEndo | 13. februar 2015

Must just be my imagination.

donandrews508 | 13. februar 2015

Good to hear that the brake lights are connected to an accelerometer. I did some tests and the lights seem to come on in city driving at regen greater than about 15kW, definitely not during all regen.
Also, new drivers getting used to the go-pedal can get graphic feedback-- look at the power meter in addition to feeling the cars response to the pedal change-- ease up on the pedal just enough to make the yellow/green line near horizontal (zero) and you are "coasting".
I love controlling the car speed entirely with the throttle, and have come to really dislike the brakes on my Toyota Hybrid. I want real brakes, with no computer figuring out whether to do regenerative or friction braking, thank you.

renwo S alset | 13. februar 2015

Last time I drove an ICE it scared the crap outta me. I took my foot off the go-pedal and it seemed to go even faster. I was thinking "How do you stop this thing?"

Watt fun | 13. februar 2015

@ BrianH " Otherwise, it would be possible to push-start an automatic, and AFAIK that is simply impossible."

Well, yes and no. Nowadays, no. Yes in the past for some. My 65 Corvairs with Powerglide had rear pumps--get the car rolling to about 15 mph, turn the key on, and shove it in low and the engine would start...or if the engine stalled, you could get some slight engine braking until the transmission overheated.

Our 67 Camaro with Powerglide did not...nothing happened if the engine was off as it only had a front (engine side) pump. Depended on the make and design of the automatic, and sometimes the version.

PBEndo | 13. februar 2015

su.ac.renwo - I feel the same thing when I occasionally drive an ICE. It feel like it is actually increasing speed when I let off the gas pedal. I also notice it on the rare occasion when I have a full battery and regen is disabled. It is a little scary!

Haggy | 13. februar 2015

When I first got my car, it didn't have the iBooster. It was installed about a month later, and I didn't feel any difference. The iBooster is specifically designed to optimize regen, and is supposed to use regen up to .3 g, and use the master cylinder "the traditional way" for anything beyond that. I don't know whether Tesla uses it as designed or has some reason not to have it work with regen when the pedal is pressed, but it is optimized for it and Bosch touts using regen as much as possible but in a seamless way as its main feature. All I know is that if it does use regen and increase it significantly when applying the brake pedal, there's no difference in feel from not having that capability. As far as I know, the main reason for the switch to the iBooster was that it was necessary for autopilot.

I didn't pay particular attention in anticipation of the change since I wasn't expecting it, so I don't remember specifically what I saw when I pressed the pedal before the swap. I know that when I step on the brakes or when TACC is slowing me down aggressively, I see regen going up near the max. With TACC off, I barely use the brake pedal except to keep the car from rolling once it's almost stopped. But when I press it moderately, regen gets stronger. Is this different for the pre-autopilot cars? Does regen appear to get weaker when the brake pedal is pressed?

Brian H | 14. februar 2015

Think of the pedal as having a range + | 0 | - so the 0 point is coasting.

Bighorn | 14. februar 2015

My wife followed me home last night and apparently I've gotten too good with the regen because she said the brake lights never came on.

Brian H | 14. februar 2015

Oh, that could be a problem. A manual switch on the wheel, maybe?

Bighorn | 14. februar 2015

@Brian
Or I could try not to overcompensate, concerned about the lights being on too much. I think I will do that because the message I got was that I wasn't giving adequate signs of deceleration.

David Trushin | 14. februar 2015

You could stick your left arm out the window with the hand down.

Bighorn | 14. februar 2015

@David
Already do that reflexively, but it was dark out.

offworld | 14. februar 2015

From the Physics, ReGen is most efficient at the lowest stopping torque/force. In the motor, Torque is directly proportional to Current(I), and the heating losses in the copper windings are (I^2)*R ... so at higher stopping rates, IE higher Torque producing more heat losses with the square of the current/torque.

With the motor only being about 86% efficient best case (optimum RPMs), and the efficiency falling completely to zero as speed approaches zero we have an interesting problem with ReGen remaining useful.

At higher speeds, ReGen recovering energy down hill, can return about 0.86*0.86=0.74 or 74% of the down hill energy can be recovered ... a 14% loss capturing, and a 14% loss returning the captured energy to kinetic motion.

At lower speeds, the motor efficiency rapidly falls off the published best case numbers ... and a smaller portion of the Kinetic energy can be captured, and used for kinetic motion later.

So it's best to use the brake pads to generate heat at slower speeds, and keep the heat out of your motor and drive electronics with low recovery percentages. Especially if you are driving in hot 100 degree weather. Your motor is already going to be struggling to keep itself cool with the 14% traction motor losses, especially in low efficiency stop and go traffic.

It would be nice to see real torque/rpm/efficiency curves for the motor, drive, and complete power train ... from stop to max rpm/mph.

David Trushin | 14. februar 2015

@offworld -- or you could just drive the car.

offworld | 16. februar 2015

@Trushin -- till it fails?

Semiconductor life is rapidly accelerated at elevated temps. Low speed stop and go in New York in the winter is probably not much of an issue ... low speed stop and go in Phoenix, AZ in the summer with ambient well above 100F every day is a LOT tougher on semiconductors.

It would be interesting to know the drive electronics junction temps while operating in Phoenix in the summer, and have a better estimate of device life ... in addition to the real efficiency curves for the traction motor and drive electronics.

offworld | 16. februar 2015

Depends a lot on cooling strategies for the high power section of the traction drive electronics ... if this is part of the cabin cooling system, then the high temps are unlikely ... and at the same time, the cooling systems power costs become part of the motor/drive efficiency calculation as it becomes a required traction drive component in high ambient temps.

IE the traction motor efficiency curves also become a function of ambient temp.

I would still like to see the real graphs.

DTsea | 16. februar 2015

Offworld, the mptor etc are actively cooled. Dont overthink.

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