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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.

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.

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

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.

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.

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.)

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%).

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.

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.

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.

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!

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!

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.

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.

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

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.

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.

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).

@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.

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.

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.

Butch, thank you, that explains it very well!

Butch +1
Theresa +1

Butch + Theresa +2

:)

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

Nyuk, nyuk!
>:\

@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).


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