Why isn't regen integrated with the brake pedal?

[derekt75]

It is possible that the amount of regen is limited by the car's speed. ie. at lower speeds, it is possible that the actual regen mechanism cannot grab a full 60 kW of power from the wheels. I'm just speculating here, but it is possible there is a technical hardware limitation, rather than just a software limitation.

Keep in mind that 0.1 g deceleration at low speed generates much less power than 0.1 g deceleration at high speed.
High speeds means more power for the same torque.

The car seems to have similar logic for acceleration as deceleration:
constant torque/acceleration/deceleration 10 mph - 40 mph.
constant power > 50 mph.

It's possible that the motor can only handle so much reverse torque or reverse current at 20 mph, or it's possible that Tesla just thought users wouldn't want more than X deceleration force from the regen. Also, it might feel a bit weird if the deceleration force kept increasing as your speed kept decreasing.

Anyway, as you say, the regen limit could be hardware or software, I'm just pointing out some possible reasons why the limits exist.

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and for what it's worth...
after you get used to it, I like that Tesla kept the pedals separate. Right pedal for electric acceleration/deceleration; Left pedal for friction brake. It's simple and it works.

 

[Cosmacelf]

Yes, my only wish is that that they had a "high regen" setting in addition to the low one. Now that I'm used to it, I would like a more aggressive regen at all speeds. And if in a later version of the car, they could have 80 kW regen or whatever, that would be great too!

 

[WarpedOne]

It is possible that the amount of regen is limited by the car's speed. ie. at lower speeds, it is possible that the actual regen mechanism cannot grab a full 60 kW of power from the wheels.

At 15 mph MS60 and MS85 can output up to 75 kW (and MSP over 100 kW) minus some loss (~10%). Regen power can be the same as output power, what is more problematic is traction, especially on rear axis because of weight transfer.

 

[dennis]

...after you get used to it, I like that Tesla kept the pedals separate. Right pedal for electric acceleration/deceleration; Left pedal for friction brake. It's simple and it works.

Agree that it is simple and that it works. It just doesn't maximize regen, in contrast to a couple of other EVs where the brake pedal increases regen to a certain level of deceleration and then seamlessly adds the friction brakes for higher levels of deceleration. The brake pedal slows that car, but the car determines how much of that slowing is through regen and how much is friction braking, depending on your speed and how hard you depress the brake pedal. Not as simple but more elegant IMO.

 

[mknox]

Yes, my only wish is that that they had a "high regen" setting in addition to the low one. Now that I'm used to it, I would like a more aggressive regen at all speeds. And if in a later version of the car, they could have 80 kW regen or whatever, that would be great too!

I'm not sure I'd want more unless it was for sure on the brake pedal. I'm concerned with loss of traction in slippery winter conditions if too much braking is applied to the rear wheels only, especially if on a curve.

 

[strider]

Agree that it is simple and that it works. It just doesn't maximize regen, in contrast to a couple of other EVs where the brake pedal increases regen to a certain level of deceleration and then seamlessly adds the friction brakes for higher levels of deceleration. The brake pedal slows that car, but the car determines how much of that slowing is through regen and how much is friction braking, depending on your speed and how hard you depress the brake pedal. Not as simple but more elegant IMO.

How do you know it doesn't maximize regen? Regen is limited by traction and the capacity of the electronics and battery back to accept power from the motor, not by which pedal it's on. I don't know how much regen other EVs generate but 60kW is a LOT. Model X may be able to go higher since it will be AWD as that would help with the friction component.

If your foot is off the accelerator then regen is being maximized at least as far as Tesla's programming will allow. I agree with another poster who states that they back off the regen to maintain a constant decel rate and not a constant regen rate as that would feel weird to people and result in a lot of people "stopping short."

I also object to your use of the word "seamless" with regards to blending regen and friction brakes. In every car I've driven that does this it is NOT seamless. There is a "jump" as it adds the friction brakes and it is very noticeable. I have not driven a Karma so maybe their implementation of this is perfect but on other Hybrids and EVs it is not. I am a huge fan of one-pedal driving. I hate having to dance my foot back and forth between pedals when driving.

 

[ken830]

Strider and others are right. Regen is already "max" (for your current speed) by letting off the accelerator pedal. Brake pedal should be just friction brakes. This is the way it is. This is the way we like it. Don't muddy our brake pedal with voodoo blending.

Also, as you slow down, you lose regenerative braking capabilities. This is a law of physics. The faster you spin the motor, the more regenerative braking force you can generate. When you slow down, you reduce regenerative braking capacity. If you apply friction brakes, you will reduce your regenerative braking capacity. You can not have regenerative braking when you are at a stand-still. This is also the reason you can not bring the car to a complete stop with regenerative braking alone.

Regenerative brakes can only resist motion. The more you have the more it can resist.

 

[dennis]

How do you know it doesn't maximize regen? Regen is limited by traction and the capacity of the electronics and battery back to accept power from the motor, not by which pedal it's on. I don't know how much regen other EVs generate but 60kW is a LOT. Model X may be able to go higher since it will be AWD as that would help with the friction component.

If your foot is off the accelerator then regen is being maximized at least as far as Tesla's programming will allow. I agree with another poster who states that they back off the regen to maintain a constant decel rate and not a constant regen rate as that would feel weird to people and result in a lot of people "stopping short."

I also object to your use of the word "seamless" with regards to blending regen and friction brakes. In every car I've driven that does this it is NOT seamless. There is a "jump" as it adds the friction brakes and it is very noticeable. I have not driven a Karma so maybe their implementation of this is perfect but on other Hybrids and EVs it is not. I am a huge fan of one-pedal driving. I hate having to dance my foot back and forth between pedals when driving.

To answer your questions:

1) I know it is not maximizing regen because I am going down a hill at 30 mph, lift off the accelerator and see 25-30 kw of regen, not 60. When I apply the brake pedal to decelerate further, regen does not increase to 60.

2) In contrast, when I run the same test in the Karma, when I apply the brake pedal regen increases. Braking from higher speeds, I have seen regen in excess of 100kw.

3) In the Karma, the transition from regen to friction braking IS completely seamless. There is no "jump". Since you are in the Bay Area, I'd be happy to demonstrate this for you.

4) I don't have to dance my foot back and forth. I can select 3 levels of regen for when I lift my foot from the accelerator. The difference is when I engage the brake pedal the software in the Karma is clever enough to maximize the amount of regen before it transitions to friction braking. In the Model S pressing the brake pedal engages the friction brakes. Period. No additional regen.

The reason I started this thread is that I had been driving the Karma for 9 months when I took delivery of the Model S. I was surprised that the brake pedal did not increase regen to the maximum. I was curious as to why Tesla had not engineered this into the Model S. I've yet to hear an answer other than "one pedal driving is better" which I don't view as a satisfactory answer, since there are no perceived downsides to having the brake pedal maximize regen.

In no way am I trying to say the Karma is a better car than a Model S. I'm seeking an "engineering" answer to why the Model S doesn't maximize regen when the brakes are applied, since I have an existence proof that it is possible. :smile:

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Also, as you slow down, you lose regenerative braking capabilities. This is a law of physics. The faster you spin the motor, the more regenerative braking force you can generate. When you slow down, you reduce regenerative braking capacity. If you apply friction brakes, you will reduce your regenerative braking capacity. You can not have regenerative braking when you are at a stand-still. This is also the reason you can not bring the car to a complete stop with regenerative braking alone.

You are missing the point that in an EV the car can be slowed in 2 different ways - regenerative braking and friction braking. When you apply the brake pedal, the software in the EV can choose how to slow the vehicle based on how hard you press the pedal and what the speed of the vehicle is. Of course if the friction brakes are slowing the car down you will eventually get less regen. But there is no reason when the brake pedal is initially applied that you can't use regen to slow the car until you max out regen and then engage the friction brakes.

 

[brianman]

(a) I was surprised that the brake pedal did not increase regen to the maximum. I was curious as to why Tesla had not engineered this into the Model S. (b) I've yet to hear an answer other than "one pedal driving is better" which I don't view as a satisfactory answer, since there are no perceived downsides to having the brake pedal maximize regen.

(a) Your use of the word "maximum" is loose here. My understanding is that Tesla's algorithm decides on a "maximum" level based on a variety of factors. You think they should use a different "maximum". That's fine but that doesn't mean that while braking the Model S isn't using the Tesla algorithm maximum. Perhaps a terminology shift would help focus the discussion in a fruitful direction.
(b) Your phrasing here makes it impossible to counter your assertion, so I won't bother. "I don't perceive why you're confused" and, equally, that's irrefutable. You can't refute my perceptions any more than I can refuse yours.

 

[dennis]

(a) Your use of the word "maximum" is loose here. My understanding is that Tesla's algorithm decides on a "maximum" level based on a variety of factors. You think they should use a different "maximum". That's fine but that doesn't mean that while braking the Model S isn't using the Tesla algorithm maximum. Perhaps a terminology shift would help focus the discussion in a fruitful direction.
(b) Your phrasing here makes it impossible to counter your assertion, so I won't bother. "I don't perceive why you're confused" and, equally, that's irrefutable. You can't refute my perceptions any more than I can refuse yours.

OK, I'll try to state my query in a simpler way:

Is there ever a case where stepping on the brake pedal causes the Model S to increase regen?

That was my original question.

 

[brianman]

Is there ever a case where stepping on the brake pedal causes the Model S to increase regen?

I'm not sure what happens when you have the accelerator pressed enough that you are not regenning and press the brake at the same time. More specifically, I'm not sure if this engages full regen + friction braking (behaving as if you released the accelerator) or not. I suspect this isn't a scenario you're interested in, though.

 

[jerry33]

I'm not sure what happens when you have the accelerator pressed enough that you are not regenning and press the brake at the same time. More specifically, I'm not sure if this engages full regen + friction braking (behaving as if you released the accelerator) or not.

I'm pretty sure it beeps and tells you that both pedals are pressed. As far as I can tell from driving, there is no more connection between the brakes and regen then there is between the brakes and the engine when downshifting in an old fashioned car and then using the brakes.

 

[brianman]

I'm pretty sure it beeps and tells you that both pedals are pressed.

Right. But does it immediately apply "100% friction + 100% regen" or something else? If yes, then this is one example where pressing the brake pedal (without doing anything with the accelerator pedal) results in the application of regen.

Dennis, if you'd like to call this a technicality I would totally agree.

 

[ken830]

Is there ever a case where stepping on the brake pedal causes the Model S to increase regen?

No. The car is already applying the "maximum" regen it allows when the accelerator is fully released. Stepping on the brakes slows down the car, and regen is reduced as a result of the reduced speed. The car is not actively reducing regen when you apply the brake pedal as you might suspect in your original post.

 

[jerry33]

Right. But does it immediately apply "100% friction + 100% regen" or something else?

If you have both pedals pressed it stops any acceleration. I was going to try it today so that I could describe it better (been a couple of months since I tried it), however traffic was too heavy for me to do this safely.

 

[brianman]

Update...

I'm pretty sure it beeps and tells you that both pedals are pressed.

If you have both pedals pressed it stops any acceleration.

Both of these were understood/agreed.

The open question (for me at least) was whether transitioning from one pedal to two pedal while at speed would invoke regen as part of the "logically lifting the accelerator as part of applying the brake".

I tested this earlier today at 60 mph with a consumption rate of between 20 and 40 kW. I pressed the brake, got the beep, saw the power consumption drop to 0 but not go below zero -- i.e. no regen.

(There were no other vehicles within at least a quarter mile of me ahead or behind.)

 

[jerry33]

That's what I found too. If you are accelerating and press the brake the acceleration stops but there is no regeneration. However, if you are already regenerating the regeneration doesn't stop (that is you have your foot on the accelerator but are regenerating). I tried this a few times to be sure that was how it worked.

 

[Charlie]

This is my first post, so I do wish to apologize if it seems out of place or if comes too late. Since I seem to finally have found the keywords that answer the one peculiarity I completely don't understand about most electric cars, I'd like to ask a question (or maybe it's more like offering an opinion).

The one question about electric cars that has always been a complete mystery to me is why regenerative braking isn't seamlessly integrated with the brake pedal. After all, IMHO, regenerative brakes should be the primary brakes. I've also never understood why there is any need for regenerative braking to activate as soon as the accelerator is released – one should, after all, coast as much as one can.

Some background. My experience with electric vehicles comes from knowing how the accelerator and brake work in these and these.

In the trolleybus, it's very primitive: The accelerator pedal is the accelerator. If one takes the foot off the accelerator, the trolleybus coasts. The brake pedal is the brake, and it controls both regenerative braking and friction braking. In typical driving conditions, friction brakes are not used at all. Usually, the trolleybus is driven in traffic and brought to a standstill using only regenerative braking. The friction brakes are only used to keep the vehicle still.
I'm saying "primitive" because the accelerator controls torque, not acceleration, and torque decreases when the power limit is reached. Integration between regenerative and friction braking is also accomplished in a very simple way: regenerative braking torque is applied proportionally to brake pedal deflection until the pedal is depressed halfway. If one depresses the brake pedal more than halfway, regenerative braking gradually cuts out while friction braking gradually activates (the transition is pretty much seamless).

In the tram, there is a hand controller. It basically has five modes:

• moving the controller forward proportionally controls acceleration (i.e. no matter what the weight and speed, acceleration at a specific position is always the same)
• there is a fixed forward position that maintains speed (basically, cruise control, applies some power so tram does not slow down)
• neutral – tram coasts
• there is a fixed backward position that maintains speed (applies some braking so tram does not increase speed downhill)
• moving the controller backwards proportionally controls deceleration (i.e. no matter what the weight and speed, deceleration at a specific position is always the same)

In the tram, which is more modern than the trolleybus, nobody cares about braking being regenerative or friction. As long as regenerative braking is available, it is used; if regenerative braking is unavailable or too much deceleration is required, friction brakes are applied. Nobody feels the need to separate friction brakes out or to apply regenerative braking when coasting.

So, basically, my question comes to this. Why doesn't lifting the foot off the accelerator simply equal coasting, why can't regenerative braking be the primary method of braking, and why aren't regenerative braking and friction braking seamlessly integrated in the brake pedal?