Mechanics of MS brake regeneration

Mechanics of MS brake regeneration

I was wondering if someone here understands the mechanics behind the brake regeneration system on the Tesla. I also have a hybrid ICE which uses brake regeneration as well. When I brake lightly, it shows me it is using brake regen to charge the battery (not using the brake pads at all). If I press on the brake harder, it simply regenerates the battery faster. If I press the brake pedal hard enough, the actual brake pads of the car kick in. This system seems very efficient and intelligent.

In speaking with reps about my MS, it seems it doesn't work that way: When you let off the gas, aggressive regeneration kicks in automatically. But if you press on the brake pedal(even a little bit), it doesn't increase regeneration but instead activates the actual brake pads.

So my questions are:
1. Do I have that correct with the differences between my hybrid car and MS?
2. If true, why would Tesla design it that way? My hybrid system makes much more sense: it uses less battery and regenerates more energy. It would work even better in the Tesla because you wouldn't need to "adjust" how aggressive you want regen to be. It would simply increase regen as you increasingly apply the brakes until it reaches the maximum and then would automatically transition to the brake pads.

Also, new MS drivers wouldn't have to "get used to" the aggressive regeneration...


jbunn | 20 september 2013

1) Yes, you are correct in so much as regen is controlled soley by the "go" pedal. The brake pedal has no part in the regen system

2) I'm assuming you do not have your Tesla S yet. I have had mine for 5 months. After a few minuites of driving, it's very natural. The S is designed to be operated more or less with one foot. And that makes even much more sense!

nwdiver93 | 20 september 2013

You are correct in how the MS regeneratively brakes. However I disagree with your assessment that the MS system is inferior. The MS does require a little more time to master but it becomes second nature very quickly. The thing I hated about pushing the brake to activate regen was that I never knew when the brake pads started to engage. In my MS if I want to slow quickly I just let off the accelerator. If that's not enough, it usually is, then I apply the brake. I hardly ever use the brake pedal.

bronto | 20 september 2013
I disagree that it makes more sense. There are many times when you need to brake a little harder or a little faster than the regen default. That is when a lot of energy is lost to the brake pads that could have gone to the battery.

Would you at least concede that the brake pedal should also tie into the brake regen system and transfer that excess energy to the battery before using the brake pads??

bronto | 20 september 2013

In my hybrid, there is a large gauge that shows you how much is going to the battery as you are braking and it kicks on the brake pads when it reaches the limit of the gauge. This is what Lexus told me anyways...

shop | 20 september 2013

The regen isn't aggressive if you just just ease off the accellerator. The regen varies by how much you ease off.

In practice, it means you end up using a single pedal for most of your driving. In normal urban driving, you can speed up quickly and then slow down as you approach a light or traffic all with one pedal. In highway driving, you just modulate your position in traffic with the single pedal.

It all feels quite natural, and the result is that you are always in control of the car and driving it. In other cars, when you lift off the accelerator, you are coasting at a fixed drag rate (the ICE engine causing most of the drag through energy losses in the pistons and tranmission), but you are not able to control the exact speed or relative traffic location of the car. You also flip from accelerator to brake and back again frequently.

I suggest you test drive a Model S. You'll find that it is very fun way to drive. I describe it as having the downshift and clutch modulation ability of a manual tranmission, except with one pedal, no noise and a heck of a lot more energy efficiency.

Brian H | 20 september 2013

The regen does not have to be on full. Lifting the goose pedal partway gives part regen. You'll get the hang of it quickly.

shop | 20 september 2013

Also, when you do use the brake, that doesn't mean the regen turns off. The regen is still occurring at the full amount consistent with the speed of the vehicle. The brakes just add more stopping power.

mrspaghetti | 21 september 2013

@shop +1

Test drive it.

Another reason not to tie regen to the brake pedal at all is safety. The more complex your brake system, the more opportunities for failure. Brakes are better off being dirt-simple IMO; push brake pedal, engage pads.

tes-s | 21 september 2013

@bronto - the MS optimizes regen.

Simply lift your foot off the go pedal to begin regen. When your foot is off the go pedal, you are getting max regen.

Whenever your foot is on the brake (assuming you drive with one foot), you are always getting max regen.

Works much better than my Prius, which regens when I press lightly on the brake. I wish there were an indicator so I would know how hard to press to get max regen, without using the brake pads at all.

The MS has the added benefit that if you need to hit the brakes in a hurry, you get the speed reduction of regen even before you foot gets over to the brake pedal. This will shorten the "real world" emergency stopping distance - and with the average age of an MS driver seeming to be around 50, need all the help we can get to our slowing reflexes...

eddiemoy | 21 september 2013


do you even know how much regen you are getting with your inferior regen? model s is capable of putting back 60KW of energy. also it isn't a mechanical system so less to wear out.

model s has the best regen system hands down. the hybrid you are driving can't even put this efficient regen in due to technical reasons.

the getting use to you are talking about is no more than a day of driving and you will wonder why every car isn't like this. only pure electric drive can be done this way,

sorry, tesla designed it this way because they were looking to design the best car. you think you are smarter than the people who do this for a living?

gocken1 | 21 september 2013

@bronto, If you haven't yet got it, think of it this way. As you let off the GO peddle your regen will increase until it maxes out the system and can't increase any more. With your foot off the peddle the MS is putting all the regen Tesla Motors feels comfortable back into the battery. At this level the car is actually breaking fairly hard. The MS is a very heavy vehicle so it takes more energy to accelerate and decelerate from a given speed than most other vehicles.

WhisperingCJ | 21 september 2013

What I like about the Tesla regen model is that when you need to brake in a hurry, before your foot has even reached the brake pedal, you are already braking.
Moving regen over to the brake pedal as in the hybrid model effectively increases your braking distance

Caveat: I'm basing this in reports, videos, and some conjecture. I've not yet had the chance to drive one, never mind have to perform an emergency stop in one :'(

jat | 21 september 2013

IMHO, the primary reason they do it this way is for safety -- you leave the friction brakes alone, and they are a completely separate, well-tested system. When I was considering building a conversion EV, I was going to do exactly the same things because I didn't want to make the brakes drive-by-wire and I didn't have the facilities to do the testing required to be confident in that.

If you really believe that "one pedal driving" is the reason, then why wouldn't you move the friction brakes to it as well? Ie, as you lift your foot off the accelerator, you brake with both regen and friction, and if you want to coast or brake lightly you simply keep light pressure on the accelerator. | 21 september 2013

The other advantage of the Tesla approach is better brake feel--its easier to modulate and control the brakes because they behave like a traditional system. The hybrid braking systems I have used on test drives always feel mushy and disconnected to me. If you read the auto mag reviews, you will also see similar comments.


WhisperingCJ | 21 september 2013

One other factor that may be a consideration is the case when the battery is full.
With the hybrid approach, the point where the physical brakes are used needs to vary

tes-s | 21 september 2013

@jat - you would not move the friction brakes to the single pedal because if your foot slipped off the pedal you would come to a screaching halt with full brakes.

cb9 | 21 september 2013

I have a Prius and a MS. Agree with what was said above. The MS approach makes more sense because it enables one-foot driving (which makes rolling hills feel effortless) and gives you complete control on when you want to regen and when you want to brake. I also feel keeping the brake system independent makes it more reliable and predictable. It does take some getting used to (especially when switching back and forth) but I prefer MS approach by far.

jat | 21 september 2013

@tes-s - so then it isn't single pedal driving.

@kribensa - yes, but since you are already doing it with software, you can adjust that automatically. With the Tesla approach, you leave it up to the driver to react to apply friction brakes when you don't have much regen braking available.

diegoPasadena | 21 september 2013

If anything, the Tesla way is smarter, because it winds up being more efficient. It does the same thing in the last few centimeters of the accelerator's travel when you release it as your hybrid does in the first few centimeters of the brake pedal's travel. In a Tesla, you hardly ever have to touch the brake pedal. But on your hybrid's system (and Ford Focus EV, I think) you don't know when you've reaching the max regeneration and the brake pads start scrubbing off energy and wasting it, because presumably they have designed it to be a smooth transition. Every time you inadvertently press the brakes a little harder than necessary, you lose efficiency. That can't happen in a Tesla.

tes-s | 21 september 2013

@jat - of course it is not 100% single pedal driving. Did you think they put in a brake pedal for decoration??

The MS approach is simple to put regen as part of the right pedal vs the Prius and other approach of putting regen as part of the left pedal.

I drive both and my personal preference is the Tesla approach.

lspitzner | 21 september 2013

To be honest, I like this for another reason, safety. On the highway I had to brake hard several times. The nice thing with Tesla is when you take your foot off the accelerator and move your foot to the brake, your car has already started to dramatically slow down, even before you start braking. That extra second of breaking even before you push down on the break could be the difference between an accident or not.

avanti | 21 september 2013

"One other factor that may be a consideration is the case when the battery is full."

Hmm, that's an interesting point. When the battery is full, where does the regen energy go? Diesel locomotives have huge resistor packs on their roofs to dump energy as heat during regenerative braking. Where does it go in the Tesla?

jat | 21 september 2013

@avanti - when the battery is full, regen braking is limited. People have complained that they expect the regen braking and are surprised that they don't slow down as much.

grega | 21 september 2013

So in the model x, if your foot fully off the accelerator is the maximum regen, I presume the regen breaking will be twice as strong on the 4wd versions as the 2wd version.

Similarly the model S engine and regen might get stronger in future releases.

Would a stronger regen on the accelerator peddle be an issue at some point, that would require the strongest regen to link to the brake, or is it simply not an issue.

(Ps. I realise the 2wd model x will probably have less regen than the model S, with 4wd being similar to model S, just trying to explore the issues)

SamO | 21 september 2013

Why use your feet at all over 20mph. The cruise is well designed and intuitive.

Brakes are for emergencies only.

Brian H | 22 september 2013

Another issue not mentioned above, is that necessarily regen "drags" on the rears only. Braking affects all 4, and the fronts mostly (inertia vectors there when slowing).

Pungoteague_Dave | 22 september 2013

grega - incorrect assumption because at max regen, the Model S already puts as much energy back into the battery that it can accept. Unless the battery and charging technology changes, I would expect regen to feel pretty much the same on future TM cars.

It is a strange feeling to have regen limited when we max charge (every couple weeks we have to max charge due to a longer trip requirement) and the car behaves differently. I have run up toward the rear of a few cars when the car did not slow as it normally does when lifting off the accelerator. I wonder if there may eventually be a suit about this.

For now the car puts a yellow line on the battery charging indicator dial to show that regen is limited, but my opinion is that a car should have consistent brake feel and behavior for safety reasons. We therefore hope that TM finds a way to make it feel like there is regen happening even when the battery is already full.

shop | 22 september 2013

I suppose the software could apply the brake in case of a full battery. But that assumes that the brake is drive by wire, and I don't think it is in the Model S. | 22 september 2013

@Pungoteague_Dave - If you want regen to be available all the time, the simple solution is don't charge to the max. A 99% charge should be enough to ensure regen is available all the time.

I've heard regen may also be limited when the battery pack is very cold, as the batteries are not in a good state to take a quick charge until they warm up. I haven't this myself, but I've never been in freezing temperatures either.

avanti | 22 september 2013

"I suppose the software could apply the brake in case of a full battery. But that assumes that the brake is drive by wire, and I don't think it is in the Model S."

You don't need drive-by-wire brakes to apply the brakes automatically. Lots of cars do that. Any car with adaptive cruise control can do it, and it is also needed for electronic stability control. I assume it is done with solenoid-driven actuators. The following is from the Tesla Model S website:

"Stability Control reacts in moments of under-steer or over-steer by reducing torque and applying the brakes to individual wheels for enhanced control when cornering."

mikefa | 22 september 2013

I don't understand what's all the fuss and why all the debate ... Adding regeneration to the brake pedal on Model S definitely makes good sense to capture as much energy as possible. The suggestion is not an attack on Tesla.

Brian H | 23 september 2013

Logically, any regen you miss out on because of max charging is charging you could have done without.

chrisdl | 23 september 2013

I've heard several people claim that 60 kW is max. regen because that's all the battery/charging system can take. That's incorrect, because you can charge at 120 kW at a supercharger. Ergo, potentially you could regen even more when you press the brakes (if a more complex system was in place connected to the brake pedal).

Personally, I like Tesla's keep-it-simple approach, but it's true that you could squeeze out a few extra miles with a more complex regen system.

I agree with what mrspaghette said: press pedal, apply pads. Don't add complexity to the brakes.

avanti | 23 september 2013

@Brian H:
"Logically, any regen you miss out on because of max charging is charging you could have done without."

Correct, but that's not the point. The two (sub)issues being discussed are:
1) Where does the extra energy go when the battery is full? (Answer: It is not harvested in the first place.)
2) Is is unacceptably strange that (because of #1), the car behaves very differently when the battery is full.

I am inclined to conclude that applying the brakes to simulate regen when the battery is full would be a worthwhile refinement. One might argue that this "wastes" brake wear. But, the question presupposes that the driver is trying to slow down, and under this circumstance, braking is the only option.

hordsterMD | 23 september 2013

I love driving with one foot. It took a little getting used to, but now I hardly ever use the brake. One of the joys of driving the car of the future.

hillcountryfun | 23 september 2013

Good thread and lots of good discussion. My question is around regen efficiency, how efficient are the regen brakes? I understand that my Prius regen efficiency is between 60 and 70%. I've also read that Ford was able to get 90% in the Focus Electric. Does anyone know what the regen efficiency is for the Model S?

Thanks -

shop | 23 september 2013

@mikefa: "Adding regeneration to the brake pedal on Model S definitely makes good sense to capture as much energy as possible. The suggestion is not an attack on Tesla."

I don't think you understand how brake regen works on the Model S. When you are pressing on the brake on the Model S, by definition, you have completely let go of the accelerator, which means that brake regen is on FULL. Pressing the brake DOESN't turn off or reduce brake regen. It continues to help stop the car at full regen while you are using the brake.

So, you don't need to add brake regen to the brake pedal since it is already there.

christurbeville | 23 september 2013

I think some are missing the point 60kw is the regen energy because the converter portion of the inverter/converter system is limited to that much power. The 120kw supercarger limit is a battery limit as it goes straight in. In regen the AC comming from the motor must be rectified by the converter into DC to be put into the battery. That converter is 60kw max. So 60kw is the max and it doesn't matter which pedal its on you won't get any more. The inverter on the other hand looks to do 360kw of DC into the AC to run the motor. A quick glace at the web says the Prius does 20kw max regen so the Tesla is 3 times as "effecient" by the odd terminology being used here. And again what pedal its on has nothing to do with how much or how effecient. I agree with most the simple brakes brake and go pedal runs the motor makes more sense and is a simpler system. | 23 september 2013

I don't know what the regen efficiency is, but there are a lot of variables that would skew what the number means. Comparing one car maker's rating with another is quite difficult without more details.

For example, is the efficiency number only putting power into the battery or is it what you get by both charging and then using the power (which will not be as efficient). My guess is all the systems have similar efficiencies if you compare them the same way (and the battery has capacity to store the energy).

Now there is a second component as to how much energy you gain from different systems. I believe the MS does far better than other systems since it can store more energy and the generator/charger mode has far more capacity than hybrids and other low energy EVs.

PaceyWhitter | 23 september 2013

I think one of the main reasons Tesla chose not to incorporate regen into the brake pedal is "brake feel." One of the main complaints concerning regen braking is that it makes the brake pedal feel "mushy." While people have grown to accept that this mushy brake feel is necessary in low price hybrids, that would not be acceptable in a high-end car. By putting all of the regen on the accelerator, Tesla elegantly avoids this problem.

NYC Realtor | 23 september 2013

I love the single foot driving in my MS but often wish for more than 60kW of regen and need to wear break pads when I would have preferred powering up my battery.

While I understand that the inertia vector will reduce traction at the rear wheels so that you may not be able to get the full 300kW that you have available for acceleration, you could surely have a lot more than 60kW. And as stated above, the superchargers already charge up to 120kW. So why limit to 60, any ideas?

shop | 23 september 2013

For people that want to know more about the engineering details behind Tesla's cars, check out the OLD Tesla blog posts. Here's one that talks about regen braking - the comments are worth reading too.

It specifically addresses why you can't regen brake to 0 MPH - the energy conversion losses in the inverter and motor are too high - you end up with a discontinuity. ie. it would probably brake too harshly at low speeds. And stability might be one reason why it can't brake any more than 60 kW. Perhaps we will see more regen braking in an AWD vehicle?

NYC Realtor | 23 september 2013

@shop: but if stability is the reason for max 60kW, then 60 would be too much already, since it will surely lock up the rear wheels driving with anything less than spike tires on snow and ice. The blog states that stability control reduces regen if wheel slip is detected and that they tested it out on that frozen lake.

So then I still don't understand why you can't regen at 120kW and have it reduced the instant a slip is detected. Would be interested to hear any ideas on that?

mikefa | 23 september 2013

Why all the fuss and debate regarding which regen method is better?... as much as we all try to avoid using the brake pedal in order to avoid wasted energy, but there are times when it is inevitable to hit the brakes due to unexpected driving conditions or unanticipated situations... so, isn't it obvious to have both methods on the Model S the ideal situation since the whole point of regen is to recapture as much wasted energy as possible?

DTsea | 23 september 2013

mikefa, when you use the brake pedal you ARE getting max regen already. What you are actually proposing is to reduce the max regen available when lifting the accelerator and 'save some' for brake application. Why bother?

Again- you are BOTH regenerating MAXIMALLY and BRAKING when you use the brake pedal.

No change required or advisable.

Brian H | 23 september 2013

The 120kW starts out as AC. Too much for the rectifier (AC→DC).

And those adapted to the goose pedal generally only use brakes for the final 5 mph at stoplights. Where are you putting on pad wear?

shop | 23 september 2013

Good point Brian. Regen generates AC current, which must be rectified to DC to charge the battery. A 60 kW rectifier is pretty beefy as it is. I would imagine a cost trade off there somewhere where the cost of doubling the regen capability just wasn't worth it. As it is, I suspect the Model S 60 kW regen is much better than anything else out there. Anyone know what the regen power output for a Prius, or Leaf is?

And yes, the brake pad wear at 5 mph is pretty small compared to what an ICE does every stop. I have heard zero comments about brake pad wear on any of the forums even though we have some high mileagers out there.

shop | 23 september 2013

To answer my own question, it seems that the Leaf maxes out at 30 kW of regen braking, while the Prius is at 20 kW.

Miggy | 24 september 2013

Question: do the broke light come on when the car is using regen braking? and if not what is the chance of getting rear-ended.

Miggy | 24 september 2013

sorry, do the brake lights come on not the broke lights.