Once you regen, you will never go back...

Once you regen, you will never go back...

When I test-drove the Model S, the regeneration mode of freaked me out (like auto down-shifting). After having my car for two days now, I don't think I could ever go back. It is amazing once you get use to it, and it takes almost little to no breaking in regular city driving. This will give us a good 75% longer use of our break-pads (which can be one of the most expensive things to maintain on any vehicle, more so on a sportcar with high-peformance pads).

Mark E | 16 november 2012

I'm also keen on regen as it is more like driving a real car with a manual transmission and using engine braking to balance the car as well as slow down. Driving properly and anticipating what is happening means your brake pads last a long time. I have put 1 (one) set of brake pads onto my 928 Porsche in 200,000km...

Looking forward to breathing that with my model s.

Nick Kordich | 16 november 2012

That reminds me, are brake pads included in the service? I thought I'd read that, but I may have been reading too much into the whole "everything but the tires" comment.

Regarding regen, it seemed very natural to me the first time I tried it on the S. In fact, I was a little surprised at how mild it was in comparison to what I was expecting.

Brian H | 16 november 2012

Sort like a motorbike, except more comfy and less windy.

Tâm | 16 november 2012

During the test drive, the first time I released the accelerator I was surprised by the Regen but I loved it immediately and I got used to it, and was able to judge the traffic in front and take advantage of Regen right away! Still wait for my car and miss that Regen feeling :)

dahtye | 16 november 2012

Regen is great in rush hour traffic!

But it's odd when I jump into my old Prius and find that I'm braking at the last minute and seeming to drive like a madman since the Prius is so underpowered compared to the Model S.

DouglasR | 16 november 2012

For those who have the S, I understand that the car does not decelerate as quickly when the battery is fully charged. Is this difficult to get used to? If you rely on the regen for braking, I would think it might be a little disconcerting when it stops working.

sergiyz | 17 november 2012


This is correct, especially if you were on full charge mode and then switched to standard mode before driving the car so you don't forget to do it later.
You can see the dotted yellow line showing the maximum regen current though, it's pretty noticeable.
If you're on standard charge, there's almost no change in regen right after charging. Same if you were on full charge mode and keep it there until the battery discharges below standard level.

Brian H | 17 november 2012

Yes, you're essentially "allowing" the regen to have access to battery cap to send current into.

jerry3 | 17 november 2012


The only times that happens are:

1. After doing a range charge.

2. For the first few minutes when the ambient temperature is cold.

3. After going down a very long mountain.

This happens in the Prius as well, and it's really not a problem. In the Model S there is a yellow line that displays on the speedometer that tells you this (no warning about this in the Prius).

So it's not something that happens frequently (other than the cold weather) and it tells you it's going to happen.

DouglasR | 17 november 2012

I'm not particularly worried, but I live on a hill, and so my first mile or so is always downhill. I wouldn't want to take my foot off the accelerator expecting to slow down (or at least not speed up), and get surprised. However, since I would probably charge in standard mode most of the time, it sounds like this will not be an issue.

RZitrin1 | 17 november 2012

I drove a rental car last weekend and couldn't figure out why the car kept going so fast when i took my foot off the accelerator.

Regeneration is terrific!

Curmudgeon | 17 november 2012

I have a Dodge Ram pickup with an exhaust brake. I think
that I will really enjoy regen when I get my Tesla. I live
on the Oregon coast and there are a lot of hills on hiway 101.
I also have a Chevy S10 that I converted to an EV. It does not
have regen and it really does coast a long way without
engine compression to slow it down.

STEVEZ | 18 november 2012

@DouglasR: That's right, charging in Standard mode means you have full regen available and plenty of battery capacity to deal with your initial downhill. Even after a Range mode charge the regen comes back to full strength quickly once you start driving. It is definitely disconcerting not to have it after you've come to depend on it, but the yellow/orange dotted line on the power meter is hard to miss, so you shouldn't be totally surprised by its absence.

Mark K | 18 november 2012

Regen is so superior it will the new norm, so it's a bit disorienting when it's even temporarily unavailable.

This can be fixed with a future software update.

Whenever the battery is full, the system can reproduce an equivalent deceleration by temporarily dissipating the power in the brakes.

The brakes are fully servo operable already because of ABS and traction control features.

It must be carefully vetted for safety, but it should be possible to replicate the feel of regen deceleration during the battery-full instances, so you have a very consistent feel every time you use it.

It would kick in only very rarely (so you won't lose much efficiency), but it would eliminate ever have to think about it.

Give TM some time to get to this. Right now, they don't have the engineering man-weeks to spare.

But I think it would be a nice finishing touch to make regen second nature for all new age drivers.

DouglasR | 18 november 2012

@Mark K,

I'm not sure I understand what you mean by "dissipating the power of the brakes." Do you mean making the brakes MORE sensitive, not less? Even then, however, regen works without touching the brakes, so how would this help?

jat | 18 november 2012

@DouglasR - he is suggesting that in the case when there isn't available battery capacity for regenerative braking, the software could apply the friction brakes automatically to provide exactly the same braking impact.

I had a Camry Hybrid and now the LEAF which does regenerative braking on the brake pedal -- ie, light braking is done entirely regeneratively, and they seamlessly blend friction braking into it as needed. I think that is better for people who aren't used to driving the car (for example, swapping driving with my wife on longer trips or if she needs to swap cars if she needs more range than the LEAF provides). I am sure I can get used to it and do just fine, but when my wife drives I will probably just set regen to low or off to make it less jarring for her). Cars have a user interface -- you step on the gas to go faster, and on the brakes to go slower -- changing that, even for good reasons, means it is going to be awkward to newcomers and especially those that switch back and forth between cars.

DouglasR | 18 november 2012

Do we know that the friction brakes can be applied by software in the Model S?

jerry3 | 18 november 2012


Depends on what you mean. ABS, VSC, etc. are all done by software (actually firmware in the two or three ECUs that controls these functions).

The brake system itself is totally conventional with the exception that the power assist pump is electric rather than hydraulic.

The parking brake is a totally separate pad and caliper so it's not linked to the standard brake system.

Regenerative braking is only in the motor/generator and is totally separate from the braking system as well.

DouglasR | 18 november 2012


My question was related whether regenerative braking could be tweaked to invoke the friction brakes when the battery is fully charged. I'm thinking it's not very likely unless there is a way to apply the friction brakes via software.

jerry3 | 18 november 2012


Ah, okay. I doubt that is possible because they are separate systems and the friction brakes are more-or-less off-the-shelf brakes from Brembo so there isn't all that much room for customization. Even if there were, you would then have to put in a system that stopped that behaviour if any of the safety systems (ABS, VCS, etc.) kicked in because those systems rely on being able to control the friction brakes absolutely. It's really tricky to get it right, whereas the system that Tesla is using is much easier to implement (and isn't nearly as finicky).

ruprecht111 | 18 november 2012

Having driven my S for three days now, I love the regen. My other primary car is a Cayenne Hybrid. The difference between the two really takes an adjustment. This is because the Cayenne turns off the engine and "glides" when ever possible. I will have to remember which car I am in when I drive, kind of like going from manual transmission to automatic and back.

Mark E | 18 november 2012

"I am sure I can get used to it and do just fine, but when my wife drives I will probably just set regen to low or off to make it less jarring for her)."

She'd get a real shock then if she drove a proper manual transmission. Regen is a lot like engine braking. When I drove a Roadster I loved it, as I particularly dislike the horrid, floaty feeling that automatic cars have.

Getting Amped Again | 18 november 2012

Regen = energy efficient
Braking for no reason = not

cablechewer | 18 november 2012

The first time I drove the S as part of the test drive tour I was disappointed the regen wasn't stronger! The rep said everyone who had driven the roadster seemed surprised that the regen was softer, but they felt that it was a more acceptable setting to most drivers.

Brian H | 18 november 2012

I also heard that the actual charge delivered was equal to the R regen, but the greater mass of the car responded less to the same braking force. And that the limit was inherent in the batteries etc. Don't have the numbers, tho'.

Theresa | 18 november 2012

BrianH, Heard the same thing although my confusion on the limiting to the same Kw recharge as the Roadster doesn't quite make sense since the S can be supercharged which tells me it isn't the batteries which are the issue but rather some other part of the recharge system.

STEVEZ | 18 november 2012

The S maxes out at 60kW of regen. I don't remember if that's the same number as the Roadster; anyone have that figure at hand?

Brian H | 18 november 2012

"Dissipate the energy through the brakes" suggests another (wild) possibility: rather than direct the power from the motor-as-generator to the battery, use it to heat the brakes -- or any other convenient heat dump. As long as the motor is "loaded", any current sink will do.

Mark K | 18 november 2012

@DouglasR -

Yes, absolutely, the brakes are already under firmware control (with a mechanical backup directly linking the master cylinder to pedal pressure).

Many Mercedes and BMW models are now brake-by-wire. My SL has electro-hydraulic brakes and they are excellent - 200-300 milliseconds faster response than purely mechanical systems.

With the advent of ABS braking, and traction control, automakers made the pump and valves servo-operable under firmware control, so just about every automaker has this now.

Of note, certain traction control systems can selectively apply individual calipers to prevent slip at the very same time other wheels are being powered. (See wikipedia on Electronic Stability Control, ESC, ESP, DSC, etc.)

The kinetic energy of the car can be dissipated by converting it into watt-hours in the battery (through regen in the motor), or by converting it into heat in the brake discs and pads.

The brake system is driven by an electric hydraulic pump (not a drive belt hyrdraulic pump like many ICE cars). This provides hydraulic fluid pressure which closes the calipers through the brake lines. The ECU (brake computer) can increase the caliper pressure by electrically controlling that pump and the valves feeding each caliper.

The Model S has a 4 channel ABS system which has a servo valve for each caliper. This is also used to implement the traction control features. The calipers are by Brembo, but the pump and valves are likely from Daimler.

This makes it possible to synthetically produce a comparable regen feel using the brakes (in those few times where the battery cannot accept further charge).

This should kick-in by default whenever the battery is full and regen is enabled, (so the response to the pedal is consistent), but you could deselect it through settings if you prefer.

The reason this is important is that regen constitutes a new user interface that is legitimately superior to the old one. With a single pedal, you can tell the car to go faster, maintain speed, or slow down.

It's very important to the acceptance of this new paradigm that the user be able to count on it working consistently at all times.

Tesla right now is driving this change to regen-style accelerator pedal operation, and this innovation builds their brand, so it's very important to get it right.

"Battery full" is the small hole in this experience that can be filled with the smart-braking suggestion.

Given what's on TM's plate, don't expect this until a little later. But like creep, I think we'll see this emerge.

@Get Amped Soon -

There's no efficiency hit here. With regen, pulling your foot off the accelerator means you want to slow down. If the battery is full, you'd have to manually hit the brakes anyway to slow down. The smart braking feature would just do it for you when it interprets your foot release gesture to execute the slow-down. If you want to coast, just keep your foot in the same place. Don't press, and don't release. In regen terms, that means "maintain current speed".

On TM's Regen Architecture - TM's regen implementation is far more elegant than the Prius. Basically, there is no guessing. When you hit the brakes, you get the brakes. When you use the accelerator, you control the motor. The smart braking suggestion above would be active only in rare cases of battery full and would be transparent to the driver. (Unlike the Prius which is constantly blending the two together and often guesses wrong at your intent.)

TM has already built the state-of-the-art in regen, and will undoubtedly continue to refine it further.

Brian H | 18 november 2012

Braking can't exactly reproduce regen, since the former is 4-wheel, and the latter is 2-wheel. Also, braking can apply much more decel, and can bring the car to a complete halt. So some emulation is possible, but perfect emulation is likely not desireable.

I was suggesting a 3rd alternative to braking vs charging: true heat dissipation by literally dumping heat. Either throwing it away through exposed heaters (e.g., in the brake area, or radiator) and/or taking over other load on the battery at that time (cabin heater, A/C, etc.). The latter strategy would delay discharge of the battery, too, and would likely have to run through some kind of buffer like capacitors. Or it might be way too complex. Don't know.

Mark E | 19 november 2012

Mark K: I have never seen a belt driven hydraulic pump for brakes. Power steering yes. Brakes no. Every brake booster that I have ever seen has been vacuum driven. I learned something today - after doing some research I learned about brakes driven from the steering pump.

To me a scary concept since its possible to drive other cars - albeit with heavy steering - with a broken power steering pump. The idea of also losing brake assist is a worry.

Glad the model s doesn't have a belt driven pump.

Tiebreaker | 19 november 2012

@Mark K: ++1

@Brian H:
The simulated regen could only brake the rear wheels.
Most diesel-electric locomotives use dedicated electric heaters for heat dissipation of the motor-braking produced energy. But you got it, too complex for a car... with Model S maxing at 60 kW on regen, that is a lot of heat to dissipate.

DouglasR | 19 november 2012

@Mark K,

Thanks for the great explanation. You might think about adding this feature to Rod & Barbara's Software Enhancement List. I would do it, but you have a much better grasp of the possibilities.

Getting Amped Again | 19 november 2012

@ Mark K - you wrote:
There's no efficiency hit here. With regen, pulling your foot off the accelerator means you want to slow down. If the battery is full, you'd have to manually hit the brakes anyway to slow down. The smart braking feature would just do it for you when it interprets your foot release gesture to execute the slow-down. If you want to coast, just keep your foot in the same place. Don't press, and don't release. In regen terms, that means "maintain current speed".

The problem with your concept is that the amount of braking is set by the vehicle, so there's no way of knowing whether it's correct for the given situation. Too much braking will require the unnecessary addition of power to speed the car back up, resulting in a slow-down, speed-up cycle that is wasteful. Even your "constant speed" scenario would be too hard, as any variation in foot pressure will cycle between "more current" and "braking".

I know this occurs with regen, but that's adding energy back into the system not dissipating it. The driver should control the amount of braking that is applied, not the car, because only the driver knows when and where the next obstacle or turn is.

DouglasR | 19 november 2012

@Getting Amped Soon,

As I read Mark's comment, he is just trying to insure a consistent regen experience, whether the car is 50% charged or 100% charged. You would release the accelerator or apply the brake in precisely the same way under both conditions. As it is now, you have a different experience at 100% than at 50%.

Theresa | 19 november 2012

Owning a Roadster and having the no regen happen to me was disconcerting the first time or two. After that I was aware of it and drove accordingly. I would not want real brakes applied unless I was applying them for the reasons getamped and Douglas gave. For most of the owners I would not expect them to be fully charging the vehicle everyday so the times that you would do it you plan accordingly.

It was not hard at all to get used to. Really no different than someone who drove a stick shift their entire life and then drove a automatic for the first time. The deceleration is different between those autos.

Getting Amped Again | 19 november 2012

@DouglasR - Agreed, but in my current car I have a different experience whether I stay in full-automatic or use manual downshifting, or whether I drive in Regular or Sport mode. When I let off the gas, I can get full coast or ranges of engine compression slowdown, and it's my choice. I don't want the car deciding for me!

mrspaghetti | 19 november 2012

You guys are getting too complicated trying to draw the friction brakes into this. Why not just dump power into some big resistors during those rare times when your battery is too full to accept the regen? That would be a lot simpler and safer than screwing around with the brakes.

Tiebreaker | 20 november 2012

@mrspaghetti... Like third-row seats barbecue? Tesla and fresh burgers, yum!

Up to 60 kW to dissipate...

mrspaghetti | 21 november 2012


You're making me hungry, stop that.

60kw does sound like a lot, but I'm thinking you only regen for short intervals - say 5 seconds at a shot. That works out to about .083kwh per 5 second regen, which would generate enough heat to raise the temperature of 1 gallon of water by about 19C or 66F.

Resistors might work if they were cooled by the battery thermal regulation system (not sure what the capacity of that is, but I suspect pretty high since it has to be able to handle supercharging). Or else they could add a separate battery just for accepting "overflow" regen, which could probably be pretty small. Or a combination; dissipate some of the heat in resistors and dump the rest in a "regen" battery.

Feel free to call BS if you find errors in my calculations.

Tiebreaker | 21 november 2012

@mrspaghetti: that's OK. Most diesel-electric locomotives use dedicated resistors to dissipate motor-breaking energy, since they have no batteries.

But I think it is impractical for a car, and complicates the design significantly. At max regen of 60kW, and (guessing) voltage of 400V, the current is 150A (400V*150A=60,000W). You need hefty conductors, even for 5 seconds. Then a place for the resistors, and a way to cool them (locomotives have them on the roof). Raising the temperature of 1 gallon of water by 19C (66F) in 5 seconds, that is some burst of energy - 60kW! Then if a long downhill is first thing out of the driveway, it will be much more than 5 seconds. All this to accommodate a short time driving while regen is not available.

So either do it in software and use the brakes, or do nothing and just get used to it.

Please do call BS if my reasoning is wrong...

Epley | 21 november 2012

I love the regen, and it's already changed the way I drive!

mrspaghetti | 21 november 2012


I don't see any flaws in your reasoning.

Maybe another solution (which could be implemented in software) would be another selectable option for charging besides "standard" or "range". It could charge the car to 98% or something, which should leave plenty of room for regen.

Or better yet, Tesla could just give us a slide bar to select our target charge level, with markers on it for "standard" or "recommended" at the 90% level, and another marker at your max charge to retain full regen.


DouglasR | 21 november 2012

I have an idea: why not get the regen to apply the friction brakes to utilize the energy?

Oh wait . . . that's where we started.

Tiebreaker | 21 november 2012

I like the 3rd-row barbecue better!

Aw fughedaboutit! Just teach the driver to apply the friction brakes....

mrspaghetti | 21 november 2012


Ok, let's turn the frunk into a barbeque pit then. That would go well with Elon's suggestion about making cross-country trips too: "If you bring food and stay with relatives, you can leave your wallet at home." With the Frunk-e-que, you don't even have to get propane :)

Tiebreaker | 21 november 2012

Frunk-gating at footbal games!

BYT | 21 november 2012

That would be frunking cool! Minus all that grease inside the frunk after?