Regen discourages coming to a full stop

[Ben W]

I believe the reason that regen cannot get the car all the way to a stop is simple physics. The kinetic energy of the car (which regen braking is harvesting) is computed as:

Ek = m * v**2

So kinetic energy is proportional to the square of velocity. That means it decays non-linearly as the car slows down. At a walking pace, the car does not have enough kinetic energy to drive the drivetrain running as a generator, and the car keeps rolling.

Better physical scientists or engineers than me (I am a neuroscientist by training) will, no doubt, refine my explanation!

I'm not sure this quite explains it. Suppose you want to use regen to decelerate at a constant rate. The kinetic energy of the car at 5mph is only 1% of what it is at 50mph, but the required "regen" (in kW) at 5mph is also 1% of what it is at 50mph, so they should balance out. In my experience the regen applies a roughly constant deceleration (perhaps 0.1g) down to about 5mph, then tapers off below that. This is probably a comfort issue; the sudden stop at 0mph might feel rather abrupt if there were no tapering.

In any case, if regen is consistently applied from 50mph down to 5mph, that means the regen is capturing 99% of the available kinetic energy. (with ~30% losses converting to electricity of course.) The benefit of regenerating the last 5mph is negligible. But I'd still be curious to hear Tesla's official answer/reasoning about this.

 

[djp]

The benefit of regenerating the last 5mph is negligible. But I'd still be curious to hear Tesla's official answer/reasoning about this.

This old Tesla blog post does a great job of explaining regen on the Roadster.

The Magic of Tesla Roadster Regenerative Braking | Blog | Tesla Motors

I should also note that the motor and controller can deliver the torque command at any operating speed, including 0 mph. This means that we can regen the car to a complete stop. But as a practical matter, the kinetic energy of a slowly moving car is low enough that very little energy is put back into the battery as the car comes to a stop. In fact, the last little bit of slowing the vehicle down generates such a small amount of energy that it does not even cover the fixed losses in the inverter and motor.

I miss the Tesla technical blogs - wish they would do the same with the Model S!

 

[pharmacydave]

I think that's probably what the i3 does, as it can come to a complete stop without stepping on the brakes.

YES,
I have an Active-E for another 45 days. I love this car!, can't stand the i3, One of the best things BMW has is the "one peddle driving". The re-gen brakes are very very strong. I hope Tesla will have this as a future setting on the car. On my test drive, this was the only thing BMW did better than Tesla. I would also like Tesla to explain cons vs. pros of this.

 

[SeminoleFSU]

I too find myself trying not to lose that last little bit of momentum and find myself rolling through stops more in the Model S than I have in previous ICE cars. it is something I noticed myself doing already, but when my GF called me out on it and said "you're gonna get a ticket or run into the back of somebody" I started tried to be more vigilant and come to 100% stop more consistently.
I have creep turned on, and I think it might make things worse... Basically, when I'm really trying to get the most out of regen, I usually let the car slow as much as it possibly can which is to the point when regen stops (and you start coasting) and then creep kicks in and you start moving forward again.. which actually makes the car start rolling forward a little faster... Right before that happens is when I start applying the brakes, or at least try to.. It's difficult to time it right when someone is in front of you because you can't predict what they'll do or how long the light will stay red... But, when you're the first one rolling up it is much easier to time just right... although I think it drives most ICE drives nuts to be behind someone approaching a stop sign or light so slowly.. They don't care as much about deficiency usually, and their idle is usually much higher causing them to lightly ride their brakes behind you... which is where their frustration comes in.
I'll admit not stopping completely is a risk for a ticket or worse, so it is something I'm going to try and consciously work on.. I already did it with my previous ICE cars, but the Model S and my desire to use the least amount of energy has compounded the problem somewhat (or so it seems)

 

[AmpedRealtor]

While I love the regenerative braking on the Model S, I really wish it would take the car to a complete stop. I find myself slowing down long before a red light and hoping it will turn green before I have to put my foot on the brake.

Actually this is the ideal way to drive if you want maximum range. The enemy to range is loss of momentum. If you have to slow down and stop at a red light, you've lost all momentum and have to start over. If you gradually slowed or coasted quite a bit ahead of the light, it should turn green and traffic should be moving by the time you get there at a decent enough speed to keep going and/or accelerate slightly to reach your cruising speed.

 

[jerry33]

Actually this is the ideal way to drive if you want maximum range. The enemy to range is loss of momentum. If you have to slow down and stop at a red light, you've lost all momentum and have to start over. If you gradually slowed or coasted quite a bit ahead of the light, it should turn green and traffic should be moving by the time you get there at a decent enough speed to keep going and/or accelerate slightly to reach your cruising speed.

This is it. Just slow down a bit further from the light so that you don't have to fully stop. I find that it allows me to not stop until the light is green three or four times out of five.

 

[AudubonB]

To some of you who have responded in this thread:

If you are in creep, you will not stop. You will creep. That is what creep is all about. Creepiness and stoppiness are mutually incompatible.

 

[brianman]

To some of you who have responded in this thread:
If you are in creep, you will not stop. You will creep. That is what creep is all about. Creepiness and stoppiness are mutually incompatible.

Creeps never stop? How gloomy for the stalked!

 

[AmpedRealtor]

I've heard of "Hollywood stops", where you roll through a stop sign. Perhaps, with Teslas, we should call those creepy stops? LOL

 

[holstein13]

Actually this is the ideal way to drive if you want maximum range. The enemy to range is loss of momentum. If you have to slow down and stop at a red light, you've lost all momentum and have to start over. If you gradually slowed or coasted quite a bit ahead of the light, it should turn green and traffic should be moving by the time you get there at a decent enough speed to keep going and/or accelerate slightly to reach your cruising speed.

I agree this is the ideal way to stop when you have the time and space to do so. Sometimes, the light stays red longer than you would hope for and you must come to a stop or hit the car in front of you. There is no way around this, currently. If the regen took us to zero, it would be better.

 

[AmpedRealtor]

I agree this is the ideal way to stop when you have the time and space to do so. Sometimes, the light stays red longer than you would hope for and you must come to a stop or hit the car in front of you. There is no way around this, currently. If the regen took us to zero, it would be better.

I agree. I would like a third and more aggressive regen option called "sport". I would also like the two existing modes to be labeled "Standard" and "Comfort". This would give regen the same three settings as steering, unifying the language, and a regen mode to satisfy every taste.

 

[Mobster]

I don't use creep and I do come to a complete stop with regen braking (eventually). You just have to gauge the distance correctly.

I DO use creep, however pertinent to your point "....come to a complete stop...(eventually)" I profoundly agree. Even if the eventual stop is against a fire hydrant. hehehe

 

[brianman]

I agree. I would like a third and more aggressive regen option called "sport". I would also like the two existing modes to be labeled "Standard" and "Comfort". This would give regen the same three settings as steering, unifying the language, and a regen mode to satisfy every taste.

While I'm generally a fan of using consistent language, for feature names there is value in having different terminology.

For your consideration, I submit that if I today say { Very High, High, Standard, Low } or { Sport, Standard, Comfort } it's pretty clear that the former is talking about suspension and the latter is talking about steering. (I grant there's some irony in my example of Standard vs. Standard.)

In contrast, if I say "range mode" there has been confusion in the past about whether the topic is charging or driving.

As such, I'd recommend using words that don't collide with the suspension and steering terminology.


All that said...

I agree there are times when something stronger than the current two settings for Regen would be a nice option. But, my understanding is that the current higher regen setting is as much as Tesla wants to go with it. Either for battery degradation type reasons, or some other issue. Also, my understanding is that the regen is as strong (or perhaps stronger) than the Roadster's regen but the vehicle weight makes it relatively speaking less effective.

 

[jerry33]

But, my understanding is that the current higher regen setting is as much as Tesla wants to go with it. Either for battery degradation type reasons, or some other issue.

Like excessive rear tire wear.

 

[AmpedRealtor]

Like excessive rear tire wear.

I use regen heavily and all the time. My rears show no sign of wear after 5,000 miles according to my service center. In fact, they deferred the rotation until they could identify visible wear.

 

[jerry33]

I use regen heavily and all the time. My rears show no sign of wear after 5,000 miles according to my service center. In fact, they deferred the rotation until they could identify visible wear.

Same here. However, if the regen was double...

 

[wycolo]

> If the regen took us to zero, it would be better. [holstein13]

I believe you are missing the point that has previously been made. If the MS stayed in 'regen mode' below the ~5mph cut off point, then battery capacity (KWHs) would be consumed in operating 'electric braking'. Better to simply step on the brake pedal and retain these KWHs in the battery.
--

 

[brianman]

If the regen took us to zero, it would be better.

I believe you are missing the point that has previously been made. If the MS stayed in 'regen mode' below the ~5mph cut off point, then battery capacity (KWHs) would be consumed in operating 'electric braking'. Better to simply step on the brake pedal and retain these KWHs in the battery.

I think you guys are talking about two different things. Holstein is talking about user experience. Some users want to use one-pedal driving almost exclusively.

 

[holstein13]

> If the regen took us to zero, it would be better. [holstein13]

I believe you are missing the point that has previously been made. If the MS stayed in 'regen mode' below the ~5mph cut off point, then battery capacity (KWHs) would be consumed in operating 'electric braking'. Better to simply step on the brake pedal and retain these KWHs in the battery.
--

Brianman is correct. I would like the one pedal experience. I like the variable braking of the current setup. But if it regenerated power stronger and took me to a complete stop, I'd be happier. I don't care if the vehicle uses a tiny bit of power to slow me from 4 mph to 0, they could even electrically deploy the brakes if that would conserve battery, but I doubt it would make any difference at all. In fact, I wish they would go even further and give us hill holding whenever we come to a complete stop, just the same way a lot of other luxury cars do.

 

[brianman]

In fact, I wish they would go even further and give us hill holding whenever we come to a complete stop, just the same way a lot of other luxury cars do.

In case you missed this...

Q7: Uphill starting. The car hold while you start. Without manual brake, making a very fine-tuned uphill start on snow is difficult. Ask you to put in a function to hold it on a steep so you can take it directly from standstill without going a few centimeters backwards.

A(EM): An active hill hold. Instead of creep. Which is generally what people want... and imprecisely achieved by creep. That is something that we're working on. Needs to have the right behavior in all circumstances.
A(JB): We definitely are working on that. We can confirm that.