Model S Regenerative Braking is Different Than Hybrid Regen..Why?

[green1]

I'm certainly expecting that from Tesla. A P85D should be capable of about 220kW of regen above the critical speed, if the pack can take it (the 60kW the car permanently does on the rear motor, plus the front motor running at its 160ish kW maximum.) That's almost half of a full on panic stop, if I did my math right - and still less load on the cells than the Volt does every day (less than 3C - the Volt's 60 kW limit is almost 4C.)

Even if Tesla doesn't want to exceed the supercharger limit, they could double the regen when the battery is in the range where it can take it - and with the new systems, seamlessly replace it with friction braking when the battery can't take the power.
Walter

What makes you think the pack can take more than the 60kW being used now? keep in mind that the superchargers can only do 120kW, and only if the pack is basically empty. When the pack is near full it uses a LOT less at a supercharger.
You wouldn't want your regen to brake a different amount depending on the battery level, you want it to be predictable, which means you make it the max that the pack can take when near full, not the max it can take at empty.

Likely the battery is the limiting factor here, you don't want more than 60kW when the pack is near full or you'll damage the pack. and you don't want it to vary with charge or it won't be predictable in normal use.

 

[HankLloydRight]

That can't happen. Regen occurs when the motor is rotating and rapidly goes to zero as you approach low speed. Can't stop the car and there is no regen when car is stopped. Unless the brakes are being lightly applied, will not stop. If it does stop on downhill, something else is stopping it.

Well, it has happened to me on this hill with no brakes applied. Next time I have a chance, I'll record a video of it, but the problem is that it's a one-lane road with a fair amount of traffic, so even though I've been on this hill a few times since then, it's not possible to just stop on the hill and block traffic. But next time I'm there late at night or with low/zero traffic, I'll try to get a video of the regen pulling the car to a complete stop on the hill.

 

[shelbri]

I think one difference may be the creep mode. If Creep is off, the car can stop without braking. However, with it on the car will creep unless you are going uphill creating resistance above Creep mode's force. Not sure why it would stop pointing downhill though.

 

[Saghost]

What makes you think the pack can take more than the 60kW being used now? keep in mind that the superchargers can only do 120kW, and only if the pack is basically empty. When the pack is near full it uses a LOT less at a supercharger.
You wouldn't want your regen to brake a different amount depending on the battery level, you want it to be predictable, which means you make it the max that the pack can take when near full, not the max it can take at empty.

Likely the battery is the limiting factor here, you don't want more than 60kW when the pack is near full or you'll damage the pack. and you don't want it to vary with charge or it won't be predictable in normal use.

With regen in the brake pedal and the new instant brakes, why do you need a predictable regen level? If the car can make the user experience exactly the same no matter what, why not take the extra regen when the battery can handle it, and use the pads when it can't?

Superchargers prove the pack can take more than 60 kW for much of the operating range - and on the lower end I expect it can take more than the supercharger gives for a few seconds at a time. The new brakes should give Tesla the ability to blend regen in seamlessly where they choose to.
Walter

 

[jerry33]

I have a hard time understanding why anyone would want regen on the brake pedal.
1) If you have to stop suddenly, you'll need more power than regen can provide so there's no gain there.
2) With regen on the brakes any time one of the safety features kicks in regen is shut off.
3) If you're just stopping from the last few MPH, there isn't enough energy to bother about.
4) Regen on the brakes is far more complex, so there is a lot more that can go wrong.

I've had cars with regen on the brakes and while I got used to the behaviour, regen on the accelerator pedal give a far superior driving experience.

 

[WarpedOne]

I have a hard time understanding why anyone would want regen on the brake pedal.
1) If you have to stop suddenly, you'll need more power than regen can provide so there's no gain there.
2) With regen on the brakes any time one of the safety features kicks in regen is shut off.
3) If you're just stopping from the last few MPH, there isn't enough energy to bother about.
4) Regen on the brakes is far more complex, so there is a lot more that can go wrong.

Simple: maximum available regen can be too strong. Lift your foot off the "throttle" a bit to fast/high and everybody in the car gets slammed forward.
Motors in P85D can provide 1G acceleration - hence they could also provide 1G deceleration.
Such powerful braking just because you stopped pressing accelerator is a security concern. A big one.

But when you start pressing the break pedal you are actively trying to slow down. Maybe even very "quickly". No excuse not to engage max regen power anymore.

 

[Saghost]

I have a hard time understanding why anyone would want regen on the brake pedal.
1) If you have to stop suddenly, you'll need more power than regen can provide so there's no gain there.
2) With regen on the brakes any time one of the safety features kicks in regen is shut off.
3) If you're just stopping from the last few MPH, there isn't enough energy to bother about.
4) Regen on the brakes is far more complex, so there is a lot more that can go wrong.

I've had cars with regen on the brakes and while I got used to the behaviour, regen on the accelerator pedal give a far superior driving experience.

When the brake system is conventional, I tend to agree with you - all the cars with blended brakes I've driven including my Volt have slightly odd pedal feels.

However, Tesla already decided that for autopilot they needed to go to an electromechanical system with artificial feedback.

That being the case, I'm struggling to understand why anyone wouldn't want regen mixed into the brakes. If Tesla does it right, it should be completely undetectable by the driver - the pedal should feel exactly the same, but you'll get free energy back, have less brake dust, and keep the discs cooler.

I'm not suggesting it as a replacement for the current system, but as an addition - keep the existing normal and low regen settings on the accelerator, but give the car the option to use more regen instead of friction brakes when the situation (including the battery charge level) is suitable.

As far as I can tell, it costs nothing (beyond the software development time,) you lose nothing, and there are potential gains to be had.
Walter

 

[Zextraterrestrial]

...
Motors in P85D can provide 1G acceleration - hence they could also provide 1G deceleration. ...

don't think this is quite true

pretty sure these batteries can output more than they can take in, especially peaks, and you wouldn't want regen to peak out then drop off and get weaker

also the weigh balance would be wrong for decel. big motor in rear capable of higher pull will be unloaded while the front will should take the majority of the down force traction

 

[WarpedOne]

We know:
- batteries can take 120kW from SC
- front motor is capable of 140kW in 85D and 160kW in P85D
- even when weight shifts forward, rear motor can help with at least "half", upping available regen power above 200kW

So, there is no shortage of "motor regen power", the limit could be the battery's 120kW.
Actual deceleration rate while braking with 120kW is dependent on current speed:
- at 50mph that is 0,24g deceleration
- at 40mph that is 0,31g deceleration
- at 30mph that is 0,46g deceleration
- at 25mph that is 0,64g deceleration

 

[green1]

We know:
- batteries can take 120kW from SC

Actually that's false. We know that an EMPTY battery can take 120kW from a SC. We also know they taper off rapidly as the battery fills up. In fact according to other threads on this site, the superchargers taper off to 60kW by half a charge, and by 3/4 they're only putting in about 20kW.
So the regen actually puts in MORE than a SC at anything above a half charge already. (good post here)

Based on that, I can pretty much guarantee that the regen limit is the battery pack.

 

[Cottonwood]

We know:
- batteries can take 120kW from SC
- front motor is capable of 140kW in 85D and 160kW in P85D
- even when weight shifts forward, rear motor can help with at least "half", upping available regen power above 200kW

So, there is no shortage of "motor regen power", the limit could be the battery's 120kW.
Actual deceleration rate while braking with 120kW is dependent on current speed:
- at 50mph that is 0,24g deceleration
- at 40mph that is 0,31g deceleration
- at 30mph that is 0,46g deceleration
- at 25mph that is 0,64g deceleration

Another limit may be somewhere in the regen path, especially electronic devices. While devices like motor/generators, resistors, inductors, and transformers are usually symmetric devices for power flow in either direction, active semiconductor electronics often are not. Often inverters and chargers are different devices. It may well be that while the Model S can generate a blinding level of power from the battery through the inverter(s) to the motor(s), there may well be an asymmetric path for the reverse regen direction that is limited to 60 kW, or maybe 60 kW with the existing rear motor/generator inverter/charger in the P85 and the front set added to the P85D will provide a little extra regen.

I do know that while full regen for short stops comes with only a few percent usage below 100% SoC, I have seen regen limits appear many times with multi-minute mountain descents and SoC's of 80% and up that more or less match the Supercharger taper. My theory is that while the battery can take some pulses of energy at the full 60 kW at 97% SoC and below. Major, multi-minute regen events follow the Supercharger taper limits; same battery, same limits.

Also, my opinion for the 120 kW limit is because of wire and connector sizes used, and not because of fundamental battery limits.

It will be very interesting to see what those first reports look like from the first P85D owners!

 

[AoneOne]

I thought of one more reason you might be able to come to a stop using only regen: due to a trick of perspective, the road is actually level, or even slightly uphill, but looks to be downhill due to a trick of perspective. I have certainly noticed some roads on which the apparent slope while driving on them didn't match the actual slope as seen from a distance. Perception can certainly play tricks on us. A spirit level could be a useful tool here.

 

[MarcG]

I'm surprised this hasn't been brought up before, but one way to optimize energy recovery AND avoid having to feather the accelerator pedal would be to have more regen levels and a more interactive/intuitive way to select them.

Example: paddle "shifters" behind the steering wheel! MB and Cadillac already do this (Smart ForTwo ED, B-Class ED, ELR) and, for someone coming from an auto-manual world, I think it's brilliant.

Use case: cruising on the freeway in low regen mode, you can just lift off the accelerator to coast or "glide" without slowing down due to aggressive regen or the need to feather the accelerator. Then to slow down at a faster rate, pull on the left paddle to increase regen a little. If the situation calls for yet more slowing down, hit that paddle again to increase regen even more. All this can be done without touching either foot pedal. When done with the slow-down situation, resume acceleration and pull on the right paddle to decrease regen levels again.

This can also be very useful in city driving, where different amounts of regen levels are needed to slow down at different rates for traffic, stop lights, etc. I wish Tesla would implement some form of this!

 

[JohnQ]

I'm surprised this hasn't been brought up before, but one way to optimize energy recovery AND avoid having to feather the accelerator pedal would be to have more regen levels and a more interactive/intuitive way to select them.

Example: paddle "shifters" behind the steering wheel! MB and Cadillac already do this (Smart ForTwo ED, B-Class ED, ELR) and, for someone coming from an auto-manual world, I think it's brilliant.

Use case: cruising on the freeway in low regen mode, you can just lift off the accelerator to coast or "glide" without slowing down due to aggressive regen or the need to feather the accelerator. Then to slow down at a faster rate, pull on the left paddle to increase regen a little. If the situation calls for yet more slowing down, hit that paddle again to increase regen even more. All this can be done without touching either foot pedal. When done with the slow-down situation, resume acceleration and pull on the right paddle to decrease regen levels again.

This can also be very useful in city driving, where different amounts of regen levels are needed to slow down at different rates for traffic, stop lights, etc. I wish Tesla would implement some form of this!

But I can use my foot to do this, which is already controlling acceleration, rather than engaging another appendage. The only thing to get used to is that your right foot controls both positive and negative acceleration.

 

[WarpedOne]

The only thing to get used to is that your right foot controls both positive and negative acceleration.

"The other only thing" is that you have got to get used to push that pedal all the time.
Cruise control was invented with sole reason you don't have to do that any more - set the speed and remove the foot off the accelerator.

What harm is there in *quick and simple* switching between standard and low regen modes? What you DON'T want such an option?
Another button on steering wheel scares you?

 

[Zextraterrestrial]

being able to set the right scroll wheel for this could work

 

[JohnQ]

"The other only thing" is that you have got to get used to push that pedal all the time.
Cruise control was invented with sole reason you don't have to do that any more - set the speed and remove the foot off the accelerator.

What harm is there in *quick and simple* switching between standard and low regen modes? What you DON'T want such an option?
Another button on steering wheel scares you?

I don't follow the analogy with cruise control. That function is designed to hold a constant speed (or separation distance if ACC) rather than relying on my foot. What is being proposed is a way to control your speed dynamically with your hand rather than your foot. Why not have an acceleration paddle to go with the deceleration paddle? Why not activate your brake with your hands? Yes, I did that in a way when I drove a manual transmission. I find this to be more elegant.

I can, of course, ignore the control if it's there. The question is whether it's a significant enough improvement for a large enough population to make it a worthwhile change.

 

[jerry33]

I'm surprised this hasn't been brought up before, but one way to optimize energy recovery AND avoid having to feather the accelerator pedal.

I don't understand why you wouldn't want to feather the accelerator pedal. Feathering it is what you do when you accelerate.

 

[jcaspar]

+1
With experience you will learn to use the accelerator pedal to modulate the regen. Want less? Let off the pedal less. For those used to driving true manual transmissions this comes naturally. I don't see how having a selector would be of any benefit.

 

[bxr140]

I don't see how having a selector would be of any benefit.

I understand it. I love the way regen works now and don't want to change if fundamentally, but occasionally I wish I could take my foot off the left button and not go to Max regen. I'd also occasionally like more than 60kw on regen. There has to be an intuitive and ergonomic solution to supplement the existing characteristics and accommodate a wider range of regen options.