Model S regenerative braking with brake pedal

[David99]

using both motors for regen is definitely a good idea. I'm pretty sure the dual motor cars are doing that.

As for more regen power. It's certainly possible but it wouldn't get more energy back. The amount of kinetic energy is a given. Harder regen won't return more energy as there isn't more. If you have to use your friction brakes to slow down in a normal situation (not a red light or not someone cutting you off), you have been driving too fast or didn't keep enough distance. Harder regen isn't the solution to an aggressive drive style. It won't make it more efficient. Driving in a way that requires friction brakes in normal traffic is what makes your car inefficient.

One thing that is sometimes hard to understand is that the kinetic energy is not linear to the speed. Double the speed is not double the kinetic energy but four times! To double your speed you need four times the energy. When you regen you have four times the energy as compared to half the speed. If you regen at say 60 kW it would feel moderate at 70 mph, but it would feel much stronger at 35 mph and it would feel absolutely brutal at 20 mph. Not what you want. To avoid that Tesla reduces the regen power as you slow down. Drive 70, take you foot off. You will see the regen power gradually drop down as you slow down while it feels to you like it's the same deceleration.

 

[green1]

As for more regen power. It's certainly possible

Actually, it isn't. Not unless you find somewhere to put it, the battery can't take more than 60kw at any charge level more than half (Or at least that's what Tesla believes because they taper their superchargers down below 60kw by the time you reach a half charge)
The only way to allow more regen would be if you varied the regen and allowed more at lower charge levels, and less at higher charge levels, that would be a disaster from a driving perspective as you'd never know how much regen you'll get until it happens. the consistent 60kw is a much better solution.

 

[Kickin]

Actually, it isn't. Not unless you find somewhere to put it, the battery can't take more than 60kw at any charge level more than half (Or at least that's what Tesla believes because they taper their superchargers down below 60kw by the time you reach a half charge)
The only way to allow more regen would be if you varied the regen and allowed more at lower charge levels, and less at higher charge levels, that would be a disaster from a driving perspective as you'd never know how much regen you'll get until it happens. the consistent 60kw is a much better solution.

And that is where an ultra capacitor would fit the bill nicely. Always dump max regen power to the ultra cap and then charge the main pack from the cap at a constant 60kW... I wish my leaf did this because the regen/brake mixture makes for a crappy transition to friction brakes, it's good, but still felt easily...

 

[green1]

And that is where an ultra capacitor would fit the bill nicely. Always dump max regen power to the ultra cap and then charge the main pack from the cap at a constant 60kW... I wish my leaf did this because the regen/brake mixture makes for a crappy transition to friction brakes, it's good, but still felt easily...

I like the concept, but that'd have to be one massive capacitor.

 

[Kickin]

I like the concept, but that'd have to be one massive capacitor.

Or a smaller pack with different chemistry (SCiB) like that in the FitEV which can take massive amounts of charging power, something like 6C with 4k cycles but very poor subzero performance...

 

[kanylbullen]

1. First, it enhances regeneration above whatever the preset default value is.
(I'd prefer to have more than two levels of control of that as well).
2. After reaching full regen, the brake pedal begins to activate the friction brakes increasingly with increasing pressure/stroke.

The problem is, when you let go of the accelerator completely, you're already at maximum regen.

 

[brianman]

Between my old P85+ and my current P85D, it "feels" like the D has less regen.

Comparing the P85 to the P85D (within hours on some days), I don't feel this difference.

 

[Ingineer]

Actually, it isn't. Not unless you find somewhere to put it, the battery can't take more than 60kw at any charge level more than half (Or at least that's what Tesla believes because they taper their superchargers down below 60kw by the time you reach a half charge)
The only way to allow more regen would be if you varied the regen and allowed more at lower charge levels, and less at higher charge levels, that would be a disaster from a driving perspective as you'd never know how much regen you'll get until it happens. the consistent 60kw is a much better solution.

I don't think this is the case. It will be limited down in steps when on the SC, but that's because it's already built up substantial heat by the bulk phase first. When driving (unless you just finished a SC session), this temp rise will not have happened, thus the pack will easily absorb a short regen session at > 60kW.

If the code was well written, you wouldn't notice in the event that regen was limited, it would just fade the friction brakes in sooner.

 

[green1]

I don't think this is the case. It will be limited down in steps when on the SC, but that's because it's already built up substantial heat by the bulk phase first. When driving (unless you just finished a SC session), this temp rise will not have happened, thus the pack will easily absorb a short regen session at > 60kW.

If the code was well written, you wouldn't notice in the event that regen was limited, it would just fade the friction brakes in sooner.

But the 60kw is already higher than what the SC allows at higher charge states, AND the battery is alerady warm from driving (it heats up both when adding, and removing electricity)

fiddling with code to use friction brakes to compensate for lower regen might help, but likely this would have negligible impact on increasing regenerative capture, AND increase use of friction brakes leading to more wear of those components. You'll also notice that anyone who's ever used a hybrid system like this has refuted the "you wouldn't notice" part, so no guarantee that Tesla could do better.

It seems unlikely that there's a good way to capture any more KW from the regen without a battery that is capable of absorbing more. This may happen in the future, but it's obviously not as simple as "tesla should just enable more regen"

 

[Ingineer]

But the 60kw is already higher than what the SC allows at higher charge states, AND the battery is alerady warm from driving (it heats up both when adding, and removing electricity)

fiddling with code to use friction brakes to compensate for lower regen might help, but likely this would have negligible impact on increasing regenerative capture, AND increase use of friction brakes leading to more wear of those components. You'll also notice that anyone who's ever used a hybrid system like this has refuted the "you wouldn't notice" part, so no guarantee that Tesla could do better.

It seems unlikely that there's a good way to capture any more KW from the regen without a battery that is capable of absorbing more. This may happen in the future, but it's obviously not as simple as "tesla should just enable more regen"

I'm not intimating Tesla SHOULD do it, just that it's possible. The pack doesn't heat nearly as much from driving as it does from Supercharging. (by an order of magnitude!) While the LEAF is a different car, it works in this exact manner already and I've seen it hit up to 50kW regen. It also has no thermal control of the pack and it's less than 1/3rd the capacity of the MS. It will get almost no regen after a DC quick charge, or coming off a full L2 charge, but it gets great regen at other times. Nissan's fade algorithm isn't the best, but it's pretty smooth and it's very hard to tell without instrumentation what % is friction vs. regen. (The LEAF's pedal-off regen is pretty anemic)

 

[green1]

Except the leaf tops out at 50, the Tesla already tops out above 60 (hard to tell how much higher due to the non-linear gauge, but it could be a fair amount), that's a non-negligible percent higher that Tesla is already doing. It's counter-intuitive to use this as proof that Tesla can in fact do more regen when we haven't seen any proof that it is possible in any consistent way. Tesla are the only ones who have enough information to be sure, but I have trouble believing that they could do it but are holding back. it's more likely that it isn't that simple.

 

[brianman]

I'm not intimating Tesla SHOULD do it, just that it's possible. The pack doesn't heat nearly as much from driving as it does from Supercharging. (by an order of magnitude!)

I'm not sure we know this to be true for all driving conditions, especially the "order of magnitude" part.

 

[Ingineer]

Except the leaf tops out at 50, the Tesla already tops out above 60 (hard to tell how much higher due to the non-linear gauge, but it could be a fair amount), that's a non-negligible percent higher that Tesla is already doing. It's counter-intuitive to use this as proof that Tesla can in fact do more regen when we haven't seen any proof that it is possible in any consistent way. Tesla are the only ones who have enough information to be sure, but I have trouble believing that they could do it but are holding back. it's more likely that it isn't that simple.

It's definitely not simple. To implement the brake pedal stuff, they have to mess with critical safety code, so lots of validation testing must be performed. I bet we'll see it at some point, but their software team is clearly overworked as it is.

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I'm not sure we know this to be true for all driving conditions, especially the "order of magnitude" part.

Well, I'm sure you CAN abuse the drivetrain and waste a lot more energy, but I'm talking average driving. In most cases this doesn't exceed 30kW average, so that would be roughly 1/3C discharge which is nothing on Tesla's stiff pack. But a 120kW SC session is over 1C, and many existing Lithium Ion chemistries are already close to about half an order of magnitude of charge efficiency loss when only at 1C as compared to 1C discharge.

 

[brianman]

But a 120kW SC session is over 1C, and many existing Lithium Ion chemistries are already close to about half an order of magnitude of charge efficiency loss when only at 1C as compared to 1C discharge.

I can't speak for the chemistries available as a whole. But my understanding of the batteries in cars situation is that the battery pack durability and lifetime offered by Tesla in cars is leading the industry. Further, my understanding is that the limitation (taper) at superchargers is related to the impact on battery health not at all related to charge efficiency concerns.