Tesla & other EV's and hybrids are dangerous!

[langwadt]

I drove a few Toyota hybrids (service centre loan cars).  The regen braking was probably the first, other than it being an automatic, thing I needed to get used to. 

This was 2018 maybe?  Yaris and Auris.  In traffic, when you put your foot on the brake instinctively, for a fraction of a second everything was normal, then regen load kicked in and suddenly you were slowing too quickly and had to modulate the brake pressure off.  If you had to stop braking and start again, this cycle would repeat.  When you got down to about 3mph, usually approaching a car length of two from the car in front, regen would instantly cut out, no decay, no warning, no feedback, it would just cut out it's part of the brake load, requiring you reapply move force for the mechanical brakes again.

Honestly, after a few days my feet just adapted it out and got used to it.

EDIT:  The best bit for me is when they give me my car back.  GT86.  A yaris feels like a small van to me.  The seat is low, bucketed, legs out nearly straight, the car rises around you, you sit down in it in the floor.  The steering is heavy and sensitive, controls responsive, if you ask the car to jerk violently it will do so.  No "smoothing out the driver inputs".

Part of me toys with the idea of putting a Nissian Leaf or Renault Zoe motor and battery in it.... put through the clutch and gear box!  That would be a dangerous EV for a whole different class of reason.  Instant flat torque in all 6 gears.

https://youtu.be/DE2oDKguy3Q?si=rih4IkKdnNbZmA9f

[tom66]

The reason EVs reduce braking power as they decelerate is the motor controller has to drive the motor in such a way to produce sufficient voltage to charge the battery.  This is typically done via constant torque command.  As the vehicle slows down the power output from the motor will decrease as power = torque * RPM.

[Psi]

The reason EVs reduce braking power as they decelerate is the motor controller has to drive the motor in such a way to produce sufficient voltage to charge the battery.  This is typically done via constant torque command.  As the vehicle slows down the power output from the motor will decrease as power = torque * RPM.

A CVT that engages on the motor for regen would be interesting.  Probably pretty hard to make a CVT handle that sort of power.

[default0.0player]

Do EV's generally already do active regenerative breaking? (ie. applying voltage to the motor from the battery, giving it more to push against to keep break force consistent regardless of speed, not just using it as a normal generator.)

Only the ones that use induction motors. The permanent magnet motors generate their own voltage and applying voltage from an external source will not increase the amount of power that can be regenerated.

The "active regenerative braking" you described is called "dynamic braking" which energy is consumed to brake and kinetic energy is converted to heat. This type of braking is most common on diesel-electric trains.

The capacitor is to build up a higher voltage. It's not in the circuit permanently, you can discharge it mostly losslessly to keep it at a given level.

https://www.mdpi.com/2076-0825/7/4/84

I was wondering why by necessity EVs would lose regenerative breaking power at low speed, as is so often said.

regenerative braking is essentially using the electric motor as a generator. The generator has a lowest RPM for it to work. There are mainly two types of electric motor used on EV. Induction motor and permanent magnet motor. An EV is a DC power system, everything except the motor windings have DC, so there's no "power factor"

Induction motor: The inverter generates a rotating magnetic field. If it rotates faster than the motor, the torque is positive, if it's slower, the torque is negative. The difference between the RPM of the magnetic field and the motor is called "slip". The inverter also controls the voltage, the higher the voltage, the lower the "slip" at a given torque, the higher the efficiency. However if the voltage is too high the winding will saturate, causing damage. To regen, the controller sets an AC frequency lower than the motor RPM, this causes the electricity being transferred from the motor to the battery. If the speed is too low, the motor RPM minus "slip" equals to a zero or negative frequency, in this case electricity is consumed rather than generated (regen becomes dynamic braking), so the controller turns off the regen and uses the friction brake.

Permanent magnet motor: The motor can be considered "electronically commutated" DC motor. The inverter controls the frequency to exactly match the motor's rotation, doing the work of the "commutator", and the voltage to power the "DC" motor. The permanent magnet generates a voltage proportional to the RPM, this voltage is called back-EMF. The output voltage of the inverter can be higher, the same, or lower than the back-EMF. The torque is the voltage difference divided by the resistance of the motor. To regen, lower the voltage so the current will be reversed i.e. from the motor to the battery. If the vehicle is too slow, the regen torque is insufficient even the input voltage is zero i.e. short-circuiting the winding (regen becomes dynamic braking).

In conclusion, the regen power is not proportional to the vehicle's speed and regen is lost at a low, but non-zero speed.
One pedal driving mode is just the vehicle engaging friction brake auto-hold before, not at, full stop, compared to "traditional" vehicles that only hold after full stop.

[tom66]

The reason EVs reduce braking power as they decelerate is the motor controller has to drive the motor in such a way to produce sufficient voltage to charge the battery.  This is typically done via constant torque command.  As the vehicle slows down the power output from the motor will decrease as power = torque * RPM.

A CVT that engages on the motor for regen would be interesting.  Probably pretty hard to make a CVT handle that sort of power.

Dual motor EVs do this well, because they tend to have different gearbox ratios for each motor, which allows them to recover energy over a wider range.  Also, some EVs can use the motor as a boost converter, which allows it to regen more aggressively at lower speeds.  Not sure of the exact physics there, but it definitely does vary quite a bit from model to model.

[default0.0player]

Also, some EVs can use the motor as a boost converter, which allows it to regen more aggressively at lower speeds.

Synchronous boost is synchronous buck working in reverse. You can simplify the power train minus the mechanical part as a voltage source(Vi) and the motor with a synchronous buck in between, the voltage applied to the motor is the battery voltage X duty cycle of the buck. If it's higher than the motor's back-EMF(Vo), the current/torque is positive and the car accelerates. If it's lower, the current/torque is negative the motor regens. In the latter case, the motor driver is working as synchronous boost, the back-EMF is boosted to the battery

Edit: It's not "some EVs", all EVs have boost converter to increase the voltage from the motor in order to charge the battery. There's no dedicated boost converter, the motor driver/inverter is being used as a boost converter during regenerative braking.

First, lets picture a simple buck converter

The input voltage is the battery, the output load is the motor instead of R, the Vout is the motor's back EMF. Ideally, the ratio between the output and the input is the duty cycle of the buck converter. The synchronous boost is just mirrored synchronous buck. The Vout is proportional to the vehicle speed due to the back-EMF of the motor. To accelerate, increase the duty cycle so the battery voltage times duty exceeds the motor's back-EMF. To regen, the duty cycle is reduced so the opposite is true. Then we can mirror the buck converter, it's exactly a boost converter with the duty cycle becomes (1-D) instead. This is an ultra-simplfied form and does not apply to a real motor driver.

Then, lets move to a real motor driver, the H-bridge for DC motor
 
Full forward is S1 and S4 on, S2 and S3 off. Full reverse is S2 and S3 on, S1 and S4 off. Coasting (zero torque) is all off. Dynamic breaking is S1 and S3 on, S2 and S4 off, OR S2 and S4 on, S1 and S3 off. Without an external resistor, dynamic braking is going to overheat the motor and/or the switches really fast so it's not actually used on EV.
Forward with speed control: S4 is on, S3 is off, S1 and S2 are switched alternatively. In this case the driver is working as a buck converter described before, the higher the S1 % the higher the speed, the higher the S2 % the lower the speed. If the speed(back-EMF) is high but the S1/S2 is low, the motor is in regen and the voltage is boosted back to the battery.

Finally, for three phase inverter used on EV

It's just like H-bridge, but three phase instead of single phase.
The 6 switches are switched on and off sequentially as the motor spins and the waveform is like this

PWM is used in the same way as above to control the output voltage and when output voltage is below the back-EMF, the inverter is being used as a boost converter to transfer power from the motor to the battery.

[Siwastaja]

Also, some EVs can use the motor as a boost converter, which allows it to regen more aggressively at lower speeds.  Not sure of the exact physics there, but it definitely does vary quite a bit from model to model.

Every EV uses the motor as a "boost converter" - it won't work any other way. For example, for any permanent-magnet motor (BLDC / PMSM), the open-circuit voltage produced by the motor will be always lower than the battery voltage, unless the speed is somewhere around 200-300 km/h or so. I don't believe there is any real difference circuit-wise between cars - of course as you say power = speed * torque and as such, at lower and lower speeds there is less and less to generate, while simultaneously the "boost ratio" gets higher and higher. Manufacturers tune their inverters in different ways, and probably the biggest concern is related to the balance / smooth transition between mechanical braking and regeneration and traction in slippery conditions, not so much trying the squeeze the last % of efficiency at very low speeds.

And regeneration is nothing else but just "short-circuiting" the motor, just at less than 100% duty cycle. Kinetic energy is converted into magnetic field stored in the motor inductance, released at higher voltage during the moments the short is removed, pushed into battery - exactly equivalent to the classic boost converter circuit. Approaching zero speed, duty cycle required to brake approaches 100%, and the boost efficiency starts to drop, where exactly, I don't know, maybe in the 95% duty cycle range the drop would be already significant. And eventually, even a fully shorted motor does not produce infinite holding force at zero speed.

[Marco]

probably the biggest concern is related to the balance / smooth transition between mechanical braking and regeneration and traction in slippery conditions, not so much trying the squeeze the last % of efficiency at very low speeds.

Ideally the car simply has enough power to use the mechanical system as pure fallback, then you can simply dedicate the majority of the pedal range to regeneration and not worry about the transition at all. Keep the mechanical range for emergencies and full stops. Any need to transition forces you into an awkward set of compromises. Do you keep the deadzone large for pure regeneration? Then the mechanical range is harsher. One pedal driving avoids that, but has its own problems.

That's probably why Porsche said they didn't believe in one pedal driving, they can recover energy 99% of the time, because mechanical breaks don't kick in 99% of the time. The luxury of a powerful drive system.

[woody]

Ideally the car simply has enough power to use the mechanical system as pure fallback, then you can simply dedicate the majority of the pedal range to regeneration and not worry about the transition at all. Keep the mechanical range for emergencies and full stops. Any need to transition forces you into an awkward set of compromises. Do you keep the deadzone large for pure regeneration? Then the mechanical range is harsher. One pedal driving avoids that, but has its own problems.

Be that as it may, I would not trade my M3 for a car that has not the same one pedal drive facilities. I don't know how they managed that but it works flawlessly without any extra flappy levers, controls, settings or what not. Just lift my foot off the accelerator, adjust if necessary and I end up right behind the car in front of me nearly every time. And does that make the car inefficient? Not that I can tell. What I do know is that it gives me a shitload of comfort for which I would gladly invest a bit of range, if any.

That's probably why Porsche said they didn't believe in one pedal driving, they can recover energy 99% of the time, because mechanical breaks don't kick in 99% of the time. The luxury of a powerful drive system.

I find that funny. It would be a first for Porsche buyers to let efficiency be a part of the decision to get one.

It might not be the luxury of a powerful drive system but the inability of some Stuttgart engineers to get this right 😀

[default0.0player]

Porsche doesn't use one pedal driving because its motors are too powerful for that, unlike Tesla's 70kW, Porsche Taycan can regen at up to 265kW, if you use one pedal driving mode, releasing the gas pedal will create a braking force similar to a panic stop on a economical car

[woody]

Ah, and it would be impossible to restrain that tremendous regen capacity with some clever software? Using my right foot on the accelerator as the governor?

After all, said foot is capable of finely controlling the same ballpark braking energy when I use the brake pedal in a regular car.

[PlainName]

After all, said foot is capable of finely controlling the same ballpark braking energy when I use the brake pedal in a regular car.

You have to actively press the brake pedal. Imagine if just having your foot slip off the throttle resulted in a crash stop...

[Marco]

Braking in one pedal driving is brake by wire as long as you don't hit the brake pedal, it can be anything the software wants up to the power limit of the drive. So a lot lower in this case, if Porsche wanted to implement it.

[woody]

After all, said foot is capable of finely controlling the same ballpark braking energy when I use the brake pedal in a regular car.

You have to actively press the brake pedal. Imagine if just having your foot slip off the throttle resulted in a crash stop...

Software! After all, there still IS a brake pedal to turn that slip off the throttle into an emergency stop if needed. There is no reason why a Porsche would need to apply the full regen capability right after letting go of the accelerator.

And as I imagine that Porsche engineers are no less capable than Tesla engineers I think not implementing a solid one pedal drive is a choice. But a (IMO) bad one, as this is an extremely usable feature. One of the two that sets Tesla apart from all (AFAIK) of its competition (the other being their supercharger network integrated in the navigation).

[PlainName]

After all, said foot is capable of finely controlling the same ballpark braking energy when I use the brake pedal in a regular car.

You have to actively press the brake pedal. Imagine if just having your foot slip off the throttle resulted in a crash stop...

Software! After all, there still IS a brake pedal to turn that slip off the throttle into an emergency stop if needed. There is no reason why a Porsche would need to apply the full regen capability right after letting go of the accelerator.

As a non-EV driver I, perhaps naively, assumed that the throttle would mimic ICE in that taking your foot off would apply (fake) engine braking. Comments here seem to suggest it is a lot more than that, hence my comment. And, thinking about it, how can it be a single-pedal thing if it doesn't apply quite serious stopping power when released? So just how hard that braking effort should be is the question, and I think it would be potentially unsafe to err on the side of 'lots'.

[paulca]

Mechanical servo brakes will provide at least full application of brake without any external or internal "power".  If the car simply turns off due to a failure the ESC etc shuts down, the mechanical brakes will still work at least to stop you.

I had a renault that once the brake servo was depleted you could NOT apply the brakes at all.  Full weight on the pedal and pulling up with my arms on the steering wheel, over 100kg of pedal pressure and I couldn't stop it from rolling at 3mph and had to use the handbrake!  In the manual it has a big red warning section that if the "STOP" light illuminates or the engine stops for any reason while travelling, "Apply the brake in ONE smooth application.  Do not release the pedal until fully stopped."

[Monkeh]

So just how hard that braking effort should be is the question, and I think it would be potentially unsafe to err on the side of 'lots'.

No more than normal driving would require. Even Porsche can figure that one out..

[woody]

As a non-EV driver I, perhaps naively, assumed that the throttle would mimic ICE in that taking your foot off would apply (fake) engine braking. Comments here seem to suggest it is a lot more than that, hence my comment. And, thinking about it, how can it be a single-pedal thing if it doesn't apply quite serious stopping power when released? So just how hard that braking effort should be is the question, and I think it would be potentially unsafe to err on the side of 'lots'.

Maybe I do not understand your question correctly, but the crux is that the amount of regen (stopping power) in one pedal drive is regulated with the accelerator. Maximum one pedal drive braking power is of course limited to the maximum regen capacity (IIRC ~75kW in a M3) but this maximum power is (depending on the anticipatory qualities of the driver) hardly ever used. If, during normal driving you find that you need to stop for the traffic light ahead, you lift your foot slowly from the accelerator and the car starts to decelerate / regen a bit. You lift your foot more and it starts to decelerate / regen more. If you miscalculate and think, hey, I am not making it to the stopline, you press you foot down just a little bit ->  less deceleration / less regen. With some training by doing (2 hours?) this enables you to stop the car exactly where you want it. Combined with the automatic application of the brake pads when coming to a nearly complete stop this makes for true one pedal drive.

[PlainName]

Maybe I do not understand your question correctly

OK, let's narrow it down a bit. Take your foot off the throttle and how much braking effort (regen or whatever) should occur? Where would it be in the range.. nothing at all, about the same as with a typical ICE, much more than that,  or anchor out the back?

[woody]

Maybe I do not understand your question correctly

OK, let's narrow it down a bit. Take your foot off the throttle and how much braking effort (regen or whatever) should occur? Where would it be in the range.. nothing at all, about the same as with a typical ICE, much more than that,  or anchor out the back?

That depends.

Drive at a constant speed = keep your foot on the accelerator in the same position.
Lift your foot a little bit = coast. No regen, you slowly decelerate due to friction.
Lift your foot a bit more = regen + deceleration from that regen, amount controlled by how far your foot is lifted.
Lift your foot completely off the pedal = over a short period (seconds) increase regen from wherever it is at to its maximum. For a M3 this will not smash your nose in the steering wheel, for a Porsche I wouldn't know. Thing is that an EV does not have to immediately put full regen on; it could (and in the case of a M3, does) soft start this.
And yes, I can see where that might not give you maximum efficiency, but that is a small (if any) trade-off for a very comfortable way of driving a car.

That is all I have to add on this subject as it is a bit off-topic and I'm running the risk of being made out a Tesla fanboy, which would be off the mark

[PlainName]

Maybe I do not understand your question correctly

OK, let's narrow it down a bit. Take your foot off the throttle and how much braking effort (regen or whatever) should occur? Where would it be in the range.. nothing at all, about the same as with a typical ICE, much more than that,  or anchor out the back?

That depends.

OK, apparently you do not understand a simple and straightforward, quite specific question  correctly.

I did actually debate with myself whether to add the line: "None of this 'just like brake pedal', 'lifting gently', etc. Just take your foot right off and what should happen?", but I figured that might be seen as too obvious to mention and probably cause a bit of kickback. But it seems it wasn't obvious at all and I should have added that line

Lift your foot completely off the pedal = over a short period (seconds) increase regen from wherever it is at to its maximum.

So what is 'maximum regen'? Not asking what the Tesla is or what that M3 thing is; I am asking what you think it should be on that sliding scale which we can all relate to.

[Monkeh]

Maybe I do not understand your question correctly

OK, let's narrow it down a bit. Take your foot off the throttle and how much braking effort (regen or whatever) should occur? Where would it be in the range.. nothing at all, about the same as with a typical ICE, much more than that,  or anchor out the back?

For normal setups, whatever amount of regen is selected, which may be quite a bit stronger than normal engine braking (but this is selectable, not least because petrol and diesel vehicles have very different behaviours and drivers will have a preference based on their experience). For one-pedal driving, it should be roughly equivalent to a fairly quick (but not emergency) stop. There's plenty of studies out there which look at driving behaviours to put numbers to that, manufacturers shouldn't find it overly hard to pull off. Unless they're Porsche.

[PlainName]

For normal setups, whatever amount of regen is selected, which may be quite a bit stronger than normal engine braking

Thanks - having the driver be able to select what suits them sounds good, within limits

I wonder, if the regen is set to 'quite heavy' and his foot slips (or he otherwise lets the pedal up), would the driver get nicked for brake testing the vehicle behind?

[Siwastaja]

After all, said foot is capable of finely controlling the same ballpark braking energy when I use the brake pedal in a regular car.

You have to actively press the brake pedal. Imagine if just having your foot slip off the throttle resulted in a crash stop...

Software! After all, there still IS a brake pedal to turn that slip off the throttle into an emergency stop if needed. There is no reason why a Porsche would need to apply the full regen capability right after letting go of the accelerator.

As a non-EV driver I, perhaps naively, assumed that the throttle would mimic ICE in that taking your foot off would apply (fake) engine braking. Comments here seem to suggest it is a lot more than that, hence my comment. And, thinking about it, how can it be a single-pedal thing if it doesn't apply quite serious stopping power when released? So just how hard that braking effort should be is the question, and I think it would be potentially unsafe to err on the side of 'lots'.

The point of "one pedal" driving modes is that you can drive 99% of the time with one pedal. Or maybe 97% or 99.927% or whatever, depending on your typical driving patterns and how the car designers designed it.

Obviously it would not brake at 265kW even if that was technically possible. Indeed this is a thing designers can control however they wish in software. I thought that would be obvious.

One pedal driving mode actually has absolutely nothing to do with regen, even. It could be available in ICE car as well, just no one did it. Even in EVs, one pedal driving mode applies mechanical brakes - again to varying degrees and in varying conditions depending on manufacturer, but I'm 100% certain every single EV with one pedal mode does it in some condition. So a modern ICE car, with automatic transmission and ESP which can apply brakes, could do that as well.

So it's 100% user experience / interface thing.

Nissan Leaf, for example, in one pedal driving mode does not diminish the regen as early (so actually does regen more, which is good for energy), but then also starts applying mechanical brakes well before the car is stopped. Mechanical brakes stop the car from ~walking speed. The amount of energy that could be gained by regenerating from 5km/h to stop is of course completely negligible, and so is wear to brake pads.

User experience and safety are probably the #1 factors, here. Just the right amount of braking force when the user lifts their foot off the pedal. Not too much for "sudden stop". Not too little so that they have to use brake pedal all the time, then it's not one-pedal mode anymore. But it's also to make drivers hit the brakes sometimes, to avoid their brain forgetting about the brake pedal in emergency situations. It's a complex UX problem.

I used one-pedal mode for maybe ~6 months, then reverted back to good old D/B modes mimicing automatic transmission (creep, need brakes to fully stop - with the exception of relatively strong regen down to ~15 km/h or so). So at intersections with YIELD instead of STOP, and not much other traffic, can usually go without touching brakes at all. But still have to use the pedal often enough so that muscle memory does not forget it.

[tom66]

As a non-EV driver I, perhaps naively, assumed that the throttle would mimic ICE in that taking your foot off would apply (fake) engine braking. Comments here seem to suggest it is a lot more than that, hence my comment. And, thinking about it, how can it be a single-pedal thing if it doesn't apply quite serious stopping power when released? So just how hard that braking effort should be is the question, and I think it would be potentially unsafe to err on the side of 'lots'.

I would say one-pedal driving on most EVs is a bit like the kind of braking you get when you put a manual ICE car in 3rd gear and let off the throttle when travelling at say 60 mph.  You will decelerate at say 0.2-0.3g.  Except unlike an ICE the regeneration force is more or less the same at any speed and in some EVs, it can regenerate down to zero speed.

For anyone who is experienced with driving a manual car you would know the idea of putting the gearbox into a lower gear when approaching stop lights, exiting the highway or otherwise.  In most driving schools this is actively taught as a recommended strategy to improve fuel economy, reduce brake wear and improve control over the vehicle.

Regen braking is just like that but everything is on the 'gas' pedal; some EVs (notably the Korean offerings) offer paddles on the steering wheel that let you dynamically control the regen force, whereas others have a fixed setting or a few options buried in a drive mode selection screen.  My ID.3 has a 'D' and 'B' mode, in 'D' mode there is no regen on the accelerator pedal, you get regen only with the brake pedal where the mapping is altered to apply regen when you brake.  In most EVs, the maximum amount of regen does increase a bit when using the brake pedal.