Is low regen setting best for highway cruising?

[CHG-ON]

After just doing an 11 hr round trip, I have now found that leaving regen in standard, especially with TACC, seems best. It works especially well for automatic slowing with TACC. So I am going to stick with that. I absolutely love TACC. It's amazing!

 

[Hover]

I came to the same conclusion. Low regen for highway, or whenever the dog is in the car!

 

[bhzmark]

You can use neutral also

 

[jerry33]

I just leave it in standard. I've never found any difficulty on highway trips--some of which are over 5K miles.

 

[FSKT]

Regen is already completely variable with fine control of the accelerator. Set it to high, and modulate with your foot. I don't need another potentiometer in the mix.

Is it, really, though? Are you really modulating the regen, or modulating your speed instead? Two different things actually.

My sense is that actually regen is 100% always on, trying to regenerate, when you've got it on the higher setting. You need to press the accelerator to overcome the resistance of a high regen setting, meaning you never truly "coast" - you're always needing to first overcome the resistance of the regen setting before you can start to move forward, and always apply energy (from the batteries) into the system to keep the car moving forward. There is never a point where you're really "coasting" . . . .

This modulating with the one pedal, then, is really a matter of pressing the accelerator pedal to achieve whatever balance of energy-in v. regen resistance energy-out you're looking for, to move the car forward. IE, with "one pedal" driving, 40mph, for example, is really this: 40mph = energy applied to turn the rotor MINUS rotor "resistance" (which is the regen setting).

I'd love to be wrong about this, but I'm not yet convinced that I am. The Volt Gen 2 addressed this issue with it's regen paddle on the steering wheel. Folks could cruise along, and coast if necessary, using a lower regen setting, but when they needed to slow down they could apply extra regen via the paddle first, and use the brake pedal as well, when needed. I'm not saying for a millisecond that I'd trade my M3 for a gen-anything Volt, but this was a feature of that vehicle that I found to be useful and helped increase energy efficiency.

 

[Silent Ludicrosy]

momentum is a better use than storing energy always

I disagree with you here. Yes, changing potential energy to kinetic energy via gravity is 100% efficient, which is much better than regen+motor efficiency. But your energy advantage in momentum will quickly be lost due to your much higher aerodynamic drag. Just store that energy in the battery where it's safe from the wind.

*Assumptions include highway speeds and grades <10%. If you were to take your tesla down a skateboard halfpipe, it would be more efficient to use momentum to get up the other side versus regen downhill and power uphill.

 

[scottm]

your energy advantage in momentum will quickly be lost due to your much higher aerodynamic drag. Just store that energy in the battery where it's safe from the wind.

Not quite. Don't forget that wind drag is stripped off from the energy moving the car forward, whether you roll that momentum out out, or stuff it in a battery with regen. In other words, what's available to put in the battery is what's left over after you subtract wind drag. That energy is gone, one way or another. Letting the car regen will store energy but the amount stored for use later will not move the car as much forward on that stretch of road as if you coasted out not storing anything.

 

[The Duke]

FSKT, perhaps I can help your understanding of how this works, I will try to explain it simply.
Your idea that regen is on always is somewhat correct, but misleading.
Consider a car that has a motor/generator to push it and at 0% throttle is at full regen and at 100% throttle is at full acceleration.
When stopped nothing happens because your regen puts out 0 volts and your throttle puts out 0 volts. As you press on the throttle the throttle volts "win", current flows through the motor and the car accelerates. Accleration continues until the air resistance equals the power you are putting in.
Now you are at speed and lift on the accelerator reducing throttle input voltage. You start getting regen, the less voltage you put in, the more regen you get. Until you stop. Or you need to speed up and use the throttle to increase voltage and start putting power back in the motor to accelerate.
If you want to say the regen is 100% all the time then you have to say the acceleration is also 100% all the time and where you sit on that line is chosen by the throttle.
At speed there is a balance of power, some in the inertia of the car, some in the battery. The direction of power flow is determined by the throttle. i.e less throttle regen volts are higher and you slow transferring inertia to battery power, more throttle acceleration volts are higher and you go faster transferring battery power to inertia (speed).

From the above you can see the more regeneration that is available the faster you can slow without wasting power on the brakes. Therefore more regen available = more efficiency. TACC is programmed pretty well to maximize regen use.

 

[Silent Ludicrosy]

Not quite. Don't forget that wind drag is stripped off from the energy moving the car forward, whether you roll that momentum out out, or stuff it in a battery with regen. In other words, what's available to put in the battery is what's left over after you subtract wind drag. That energy is gone, one way or another. Letting the car regen will store energy but the amount stored for use later will not move the car as much forward on that stretch of road as if you coasted out not storing anything.

More is lost if you coast than if you regen since drag increases with the square of speed. I did a bar napkin calculation to show the difference:

A P100D is on flat ground doing 60mph at 280 wh/mile. It arrives at a 3% downhill grade. The driver can keep TACC at 60mph, which will result in ~5kw regen downhill, or let the car accelerate to 70mph which will result in 0kw regen. The coasting car will initially have an energy advantage (kinetic plus battery) of about 30kJ. But at 70mph, the coasting car's drag is 36% higher than the drag at 60mph. That 30kJ total energy advantage from coasting will be lost in less than 10 seconds due to higher drag.

If you have a very short/steep hill, coast it out. Under most conditions regen is better.

 

[The Duke]

Correct, AP with max regen is the clear winner when ease and efficiency are accounted for.

 

[scottm]

More is lost if you coast than if you regen since drag increases with the square of speed. I did a bar napkin calculation to show the difference:

A P100D is on flat ground doing 60mph at 280 wh/mile. It arrives at a 3% downhill grade. The driver can keep TACC at 60mph, which will result in ~5kw regen downhill, or let the car accelerate to 70mph which will result in 0kw regen. The coasting car will initially have an energy advantage (kinetic plus battery) of about 30kJ. But at 70mph, the coasting car's drag is 36% higher than the drag at 60mph. That 30kJ total energy advantage from coasting will be lost in less than 10 seconds due to higher drag.

If you have a very short/steep hill, coast it out. Under most conditions regen is better.

..make it simpler to demonstrate laws of physics (conservation of energy) and imperfect energy conversion of modern day electrical systems.

Flat ground. Any speed, but 60 mph is good. Get in any Tesla and drive it.

Sample 1: standard regen.
At point X lift foot off pedal completely.
Begin computing battery charge power. (hard to do but say you could measure this precisely)
Car comes to rest at point Y.
Using only the power returned to battery from this decel cycle, resume driving in any fashion you choose under the car's own power, until that charge amount is depleted. Let's assume (best case) once the energy is depleted, allow the car to roll out rather than halting abruptly.
Car comes to rest at point Z.

Sample 2: zero regen.
Re-do the run arriving at point X at exactly the same speed as first sample. Car can be at any charge level at the beginning of the run.
Flick the car to neutral. No charge is returned to battery.
Car rolls past point Z. Every time.

Repeat at any speed.

Repeat at low regen setting instead of standard.

Sample 2 always surpasses point Z.

 

[The Duke]

..make it simpler to demonstrate laws of physics (conservation of energy) and imperfect energy conversion of modern day electrical systems.

Flat ground. Any speed, but 60 mph is good. Get in any Tesla and drive it.

Sample 2 always surpasses point Z.

Certainly a correct example of physics, but not actual driving. Coasting is best if you are alone on a straight flat road.
Regen wins in real life because we often have to choose between putting power back into the battery (regen) or waste the power to heat with the brakes (coasting). Traffic, curves, up hill, down hill and speed control conspire to make full regen the best choice.

 

[scottm]

Certainly a correct example of physics, but not actual driving. Coasting is best if you are alone on a straight flat road.
Regen wins in real life because we often have to choose between putting power back into the battery (regen) or waste the power to heat with the brakes (coasting). Traffic, curves, up hill, down hill and speed control conspire to make full regen the best choice.

this thread is about highway cruising... we're not talking about using friction brakes vs. regen (the obvious choice is use regen instead of brakes)

it would suck to be on highway where you had to cruise using brakes

if your foot doesn't need to come near the brake pedal for ... hours on end... then regen is simply not needed and you are literally further ahead not using any

 

[Pezpunk]

i feel like the people who think regen is too strong on the highway are in the same category as ICE drivers tapping their brakes every quarter mile. just learn to use the pedals properly and it's not a problem.

 

[animorph]

Yeah, an ideal flat road has no need for regen, coasting, or braking. The reason ICE cars run more efficiently on flat roads usinAtg an accelerate/coast pattern is because ICE is more efficient at wide open throttle. If WOT gives you exactly your target speed, that's great. But if not, then you use WOT to your target max speed and then turn off the ICE and coast to your target minimum speed. Then repeat as necessary. Without an unusually steep hill I'm not hitting regen while I'm traveling at 70 MPH. Nor am I willing to exceed my normal freeway speed by more than a few MPH. So coasting is pretty much not happening for me.

Regen is not actively slowing the car unless you let off the accelerator enough to enable it. It does not cause a drag on the car while the motor is actively pushing the car. It does not cause a drag on the car if you slowly reduce your drive power in order to slow down gradually (while you still have an orange line on the power graph). Regen only adds drag and charges the battery if you let off on the accelerator enough to get the green line on the power graph. And you can control that drag using the accelerator pedal as needed, from approximately coasting to the maximum the car will allow. We don't know how efficient regen is, but hopefully something around 60% - 80% of the regen braking energy is saved in the battery. If you need more braking then regen provides you can add in the friction brakes. Regen remains fully active and charges the battery while you use the friction brakes, but the friction brakes are wasting all their braking energy.

Coasting doesn't waste any energy directly. Regen wastes some because it is not 100% efficient in charging the battery. Friction braking wastes all the energy. The mechanical and aerodynamic drag of the car also wastes energy. Aero drag increases with speed. So it is quite possible to coast downhill and pick up enough speed that the additional aero power loss exceeds the regen losses that would have been incurred by keeping a constant speed with regen. Someone needs to work the math or run some experiments to figure out if coasting at increasing speed or regen is better for specific downhill cases, but I don't think it's a clear win across the board for either at freeway speeds.

At slower speeds, like city driving stopping for a stoplight, coast is probably more efficient most of the time, but I'm not going to try to do that in traffic. I do use regen as much as possible, modulated to hit my stopping point and with brakes just at the end, just like I used to use engine braking and downshifting in previous cars.

 

[Shock-On-T]

Yes it is more efficient to gain speed on the downhills than to charge the battery.
The very best way, if you have sufficient foot dexterity, is to apply the throttle just enough to avoid the green line on the power meter.

 

[cpsss]

I wish there is a "no regenerate" mode, and more, I want the regen to be available on the brake pedal, so it will drive like an ice car, but regenerate while decelerating.

 

[scottm]

I wish there is a "no regenerate" mode, and more, I want the regen to be available on the brake pedal, so it will drive like an ice car, but regenerate while decelerating.

I wish there was a regen off setting too.

At least it's possible to shift to Neutral which accomplishes the same effect... but it's tedious using the shifter... an OFF setting would be better.

Some hybrids do what you say with two-purpose brake pedal. You can end up with a two stage feel to the brake pedal as it transitions from regen to friction. The best ones are more seamless transition so it's hard to detect when it's doing what by feel alone. The worst ones have a grabby transition that lurches a bit when you finally hit friction.

 

[cpsss]

Right. I say there should be either no regen or strong regen that is able to bring the vehicle to a complete stop.

Current "standard" regen is sort of awkward: slows you down dramatically but lose the drag at low speed, plus maybe not working in low temperature, which makes it unreliable in tense urban drive. In many situations, my right foot can't decide while to sit: if I sit on the brake, the car will not coast; if I sit on the accelerator, I will not able to hit the brake fast enough in case I need to.

 

[The Duke]

it would suck to be on highway where you had to cruise using brakes

I live in California and this is often the case and you are right, it sucks.