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Regen Fast or Slow?

mfan

Member
Aug 23, 2019
15
2
California
Just wondering which method of slowing down provides better regen. If you're going 45 mph with a stop sign coming up, does it provide better energy recovery if you start to slow from far away and gradually come to a stop, or if you continue at 45 mph and fully release the power pedal (ignoring the whiplash) at the last minute, or is there no difference from the regen perspective? In both cases, the energy from 45 to 0 mph is partially recaptured and partially dissipated as heat. Is the regen more efficient at higher or lower rates?
 

jcanoe

Active Member
Oct 2, 2020
4,382
4,564
Maryland
Coming from driving a GM Chevrolet Volt vehicle this has been explored on the GM-Volt forum and other forums. GM's engineers have stated that the most efficient method to maximize driving efficiency when approaching a traffic signal or stop sign at a distance is to coast until you are right at the intersection. Coasting is an example of Newton's First Law of Motion and is only affected by the friction of the wheels, the tires on the road and the air resistance. Regenerative braking recovers mechanical energy by converting this into electrical energy and then storing the energy in a storage battery or a capacitor. There are conversion losses at each step of the process.

Low level regenerative braking is the closest to actually coasting. Standard or full regenerative braking is less like coasting and somewhat less efficient; of course applying the friction brakes on a vehicle is the least efficient method of stopping. In actual driving the efficiency of either low regenerative braking or standard regenerative braking will be the same for all practical purposes. Starting to slow well ahead of a traffic signal may earn you nasty looks from the drivers who are behind you.
 
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mfan

Member
Aug 23, 2019
15
2
California
Thanks. Yes, coasting is obviously most efficient, but that's assuming you can come to a complete stop at exactly where you want without having to apply any braking either by regen and/or friction. In real life, that's not practical. With Tesla's low rolling resistance, that could take miles depending on your original speed.

My curiosity is on the regen process itself. As you said, regen recovers some of the mechanical energy by converting to electrical. Does that recovery and conversion process work more efficiently at high resistance (whiplash aside) or low resistance (nasty look from other drivers aside)?
 

Rocky_H

Well-Known Member
Feb 19, 2015
7,310
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Boise, ID
My curiosity is on the regen process itself. As you said, regen recovers some of the mechanical energy by converting to electrical. Does that recovery and conversion process work more efficiently at high resistance (whiplash aside) or low resistance (nasty look from other drivers aside)?
It should be very little difference either way, but since you are asking, the lighter, more gradual regen process probably is slightly more efficient with less losses to heat since it would have lower current in the motor regen processing, and current squared is what relates to heat losses in wiring.
 

jcanoe

Active Member
Oct 2, 2020
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Maryland
Thanks. Yes, coasting is obviously most efficient, but that's assuming you can come to a complete stop at exactly where you want without having to apply any braking either by regen and/or friction. In real life, that's not practical. With Tesla's low rolling resistance, that could take miles depending on your original speed.

My curiosity is on the regen process itself. As you said, regen recovers some of the mechanical energy by converting to electrical. Does that recovery and conversion process work more efficiently at high resistance (whiplash aside) or low resistance (nasty look from other drivers aside)?
Bottom line up front (BLUF): In real world driving it makes little difference whether you use Low or Standard regenerative braking as long as you use some form of regenerative braking to recover as much of the vehicle's momentum as possible. Low regenerative braking, moot actually since Tesla removed this setting option, is closer in function to coasting than Standard regenerative braking. Low regenerative braking is theoretically more efficient because the vehicle is covering more distance before it stops. When Standard regenerative braking is used the vehicle covers less distance before stopping.

Anecdotal Reference: The Chevy Volt offers Drive mode (D) and Low (L); D is the most like Low regenerative braking in the Tesla and L is most like Standard regenerative braking in the Tesla. In actual driving tests by Volt owners the results were split. In some driving tests over some terrains the L setting had a slight advantage. In other tests the D setting provided slightly better driving efficiency. None of this was performed under test track conditions.
 

Rocky_H

Well-Known Member
Feb 19, 2015
7,310
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Boise, ID
Low regenerative braking is theoretically more efficient because the vehicle is covering more distance before it stops. When Standard regenerative braking is used the vehicle covers less distance before stopping.
If I get where you're going with that, I don't think that's how the theory works. This is calculus about integrals and the "area under the curve", which is math stuff I really don't want to remember how to calculate exactly. But either with recovering the energy faster over a shorter time, or by recovering it slower over a longer time, it is still the same amount of energy recovered (theoretically). And that's because you are starting with a certain amount of kinetic energy, and by the time you get to velocity = 0, that's all gone. But that's obviously mathematical ideal cases, assuming always the same efficiency of the motor and no difference in resistance losses in the wires. And that is where things are not ideal and have a little bit of difference.

Although maybe you're just talking about low power resistive heating in the wires for a longer time rather than high power resistive heating for a shorter time? I'm not sure which way that would go exactly, but since heat losses are based on current squared, not linear, I think the high current losses would be greater, even if they are for a shorter time.
 
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jcanoe

Active Member
Oct 2, 2020
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Maryland
It is theoretically more efficient to use low regenerative braking because you introduce less of the losses you noted. If we agree that pure coasting is the theoretical ideal way to maximize the distance that can be covered after achieving a steady rate of speed then the less energy routed through mechanical to electrical energy conversion processes remains closest to this ideal.
 

user212_nr

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Aug 26, 2019
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If I get where you're going with that, I don't think that's how the theory works. This is calculus about integrals and the "area under the curve", which is math stuff I really don't want to remember how to calculate exactly.

I think it is nonsense. Lets say that you go 45 -> 20, then you recapture 25 mph worth of energy. Then you will coast at 20 mph to your stop.

Alternatively, with slower regen, you will have 45 mph of wind resistance (between 45 and 20 gradually) and so you would recoup less energy. The lost energy is gone and cannot be recaptured from the air.

This effect is much easier seen at 80 mph where the wind is higher. If you go 80-60 then drive for an hour at 60, then the faster you went to 60 the better.

I notice this on autopilot, and it has been annoying me. They are clearly trying to recapture the max energy by hard braking... no other excuse for it.


Although maybe you're just talking about low power resistive heating in the wires for a longer time rather than high power resistive heating for a shorter time? I'm not sure which way that would go exactly, but since heat losses are based on current squared, not linear, I think the high current losses would be greater, even if they are for a shorter time.

The current losses are probably completely negligible given that the wires to the motor are surely gauged accordingly for their purpose.
 

jcanoe

Active Member
Oct 2, 2020
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Maryland
Energy captured through regenerative braking can be expressed as (-) kW as it is power that is being sent back into the battery. If, just for example, Low regen has a maximum power level of -40kW and Maximum regen has a maximum of -80kW then if we apply Low regen for 10 seconds to slow from 60 MPH to 15 MPH and Maximum regen for 5 seconds to slow from 60 MPH to 15 MPH the amount of total energy that is captured is going to be the same.

Other factors include that Low regen will experience mechanical energy to electrical energy conversion overhead losses for twice as long. Maximum regen will experience higher current, increased losses due to resistance as heat losses but over a shorter period of time.

Examining the vehicle's overall efficiency includes how far the vehicle would travel during regenerative braking. For Low regen, assuming starting at 60 MPH and slowing to 15 MPH, I estimate the vehicle would travel a total of 550 ft in 10 seconds. For Maximum regen the vehicle would travel just 275 ft in 5 second. Assuming 250Wh/mi energy consumption when driving at 60 MPH the extra 275 ft traveled represents ~5% or about 12.5Wh. Over the additional 5 seconds this is insignificant at less than 2W but it does add a small amount to the vehicle's overall efficiency while using Low regenerative braking versus Maximum regenerative braking.

I don't have the source but studies have been performed that show that regenerative braking can recover, convert up to ~70% of the vehicle's momentum and store this energy in the battery.
 
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Rocky_H

Well-Known Member
Feb 19, 2015
7,310
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Boise, ID
The current losses are probably completely negligible given that the wires to the motor are surely gauged accordingly for their purpose.
That's not the question, though. This wasn't asking if either one was significant or negligible or harmful or detrimental, etc. etc. You would be right that neither case would be bad, but the question is just comparison: which would have more loss, and there has to be an answer there. So even if both cases are negligible, one does have more current, which is squared in the calculation, and so would be generating more heat.
 

jcanoe

Active Member
Oct 2, 2020
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Maryland
Despite additional heat loss during Standard (higher level) regenerative braking something about the way the EPA testing is conducted favored Standard regenerative braking over Low regenerative braking. At least this is one theory as to why Tesla removed the user option for Low regenerative braking from newer Tesla vehicles.
 

srlawren

Active Member
Aug 3, 2020
1,167
806
Vancouver, BC, Canada, Eh?
Regen is easily controlled by the power pedal (commonly called the "accelerator" in a gas car). No braking required, regen on high, power pedal allows for fine tuning where you'll stop, to then let off the pedal. It takes about two stops to figure it out and wonder at its simplicity.

@robl45 I would say in gas cars it's often called the "gas pedal". "Accelerator" is also common as you say, and in fact, in the Model Y owner's manual they refer to it as an accelerator pedal. That said, I like "power pedal" (first time I've seen that term used) for EVs with regen as it really is about modulating the power rather than just accelerating.
 

user212_nr

Active Member
Aug 26, 2019
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That's not the question, though. This wasn't asking if either one was significant or negligible or harmful or detrimental, etc. etc. You would be right that neither case would be bad, but the question is just comparison: which would have more loss, and there has to be an answer there. So even if both cases are negligible, one does have more current, which is squared in the calculation, and so would be generating more heat.

I'm not sure how you are reading what I said, but the "hard braking" will be more efficient. The energy is lost to air resistance which is exponential from speed. Once lost, it can't be recovered. The car recovers the energy, travels at a lower speed and has less air resistance. It then has more energy to recover when coming to a stop.

How significant is unclear, but depends how early you brake, how fast. Definitely on a highway with high air resistance it will make a big difference.
 

Uncle Paul

Well-Known Member
Nov 1, 2013
6,299
7,615
Canyon Lake,CA
While some will obsess over tiny efficiency deltas, most of us will just drive the cars the way we are most comfortable.

Cannot image any significant energy savings to be gained by either coasting and using less regen over a longer distance, than using full regeneration over a shorter distance.

Most just drive the cars as they wish and let the computers figure out the details. Tesla has far more data on driving efficiencies that the average driver will ever know.
 

jcanoe

Active Member
Oct 2, 2020
4,382
4,564
Maryland
Moving beyond EVs for the moment, this would an interesting test case for a hypothetical hypertube. In a tunnel, in a partial vacuum there would be very little air resistance. The hypertube capsule floats via magnetic levitation above the rails. The capsule accelerates until it reaches the maximum speed. At some point you need to start to slow down. Do you coast, use low levels of regenerative braking or apply more magnetic force to brake later and harder as you approach the terminal?
 

Rocky_H

Well-Known Member
Feb 19, 2015
7,310
9,006
Boise, ID
I'm not sure how you are reading what I said, but the "hard braking" will be more efficient. The energy is lost to air resistance which is exponential from speed. Once lost, it can't be recovered. The car recovers the energy, travels at a lower speed and has less air resistance. It then has more energy to recover when coming to a stop.

How significant is unclear, but depends how early you brake, how fast. Definitely on a highway with high air resistance it will make a big difference.
You're talking about air resistance at highway speed. Given that the context of the question was about slowing to a stop, I don't really consider that a "highway" situation. I'm thinking this is like the situation from about 40 mph or lower, where the air resistance differences aren't very much, and I wouldn't be so certain that it's a much bigger factor than the wire heating factor.
 

user212_nr

Active Member
Aug 26, 2019
1,407
875
US
You're talking about air resistance at highway speed. Given that the context of the question was about slowing to a stop, I don't really consider that a "highway" situation. I'm thinking this is like the situation from about 40 mph or lower, where the air resistance differences aren't very much, and I wouldn't be so certain that it's a much bigger factor than the wire heating factor.

Drag force at 20 mph is about 40N, and 140N for 40mph, but actually we are probably talking about more like 5-10mph for coasting.

If you have a wire that is below its capacity it might have 0.01% losses, but when it reaches the capacity it has maybe 3-5% losses (exponential, but still not very much). If Tesla could increase the size of their wires and thereby generate more power, it reasons that they would have done so already. Or, alternatively, the motors could be capable of increased braking power than they currently are providing at max regen. That is to say, Tesla has surely limited the inefficiency of regen as much as possible.

Tesla mysteriously removes regenerative braking strength option in new cars - Electrek

You can see clearly that Tesla is discouraging light regen. I think that says as much as doing the calculations by hand.

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