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You have a lead foot; how does it affect your EV?

08/31/2022 10:22 PM

We know that having a lead foot on the gas and the brakes will cut the MPG on our ICE car. But how will such behavior affect an EV? Does it take more energy to get to a given cruising speed quickly than if we take it easy? Why or why not? And EVs have regenerative braking, so does hitting the brakes hard and often still hurt mileage a lot?

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#1

Re: You have a lead foot; how does it affect your EV?

08/31/2022 10:39 PM

I'm not sure a heavy accelerator foot will make any difference in an Electric Vehicle. A heavy brake foot will have less regenerative braking putting charge back into the batteries. Batteries take time to properly charge.

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#2

Re: You have a lead foot; how does it affect your EV?

08/31/2022 10:53 PM

One foot on the brake and one on the gas, hey.

Well there's too much traffic, I can't pass, no . . .

Maybe Sammy can re-do that video with a Tesla S?

I believe the results will be similar. There is a practical limit to the amount of regenerative braking and the triple loss of efficiency in generating the power (regen), storing it in the battery, and then using the power again when you hit the gas. Maintaining the 0.5mv2 energy to the extent possible results in fewer conversion losses.

Batteries are typically more efficient when charged slowly, so again a fast charge creates more heat and more losses, so slowly braking puts less charging strain on the battery. Similarly, high discharge rates from the batteries are generally less efficient (unless the battery is so cold, that the self-heating actually helps its efficiency).

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#3
In reply to #2

Re: You have a lead foot; how does it affect your EV?

09/01/2022 2:09 AM

Like my new ride?… end of the day, both feet are heavy…

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#4

Re: You have a lead foot; how does it affect your EV?

09/01/2022 7:46 AM

If you drive like a maniac you can expect the same results in an electric car as in a gasoline engine car, about half the mileage as your hypermiling nerd neighbor....

https://www.electric-cars-are-for-girls.com/hypermiling.html

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#5

Re: You have a lead foot; how does it affect your EV?

09/01/2022 9:01 AM

IMHO...

Mathematically, when you accelerate, you are storing energy in your car, some of which may be recovered by regenerative braking. The faster you drive, the more energy is lost to air drag and other friction. Air drag increases as the square of the speed, so the energy lost per mile (drag force x distance) would be 4 times as much at 40 mph as 20 mph. So traveling at a slower speed should be more efficient.

An ICE vehicle has a gearbox because the engine is only efficient over a range of RPM. The result is that higher vehicle speed in this type of vehicle is more efficient due to a lower RPM of the engine in high gear.

"... the friction power (FP) of an engine running on lubricant SAE10W-30 increased by 4 to 5 times, with the increase in engine speed from 1000 rpm to 2000 rpm although load was kept constant at 50 Nm..."

https://www.hindawi.com/journals/at/2014/928015/

Bottom line: I would expect that driving an EV conservatively (lower speed) would give a greater return in efficiency than an ICE vehicle.

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#6
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 9:17 AM

True but I was considering if one gradually reached a cruising speed or the lead foot of flooring it to reach the same cruising speed. Cruising at a higher speed will surely consume more energy.

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#7
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 10:17 AM

So does the time required to get to a given cruising speed have any effect on the energy required? Show your work for extra credit.

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#8
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 12:31 PM

"Higher torque at the shafts increases frictional losses in the reduction gear.

Full acceleration exposes the motor to very high currents, and before the acceleration is achieved, individual motor windings are exposed to those very high currents for longer, which increases thermal losses in the motor."

" Efficiency is simply output (useful) power divided by input power, with the difference being losses due to imperfections in design and other inevitabilities."Mar 31, 2021

Real world test here noted a 7% decrease in range...

https://sciencing.com/calculate-motor-efficiency-6030463.html

https://cleantechnica.com/2018/04/16/how-ev-range-is-affected-by-quick-acceleration/

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#9
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 12:58 PM

Excellent article, thank you. I've had a thought: the energy delivered to accelerate the car is amps*volts*time. If we double the amps, the time should be halved. But there is some resistance in all the wiring. And energy loss is I²R*time. So loss goes by the square of the current. So faster acceleration increases energy lost to resistance, but by the square instead of just proportionally. Now EV wires are big and short and will have a low resistance, so the loss may be small compared to the mechanical losses.

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#10
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 1:45 PM

What equations do you want to see that I'm not sure if rapid acceleration will make a difference? Solareagle's excellent article wasn't sure either but it did perform a reasonable test.

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#11
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 2:43 PM

I think we've pretty much done all the thought experiments, and the tests confirmed suspicions that lead footing will hurt mpg of an EV, but not as much as it does an ICE car. It's been a good discussion. I anticipate EVs will get even better with new battery technologies, new electric motors, and autonomous features (robots are better drivers than humans). It's a good time to be an engineer!

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#15
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 6:05 PM

Until we have superconducting windings in our motors, we are probably at the point of diminishing returns on motors and for that matter, batteries as well. Some people (cheerleaders) want to apply Moore's Law of semiconductors/processors to basic chemistry and physics. Sorry Charlie, only good tasting tuna . . .

I believe there will only be very incremental improvements in motors, power electronics (motor drives/inverters) and batteries. The key to EV'ing the world will be accessibility to charging and reducing the charge time unless we go with some sort of chemical replenishment versus electrical storage mechanism. It's hard to beat the energy to volume ratio for petrol.

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#16
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 7:41 PM

You know, they are trying all kinds of materials and configurations in motors. But yep, with present efficiencies around 90%, there's not a lot of room for improvement, except for lighter, smaller, cheaper motors. I predict that we will migrate to a motor at each corner, part of the hub, eliminating half-shafts, U-joints.

Now batteries are stuck at Li-ion, but many folks are working on new chemistries that could mean cheaper batteries, higher energy density, faster charging, longer life...solid-state, anyone?

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#19
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 9:33 PM

All the good ones have been 'taken'.

I liken this to improvements in power electronics with the somewhat recent developments with wide-bandgap semiconductors (SiC and GaN) that permit faster switching and higher voltage ratings. It makes things a bit more efficient but like you say, the thing that will drive down costs and increase adoption is going to be manufacturing improvements.

Fundamentally, there are a finite number of elements and once the combinations and permutations are all tried, the chemistry is what it is. To get beyond where we are today, the energy storage is going to have to be something other than a static electro-chemical cell. Flow batteries, fuel cells, who knows?

Or keep pumping oil out of the ground and kick the can down the road a couple more centuries.

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#12
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 2:55 PM

Acceleration is proportional to motor torque which is proportional to current. If you accelerate to cruising speed in half the time, there will be twice the acceleration and twice the current (for half the time).

It seems you would be drawing the same amount of amp hours from the battery during the acceleration phase, resulting in the same kinetic energy of the vehicle. So it's a wash.

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#13
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 3:40 PM

Don't you think that back EMF from the electric motor will play into this at some point? It's not a resistive load here.

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#14
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 4:47 PM

Don't you think that back EMF from the electric motor will play into this at some point?

Back EMF is proportional to the RPM of the motor. Current is proportional to the torque of the motor.

Electrical Power = Back EMF x Current,

Mechanical Power Out = torque x RPM

Electrical Power = Mechanical Power (ideally)

The motor controller has to overcome the back EMF. Driving voltage in excess of back EMF increases current, which increases torque, which increases vehicle speed, ... etc.

I don't think back EMF affects the amount of energy needed to accelerate to cruising speed, whether it takes 30 seconds or 60 seconds. Energy = power x time. A shorter acceleration time requires more power but the same amount of energy.

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#20
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 9:57 PM

Yes, the current is proportional to the torque produced by an electric motor. This is why a locked rotor condition produces the maximum current. After that initial (t0) condition the motion of the rotor causes the current and torque to drop due to the back EMF.

Also, your diagram and equations only show the armature circuit for an induction motor. The stator circuit is not included. The Steinmetz equivalent circuit includes both the stator and armature circuit.

All of the variables are identified in the Wikipedia link.

The simplified thermal losses of an induction motor are proportional to i^2*t. Thus if one takes twice as long to reach cruising speed with half the average current drawn to reach cruising speed the thermal losses will be half of the quickest "lead foot" response. Self-heating of wiring will also increase winding resistance but I expect it to be a lesser extent.

This agrees with the real-world test Solareagle located.

I thank one and all. It's been a while since I delved into the mechanism of an induction motor, the likely type of motor used in an EV. That was fun.

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#27
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Re: You have a lead foot; how does it affect your EV?

09/02/2022 3:07 PM

It will always require more power to accelerate faster.

Simple physics.

1 HP =735.5 watts=33,000 pounds lifted 1 foot in 1 MINUTE.If you lift it in 30 SECONDS,it will require 2 hp.etc.More hp requires more energy,regardless of the source.Time is a factor in the calculation.

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#28
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Re: You have a lead foot; how does it affect your EV?

09/02/2022 3:14 PM

True but many people regularly confuse power with energy.

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#17

Re: You have a lead foot; how does it affect your EV?

09/01/2022 7:54 PM

The 22% recovered from regenerative braking seems overly optimistic to me, particularly if you're cruising on the hwy...If you are in stop and go traffic then you are experiencing higher losses do to drivetrain and heat losses...I would put this figure more like 5%...accessory losses also seems low, would put that more towards 10%...

https://www.fueleconomy.gov/feg/atv-ev.shtml#:~:text=Electric%20vehicles%20(EVs)%20are%20more,75%25%20for%20a%20gasoline%20engine.

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#18
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 8:40 PM

Great link to more info, thank you Eagle! I do get a kick out of claims for regen braking; I've seen numbers up to 70% recovery. And maybe that is not so crazy, depending on how one defines recovery. Simplistically I would define it as what percentage of the kinetic energy of the car is returned to the battery. Any ideas re other definitions?

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#22
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 10:20 PM

There are four conversion losses going from kinetic energy to electrical energy to chemical energy to electrical energy and to kinetic energy again. I'm surprised to see the graphic at 22% as that seems pretty low (if I'm assuming city kind of driving) but I would be equally surprised by numbers as high as 70%.

But then again, if I take the fourth root of 22% (four efficiency conversions), the answer is that if each step in the conversion process were 68.4% efficient, then after four conversions, you are only left with 22% of your starting value. So maybe 22% regen efficiency may not be that far off.

Motors and generators might be about 90% efficient in this size and application. The motor drive/inverter is probably about 95% efficient and the battery charger is probably somewhat lower efficiency at maybe 90%. If charging a battery is 80% efficient and discharging a battery is 80%, then the end result is 44%.

Driving profile probably influences that number significantly.

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#21
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Re: You have a lead foot; how does it affect your EV?

09/01/2022 10:01 PM

Recovering 22% of the energy is better than recovering none of the energy.

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#23

Re: You have a lead foot; how does it affect your EV?

09/02/2022 4:12 AM

My only experience of driving an EV is when I had a Prius as a loan car for a couple of days. In that, even if I planted the 'gas' pedal to the floor, it still accelerated like a slug towing a heavy wagon.

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#24
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Re: You have a lead foot; how does it affect your EV?

09/02/2022 12:35 PM

Very interesting topic and thanks to those who provide such detailed answers.

We use C Max and Prius hybrids and Bolts at my work - no fancy Teslas or Rivians here. C Max is a pretty aggressive hybrid and the Bolt is pure electric. The C Max has a regenerative braking "guide" that shows you how much to brake to get the best return from the braking force. In order t o get near 100% of a good regenerative braking score, it seems to be around 40 to 60% of what I would call very hard braking (emergency?). Its very subjective of course but even a 100% score is totally a made up number and in reality we will never know how much energy that is. The Bolt can tell you how energy it gathered from the regenerative braking.

As to performance, the Bolt and C Max accelerate pretty well imo for such cars. The Bolt can be quite fast but I have not put it down in that car. The C Max will outpace a Prius easily and handles way better in the corners.

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#25
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Re: You have a lead foot; how does it affect your EV?

09/02/2022 12:53 PM

I gotta relate my recent test drive of a Nissan Ariya AWD. There was a course consisting of an acceleration demo and then a slalom. The Nissan guy riding shotgun said "now floor it and hang on until I say stop." I did and both my wife and I were screaming mercy. 0-60 in under 5 seconds in a big SUV. Felt like we were atop a Saturn 5 rocket at liftoff, except it was smooth and quiet and astronauts don't scream (that we know of). Then the slalom and the guys says "go as fast as you can." The course was tight and I was stoked with adrenaline, and my poor wife was being slammed side-to-side in the rear seat. EVs are quite an experience.

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#26
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Re: You have a lead foot; how does it affect your EV?

09/02/2022 1:58 PM

You just cannot drive non-stop from Washington D.C. to New England; a trip I used to do quite frequently when my parents were still alive.

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