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ICE cars trained us all to avoid a heavy right foot if we want economy. Accelerate hard and revs are high.

Most people seem convinced the same applies to EVs.

But why? Obviously speed matters. And regen isn't 100%. But is the acceleration itself "expensive" in efficiency terms? If you intend travelling at 60 mph, is there a cost of getting to that speed in 5 seconds vs 15?

What would be happening inside the motor to cause inefficiency?
 

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ICE cars trained us all to avoid a heavy right foot if we want economy. Accelerate hard and revs are high.

Most people seem convinced the same applies to EVs.

But why? Obviously speed matters. And regen isn't 100%. But is the acceleration itself "expensive" in efficiency terms? If you intend travelling at 60 mph, is there a cost of getting to that speed in 5 seconds vs 15?

What would be happening inside the motor to cause inefficiency?

In a word, I²R losses. The motor windings, cables and the inverter all have some resistance (R), which is why motors and inverters need cooling systems, the resistance causes them to get warm when they pass current. The more current they pass (i.e. the more power the motor is delivering) the greater the losses, and because these losses are proportional to the square of the current (I) if you double the current (so double the power) then the losses increase by a factor of four.
 

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Higher acceleration does mean more current, and that does mean more losses in the wire, but it is tiny in the scheme of things.

With higher acceleration, the higher current is there for a shorter time, so the "squared" part of I2R losses is negated. High acceleration is lossy, but less than you might think.

If you're after the biggest possible range, you might look at deceleration rather than acceleration. Regen is inefficient (though much better than not having it!) Winding losses, electronics losses, losses charging and discharging the battery, then losses accelerating again. The best thing to do is to spot that you have to slow down well in advance, then hold the throttle so that your power meter is at (or close to) zero. That way you're neither putting power in to the motor nor regenerating power from it. That's the most efficient way to slow down and will save you multiples of your acceleration losses.
 

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Higher acceleration does mean more current, and that does mean more losses in the wire, but it is tiny in the scheme of things.

With higher acceleration, the higher current is there for a shorter time, so the "squared" part of I2R losses is negated. High acceleration is lossy, but less than you might think.

If you're after the biggest possible range, you might look at deceleration rather than acceleration. Regen is inefficient (though much better than not having it!) Winding losses, electronics losses, losses charging and discharging the battery, then losses accelerating again. The best thing to do is to spot that you have to slow down well in advance, then hold the throttle so that your power meter is at (or close to) zero. That way you're neither putting power in to the motor nor regenerating power from it. That's the most efficient way to slow down and will save you multiples of your acceleration losses.

Having built several electric bikes, and electric motorcycle and an electric boat, I can absolutely confirm that I²R loss is far and away the greatest source of drive train inefficiency in any EV, and that, as the power equation predicts, things get a lot hotter when there is a higher current flowing, even if only for a short time. In addition, as the resistance of most conducting materials increases as temperature increases, the warmer the conductors, the greater the I²R losses.

The resistive losses are everywhere, within the cells, in the wiring, the inverter and the motor windings, and these make up most of the overall losses in the drive train. There are some eddy current losses at high motor speeds, but generally these tend to be a lot lower than the I²R losses, as motor design has pretty much reduced these losses to a negligible level over the normal motor RPM range.

Regeneration can never recover energy lost as heat, and regen itself has I²R losses that create more heat.
 

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Motors seem to be most efficient in mid range both in terms of torque and speed so in the question 10secs might be the most efficient.

My choice for the most efficient is constant power setting. This does feel a bit slow initially but that i think is just a conditioning of a lifetime of ICE driving. Interestingly on my E-Golf if in ECO+ mode that is what it does from a standing start so that gives some approval.
 

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EVs are much more consistently efficient in delivering their part load power than ICE. Below is an efficiency graph for a LEAF which shows variation only from 85% to 95%. So whilst it is more efficient in terms of the motor to accelerate hard the difference is minimal (10% at most from virtually no load to max load at 6,000 RPM), and as @Jeremy Harris states the losses in the rest of the electrical system are higher at higher current more than wiping out the gain of moving the motor into a more efficient operating zone.


 

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But how significant are those I2R losses compared to the energy spent on accelerating the car and overcoming the many mechanical losses?
 

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But how significant are those I2R losses compared to the energy spent on accelerating the car and overcoming the many mechanical losses?

It's all about energy, really, and where it goes. In general, the energy used to accelerate the car isn't wasted, but if the motor has to deliver more torque to accelerate the car faster, then the losses will be a lot higher (because of the square law relationship of the copper losses). When driving at a constant speed all the energy used to maintain that speed is lost as heat (a mix of heat losses from the drive train, air frictional heating from drag and tyre and road heating from rolling resistance, mainly, plus some minor bearing friction heat losses). Some of the energy used to accelerate the car to a constant speed will be recovered by regen braking, but again the efficiency of that depends on how hard the regen is - the harder it is, the higher the current and the much higher the losses, just the same as with acceleration.

Keeping the motor and drivetrain in the efficiency sweet spot will maximise overall efficiency, so no hard acceleration, and no hard regen, try to keep things inside the red area on the graph posted by @dk6780
 

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Anecdotally, watching the GOM drop by three miles when having to accelerate from roundabout speeds to NSL up a steep hill over the course of a quarter of a mile or so makes me say "Yes!"
 

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If you accelerate rapidly, your average speed will be higher and so wind resistance will be higher overall.
 

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ICE cars trained us all to avoid a heavy right foot if we want economy. Accelerate hard and revs are high.

Most people seem convinced the same applies to EVs.

But why? Obviously speed matters. And regen isn't 100%. But is the acceleration itself "expensive" in efficiency terms? If you intend travelling at 60 mph, is there a cost of getting to that speed in 5 seconds vs 15?

What would be happening inside the motor to cause inefficiency?
In addition to other replies, internal losses in the battery are higher under heavy current discharge. So a slow steady motor output will be best for economy.
 

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If you accelerate rapidly, your average speed will be higher and so wind resistance will be higher overall.
This is probably the main real world reason. I cant resist a traffic light GP and I've still managed in excess of 6 miles per kWh in my Ioniq over summer. Your overall average speed and aircon/heat use have a much bigger impact on range.
 

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Discussion Starter · #14 ·
Thanks all. Jeremy's I²R is bringing the faintest of O-Level memories to me across an almost unimaginable vastness of time.

Does the wide range of efficiency figures in the EV Database, from Tesla to e-Tron, almost double, reflect different motors? It feels odd to have Tesla as one of the most acceleratey cars and also one of the most efficient. Does their motor have less R in the wiring and loses less to heat?

Or is it all in the body shape?
 

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Thanks all. Jeremy's I²R is bringing the faintest of O-Level memories to me across an almost unimaginable vastness of time.

Does the wide range of efficiency figures in the EV Database, from Tesla to e-Tron, almost double, reflect different motors? It feels odd to have Tesla as one of the most acceleratey cars and also one of the most efficient. Does their motor have less R in the wiring and loses less to heat?

Or is it all in the body shape?
Aerodynamics and weight have the biggest effects. The Tesla have relatively small frontal areas as well as efficient shapes, and the e-Tron is an overweight brick based on an ICE with the internal airflow details associated with that. But there is no doubt an element of efficiency in the electrical design as well with Tesla and Hyundai being at the top.
 

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ICE cars trained us all to avoid a heavy right foot if we want economy. Accelerate hard and revs are high.

Most people seem convinced the same applies to EVs.

But why? Obviously speed matters. And regen isn't 100%. But is the acceleration itself "expensive" in efficiency terms? If you intend travelling at 60 mph, is there a cost of getting to that speed in 5 seconds vs 15?

What would be happening inside the motor to cause inefficiency?
Someone once explained it to me like this, the quicker you get to a speed, the longer you spend at that speed, so the more energy you use. So get to 70 in a EV in 6 seconds compared to a leisurely 1m 6 seconds mean 1m longer at higher speed which drain more of the battery. Unless making a beyond range journey, i have never worried about it and treated my EV's like cheaper to run hot hatches.
 

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If you accelerate rapidly, your average speed will be higher and so wind resistance will be higher overall.
Yes but you will arrive quicker! :rolleyes:
Is time more precious than range? 🚗
Thankfully the days of driving your EV with the heating and music off, and you wearing a big coat, scarf and gloves are gone?⛄❄❄❄
 

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Most electric motors are highly efficient, I wouldn't expect significant differences between permanent magnet motors from Tesla/Jaguar/VW/Hyundai etc etc. Mass of vehicle & aerodynamics cause far greater variations in m/kWh. You want a really efficient EV? Get a Hyundai 28 kWh. Compact, light, really slippery aerodynamics, fast on Rapids, what's not to like!
 

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I don’t think that It has much to do with motor/drive efficiency. I suspect that it’s the battery performance which is highly susceptible to excessive discharge rates when accelerating hard.
 
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