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.

 

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.

 

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.

 

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.

 

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.

 

A closer look at the losses in EV motors - Charged EVs

In the bad old days when the only batteries suitable for traction applications were flooded lead-acid and Ni-Cd types, the battery was the least efficient component of an EV powertrain, routinely delivering back only 50-70% of the amp-hours that went into charging it (aka the Coulometric...

chargedevs.com

 

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

 

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!"

 

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.

 

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.

 

Lithium Ion cell columbic efficiency is very close to 100% even at relatively high charge/discharge rates, so these cell losses are almost entirely regular I^2*R losses (internal cell resistance) as already discussed. (If you consider "battery" to include all the interconnects between cells those have I^2*R losses as well)

Not the case with some other battery types - for example columbic efficiency of Lead Acid cells goes WAY down at higher discharge rates, even 1C, so the increase in losses is a lot more than the measured internal resistance would suggest so a golf cart with Lead Acid batteries would see a much heavier range penalty for being hammered than one using Lithium Ion cells with the same Ah capacity as Lead Acid just isn't efficient at high discharge rates.

 

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?

 

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.

 

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.

 

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!

 

At lower RPM this is true, but at high motor RPM near top speed bog standard Permanent Magnet Synchronous motors as seen in cars like the Leaf are not very efficient, with significant fall in efficiency at the top end, hence poor efficiency at high speed, (beyond just aerodynamics) and also a relatively limited top speed of about 100mph to avoid gearing it too high and killing low speed performance.

This is why EV's are starting to move to hybrid designs like the Switched Reluctance motor used in the Model 3 - which depending on the phasing of the current relative to the rotor position can operate anywhere between permanent magnet mode and reluctance mode. (In practice it is always operating somewhere between the two but the operating point is varied with motor speed to stay at optimal efficiency at all speeds)

A motor like the Model 3's has a much wider RPM/torque operating range and maintains a high efficiency over a much wider RPM range. It can give all the benefits of a PMS motor at low speeds without any of the drawbacks at high speeds.

 

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.

 

The discharge rate, even for something like a Model 3P, is pretty trivial. 20 years ago I was building battery packs that would happily support a 20C rate of discharge with no problems or cell overheating. A Model 3P, at absolute max power, is not even hitting close to half that.

 

For the sort of small discharge rates from an EV battery it isn't really a significant issue. Tha capacity isn't affected directly, all that happens is that the I²R losses from the pack internal resistance generate a bit more heat losses, so the overall efficiency is a bit lower if a lot of current is pulled. There's nothing like the voltage drop seen on a lead acid battery at modest discharge rates, as even really small and cheap lithium cells will comfortably deliver way more current than a lead acid car battery (hence the reason that there are pocket sized lithium booster packs that will crank an engine happily).

 

There's nothing like the voltage drop seen on a lead acid battery at modest discharge rates, as even really small and cheap lithium cells will comfortably deliver way more current than a lead acid car battery (hence the reason that there are pocket sized lithium booster packs that will crank an engine happily).

I didn't know that.
Thank you

 

We know that Lithium batteries can support high discharge rates but we also know from research that lifetime is adversely affected in the same way that high charging rates of 2C and above have a detrimental affect on the life. Motor and drive efficiencies are extremely high so I am simply suggesting that reduced range my be more to do with battery performance than drive components, however, I haven’t seen any curves of discharge rate v capacity.

One thing I do know, if I put my foot down hard in Eco modes I get flashing warnings on the dash!

 

The effective internal resistance of lithium cells is very low, much lower than the resistance of all the stuff the battery is supplying. Taking a Tesla Model 3 LR pack as an example (only because the data is available) each cell has an internal resistance of about 12mΩ, and there are 46 cells in parallel in each cell group, so that gives an effective cell internal resistance of 0.2609mΩ. There are 96 cell groups in series, making the total pack internal resistance ~25mΩ.

I ran TeslaMate with my Model 3LR and the highest power I ever managed to record was about 235 kW, and then only for maybe 2 seconds (at the power level the car hits the legal speed limit rather quickly). The typical battery pack voltage is around 355 V, so that equates to a battery current of ~662 A. The battery I²R losses will have been about 11 kW for those couple of seconds. The motors/inverters combination is around 85% efficient at that power level (they are much better at lower power levels, up to about 94% efficient), so the losses in the motors/inverters would be around 35 kW.

Using more sensible numbers, the average current from the battery pack is around 50 A, and at that current the battery pack I²R losses would only be around 63 W, whilst the motors/inverters would be around 94% efficient, so their losses would be 1.065 kW, massively higher than the battery pack internal resistance losses.

 

I wonder if we'll see any capacitative diractance/magneto reluctance hybrid motors

 

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?

It's 'different' to ICE, the full low-down here if you want the master-class;-

Getting that bit more range from donald-style driving.

I am using a generalised efficiency map from a public document here, it is quite representative of a lot of motors/inverters, obviously there will be variation. You will notice it is rpm versus torque. Bear in mind the highest point of the torque is the maximum power, so for example at 1000rpm...

www.speakev.com

 

Donald,
I practice what you preach and believe it does pay in efficiency.

In my Golf i accelerate at power setting 1 if i can, which i know from experience gives a 5 mls/kwh, I can hold this setting up to c50mph and then the air drag starts to kick in.
I have no idea how the power setting i see ties up with the torque i cannot see and so wonder how this would appear on the torque/speed map.

 

Great thread. 👍👍👏