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Published on April 16th, 2018 | by Andy Miles

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How EV Range Is Affected By Quick Acceleration

April 16th, 2018 by  


Originally published on EVObsession.

I was recently involved in a discussion about the effect of levels of acceleration an electric vehicle range. This was below one of the articles on CleanTechnica. I was arguing that heavy acceleration will reduce range, while another person was saying that it made no difference whatsoever.

The Science

The science is fairly straightforward. Acceleration of a vehicle represents an increase in its kinetic energy. During acceleration, electrical energy from the battery is converted, via the electric motor, into the kinetic energy possessed by the vehicle. The formula for kinetic energy is ½ MV2, where M = mass and V = velocity. The mass remains the same, but the velocity increases with acceleration, and the kinetic energy increases by the square of the velocity. So, to reach 8 mph requires four times the energy it takes to reach 4 mph, not just double.

If you want to work it out, 8×8=64, and 4X4= 16, and 4X16=64.

This is why the faster your car is going, the harder it is to stop. However, time does not come into it, so that to increase speed to 60 mph in one second takes the same amount of energy as it would to increase speed to 60 mph in 1 min. It just needs a much more powerful motor to deliver all that energy within such a short length of time.

The Engineering

The science seems to indicate that how quickly a car accelerates should not make any difference to energy consumption. It almost seems too good to be true that we can drive about with wild abandon, with our foot to the floor, enjoying the exhilaration of an EV’s rapid acceleration, without incurring any penalty. (Except perhaps a speeding ticket). In my opinion it is too good to be true, because fast acceleration causes losses in the system. I think this is a good illustration of the difference between science and engineering.

The losses are as follows:

  1. Faster acceleration leads to higher average speed, which leads to higher air resistance losses.
  2. Rolling resistance also increases with speed.
  3. Higher torque at the shafts increases frictional losses in the reduction gear.
  4. 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.

That is as much as I can think of at the moment.

Measuring the losses

The next question then is by how much would these losses affect energy consumption during high levels of acceleration. I suppose there might well be sophisticated testing devices and instrumentation to answer this question precisely, but lacking these, I decided I would do a road test to get some kind of rough answer to the question.

Road Test – Day 1

initial charge to 80%

initial charge to 80%

The method I employed was similar to tests I have done before. I drove to a motorway (freeway) service area on the M1 near where I live, and connected to the fast-charger to bring the battery charge to a precise 80%. It is convenient that my car’s system interrupts the charging when it reaches 80%, so there is no chance of it overshooting. This provides me with a precise starting point that is repeatable on any number of occasions. I chose a destination about 15 miles away, using ordinary rural roads, with only about a mile between the service area and the junction leading off the motorway. I then drove to that destination and back to the service area on the opposite side of the motorway, thereby undertaking a journey of about 30 miles.

Battery % on return

33% battery level on return

I adopted a driving style using maximum acceleration wherever I could safely do so, but kept to the speed limits and avoided any heavy braking. I maximized utilization of regenerative braking by not using the foot-brake pedal where possible, and only slightly depressing it when necessary. On most EVs, a slight depression of the foot brake engages additional regenerative braking rather than the friction brakes, which only operate on further depression. (I learned this from a reader named Frank, through a discussion following another article. At the time I had made a not unreasonable assumption that all regenerative braking was available on the accelerator pedal.)

On the whole of the 30-mile journey, the only difference in my normal driving style was the heavy acceleration. I also avoided any slipstreaming of vehicles, knowing what a difference that can make. On returning to the service area, I connected to the fast charger again and photographed the display on the charger, showing the state of charge on my return.

Road Test – Day 2

initial charge to 80%

initial charge to 80%

The next day, I repeated the exact same journey using the exact same procedure, but this time avoided any heavy acceleration and drove smoothly and gently at all times.

The state of charge on the first day on my return was 33%, and the state of charge on this next day on my return was 40%, making a difference of 7% in the state of charge over a 30-mile journey.

Battery % on return

40% battery level on return

Conclusion

This translates to a significant difference on any longer journey, so that in a 90-mile trip, for example, over 20% of the battery capacity is lost through high levels of acceleration. In some circumstances, such as long distance motorway (free-way) cruising, there would not be any accelerating and decelerating, so these conclusions would not be relevant.  It only applies in city driving and driving on two lane roads with bends, where there is constant slowing and speeding up.  There is no precise figure really, as it depends on so many variables.

I include pictures in this article showing my car charging up at the service area with the Bridge restaurant in the background. When this was built in May of 1966, it was a novel idea to have the restaurant housed on a bridge over the motorway so that people could watch the traffic over their cup of coffee. It is probably less of a novelty now, and people might prefer somewhere more peaceful and quiet to take their break from motorway driving. I also include pictures of the display on the fast chargers, illustrating the percentage of charge of the battery at each stage.

I am aware that this is not a particularly scientific experiment, but I hope it provides a satisfactory answer to the question about acceleration, and is certainly preferable to the “’tiss so – ’tiss not so” kind of discussion I was having with my fellow CleanTechnica reader.

Reprinted with permission.



About the Author

As a child at school, I had the unrealistic expectation that I would learn about, and understand, absolutely everything, during the course of growing up. Now, at the other end of life, I am fully aware of how much I have not learnt, and do not understand, and yet, I remain interested in everything. My education, starting with an arts degree, and going on to postgraduate studies, in everything from computer science, to hypnotism, reflected my broad interests. For work, I ended up working as a senior officer in local government, and built my career on being a legal eagle for the obscure branch of law, governing the work of my department. I am retired now, and am currently living in North Leicestershire in the United Kingdom, with plenty of time for doing whatever I like. I have always had a keen interest in everything alternative, which includes renewable energy and energy efficiency and, of course, electric vehicles. So, naturally, I have taken ownership of an EV, now that they are affordable and practical forms of transport. Writing is also one of my great pleasures, with various articles and a novel to my name, so writing about EVs is a natural evolution for me.



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