Range Confidence: Charge Fast, Drive Far, with your Electric Car

By David Herron

Energy storage, energy consumption, weather and electric car driving range

Understanding electric vehicle range Total driving range in electric cars: plug-in hybrid versus fast charging
How far can you drive an electric car? The official estimate is approved by government regulators, but electric car drivers know the number on the dashboard is a guess, and that we often do not get to drive the distance we're told is possible.

Battery pack size, or for that matter gasoline tank size, is the primary determiner of driving range. It's a simple equation: Divide the energy stored on the vehicle by the energy consumption per mile, and you have the driving range:

range = energy stored / energy consumed per mile

In an electric car the quantity of stored energy is measured in kiloWatt-hours. One car might have a 24 kiloWatt-hour pack, and another have an 85 kiloWatt-hour pack, or well over 3 times the amount of energy. Assuming the two cars consume energy at the same rate, the second car will have 3.5 times the driving range.

Here's two concrete ways to correlate energy storage, energy consumption, and range.

range = gallons gasoline / gallons consumed per mile
range = kiloWatt-hours / kWh consumed per mile

In practical reality it is far more complex than that. For example the car with the bigger battery pack probably weighs more, requiring more energy to drive down the road, and therefore achieving less efficiency in terms of kiloWatt-hours to drive a given distance. Or the same car driven up a mountain will consume more energy than when driven on flat land. Or a car driving into a steady wind will consume more energy.

For example, that 85 kWh car weighs a lot more than the 24 kWh car, and achieved a 265 mile range to the 84 miles range estimated for the 24 kWh car. The higher weight means a higher energy consumption per mile simply because of the weight. Therefore it doesn't get 3.5x the range of the 24 kWh car, but more like 3.1x.

Many people are asking when there will be an electric car with 400 miles range. The answer is a matter of balancing the kiloWatt-hours of battery, against the kiloWatt-hours consumed per mile, against the battery pack weight, and other factors like cost. An electric car can be built with enough energy storage to drive 500 miles, for example, but would it be too heavy to be efficient, or would it cost too much?

The official range estimate was measured in a laboratory based on standardized testing protocols, and not in the real world. Remember that it is an estimate, and nothing is better than your experience with your car on the roads you typically drive.

Even so, battery pack size is a great first order estimate of range. For example, for several months before GM released details of the Chevy Bolt we knew it would have a 200 mile (or so) range. Making the assumption GM meant the EPA range would be 200 miles, that meant the car would likely have a 60 kiloWatt-hour battery pack. How did we derive this guess? Simple, the 60 kiloWatt-hour Tesla Model S had a 208 mile EPA range. It's a rough extrapolation that 60 kWh gives about 200 miles range. But what really counts is the officially sanctioned EPA estimate, not the number tossed around by the automakers marketing department.

Another complication is driving habits - the hotrodder tends to consume more energy than the sedate driver.

Driving habits, energy consumption, and driving range

It's not all about how big your battery pack is, but what you do with it.

It doesn't matter what kind of car you have, it carries in its gas tank or battery pack a given quantity of energy. Your driving habits directly impact the rate of energy consumption. You can drive fast and hard, using up that energy more quickly, giving you less driving range, than if you hypermiled your way around town.

The story of the Tortoise and Hare is apropos. One day the Tortoise challenged the Hare to a race. The Hare, thinking he can outrun anybody, especially a pokey old creature like the Tortoise, gleefully accepted the challenge. You no doubt heard this story as a child, and know that the Tortoise ended up winning while the Hare could barely huff and puff his way across the finish line.

How does this apply to electric car drivers? Or, for that matter, to gasoline car drivers?

The Hare is like the hotrodder, squealing their tires at every chance, slamming the brakes hard at every stop, driving over the speed limit, etc. It's well known driving this way consumes energy like there's no tomorrow.

Optimizing your driving range means adopting some hypermiler techniques, rather than hotrodding your way around town. More aggressive driving habits means consuming more energy per mile, and therefore getting fewer miles of range. Such people, while they might enjoy the speed, especially the thrill of 100% torque at 0 RPM, will have to stop to recharge more often. Just like the Hare had to stop and recharge, and get beaten by the Tortoise.

Here's some numbers pulled out of thin air:

24,000 Watt-hours / 300 Wh per mile = 80 miles range
24,000 Watt-hours / 400 Wh per mile = 60 miles range

Take a 24 kiloWatt-hour car, drive it aggressively (400 Wh/mile versus 300) and your total range is slashed.

There's no magic to this, just simple physics. Consume more energy to drive, and your fixed quantity of energy will disappear more quickly. A hotrodding gasoline car driver faces the same fate.

Does driving in the rain or snow reduce electric car driving range?

Recently we had a pair of days where we drove 70 miles to an event, then returned home, and then repeated the trip on a 2nd day. There was a serious rainstorm on the way home the first day, and the rainstorm continued the next morning.

This trip is drivable with our car, with 25 miles range remaining upon arrival. On the morning of the first day that's what happened - we drove down the highway, and easily arrived with 25 miles remaining range. Then I went to the nearby fast charging station, and charged the car to 100% so we could drive home that evening. That was the plan, but what happened is entirely different.

By the time we returned home, there was standing water on the road in many places, and it was raining fairly strong. About halfway home the remaining range on the dashboard looked to be low enough we might not make it home. I'd thought ahead and located a fast charging station we could use Just In Case. We stopped at that station, charged for about a half hour, then drove the rest of the way home. We arrived with few enough miles range to validate the worry that we might not have made it home.

The next morning was similar. For half of the trip it was raining heavily enough we even considered aborting the trip and returning home. After awhile the sky cleared and the rain stopped. But the remaining range looked small enough to make us think we might not make it to the destination. So we stopped at a fast charging station and charged for about a half hour, then drove on to the destination.

The next afternoon had occasional light rain, but was otherwise clear. We arrived home with the expected 25 miles remaining range.

Bottom line - when it was raining the effective range seemed smaller than when it was not raining.

Assumption - somehow driving in the rain increases energy consumption.

It's nice to have this hand-waving observation. Is it true that driving through the rain, or for that matter in the snow, going to increase energy consumption? Have other people noticed their range decrease by driving in the rain?

Turns out that lots of others have noticed this as well. In (teslamotorsclub.com) Rain and Energy consumption there is a discussion on the Tesla Motors Club about this very issue. Many people chimed in with agreement.

The reasoned theory is a combination of factors would increase energy consumption while driving in the rain, and therefore decrease driving range:

  • The contact between rubber and road is less efficient on a wet (or snowy) road
  • Driving through standing water increases road resistance
  • The weather tends to be colder, hence the battery pack will be less efficient
  • The weather tends to be windier, increasing energy required to make it through the wind
  • The falling water itself (the rain, that is) also presents a resistance

Innate energy consumption

It's not just your driving habits, but the car's design, that affects energy consumption. To demonstrate this let's look at the energy efficiency of a few cars:

Car Consumption MPGe Range Battery
2014 BMW i3 BEV 270 Wh/mile 124 MPGe 81 miles 22 kWh
2014 Chevy Spark EV 280 Wh/mile 119 MPGe 82 miles 19 kWh
2014 Honda Fit EV 290 Wh/mile 118 MPGe 82 miles 20 kWh
2015 VW e-Golf 290 Wh/mile 116 MPGe 83 miles 24 kWh
2015 Nissan Leaf 300 Wh/mile 114 MPGe 84 miles 24 kWh
2015 Kia Soul EV 320 Wh/mile 105 MPGe 93 miles 27 kWh
2014 Ford Focus Electric 320 Wh/mile 105 MPGe 76 miles 23 kWh
2015 Tesla Model S 85D 340 Wh/mile 100 MPGe 270 miles 85 kWh
2014 Tesla Model S 60 350 Wh/mile 95 MPGe 208 miles 60 kWh
2014 Tesla Model S 85 380 Wh/mile 89 MPGe 265 miles 85 kWh

The BMW i3 has a smaller than usual battery pack, and most of the car's structure is made from carbon fiber. Because carbon fiber has an extremely high strength to weight ratio, the BMW i3 weighs very little but has an ultra-rigid structure to keep passengers safe. As a result it has the lowest energy consumption per mile of any electric car. How? A light weight vehicle takes less energy than a heavy vehicle.

Both the BMW i3 and Chevy Spark EV have a smaller-than-typical battery pack, but achieve the same range. The trick is that both are light-weight, the i3 because of carbon fiber and the Spark EV because it's simply a small car.

By contrast, while the Tesla Model S uses lots of light-weight aluminum, the ultra big battery pack is simply heavy. The weight negatively impacts energy efficiency, and it has the highest energy consumption of any electric car.

Note the direct correlation between high energy consumption and the lower the MPGe value.


Range Confidence is Copyright © 2016-17 by David Herron

Energy storage, energy consumption, weather and electric car driving range

Understanding electric vehicle range Total driving range in electric cars: plug-in hybrid versus fast charging
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