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

By David Herron

Driving habits, energy consumption, and driving range

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

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.

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

Driving habits, energy consumption, and driving range

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