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

By David Herron

Understanding electric vehicle range

What does it mean when we say the 85 kiloWatt-hour Tesla Model S has an EPA certified 265 mile range, or the Chevy Volt has about a 40 mile electric range. How are these numbers derived? How accurate are they?

We, as electric car consumers, need accurate figures not only about the estimated range, but the currently remaining range. To plan our trips we need to know how much range we really have. And, because there isn't enough charging infrastructure in most areas, an accurate range prediction is more critical than for gasoline cars.

Gasoline car drivers can get along with a vague fuel gauge, relying on ubiquitous gasoline stations for a quick refueling. Even so, there is a similar problem with gasoline cars. The "Miles per Gallon" supposedly makes it easy to calculate the range: Multiply the gas tank capacity by the MPG rating. But sometimes automakers get caught making inaccurate MPG claims.

Accurate MPG or range estimates are not just for the benefit of drivers. Regulators need to know how clean or dirty these cars are, so they can enforce clean air laws and fuel efficiency standards. Are clean cars producing the benefit we require? And precisely how dirty are the non-clean cars? Accurate efficiency and emissions measurements are crucial to answering those questions.

"Range" is misleadingly simple

The idea is very simple - how far can you drive on a full charge (or a full tank of gas). The math is simply to divide the energy consumed per mile into the total energy carried by the vehicle, giving you the miles of range. But there are many variables that make this difficult. After all, these ratings are estimates based on idealized scenarios meant to emulate the average driver in average conditions. In other words, the estimates are different from your real world conditions with your foot on the pedal.

Range or MPG or any other way to measure efficiency can be skewed in several ways:

  • Your driving habits - you could be the perfect hypermiler, and record an ultra-high efficiency, or the perfect leadfoot, and record an ultra-low efficiency
  • Terrain - Hills, mountains, etc all take more energy to drive than on flat land.
  • Traffic - Stop-and-go traffic is the worst for gasoline cars, and as a result get lower City MPG scores than Highway MPG. Electric cars do great in this traffic because they don't have to keep the engine idling wasting gasoline at stop lights. As a result City MPGe scores for EV's are usually higher than their Highway MPGe scores.
  • Malfunction - Cars, especially gasoline cars, start failing as they age and can run less efficiently. In an electric car, an old battery pack holds less energy than a new battery pack.

That acronym, MPGe, what does it mean? Think "Electric MPG" and that'll be close enough. The number is meant to help us compare electric and gasoline car efficiency when we're accustomed to "MPG". There's a calculation behind the MPGe figure that supposedly equivalences electricity to gasoline. The problem is that electricity doesn't come in gallons but in kiloWatt-hours, and the MPGe figure is a bit of a smokescreen.

The Guess-o-Meter

Another problem is the range estimate on electric car dashboards is often inscrutable. Most of us call it the "Guess-o-Meter" because it looks like the car is guessing (badly) the remaining range.

Electric cars usually estimate remaining range by averaging energy consumption over your recent driving history. Your actual range is extremely dependent on how much energy you consume. If you run the heater or air conditioner a lot, or if you have a lead foot, you'll have high energy consumption and a short driving range estimate. On the other hand, if you're carefully hypermiling, minimizing energy usage, you'll have a longer driving range estimate.

You may typically, on most days, drive in one style and the car will develop an average energy consumption matching that style. If you normally drive around town your energy consumption will be lower than on the highway, because lower speed driving simply uses less energy. The day you then break out of your habitual pattern, say jump on the highway for a long trip, it'll throw the car off and will give a bad range estimate.

The range estimate will also jump up and down if you turn the cabin heater on or off. That's simply because the cabin heater uses a lot of energy, and keeping it turned off will preserve energy for driving. How do you stay warm in the winter? Make sure your car has seat heaters and use those.


Range Confidence is Copyright © 2016-17 by David Herron

Understanding electric vehicle range

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