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

By David Herron
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 gas tank capacity by MPG. 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.

An example is the Volkswagen Group Dieselgate scandal. In September 2015 it was revealed Volkswagen had fraudulently lied about NOx emissions from its TDI Diesel cars. These cars were introduced in 2009 to great fanfare thanks to claims the TDI Diesels did the impossible, being both extremely clean AND powerful and fun to drive. Governments around the world gave VW's TDI Diesels all kinds of benefits and tax breaks and subsidies. Turns out the Volkswagen Group (including Audi and all the other VW brands), as well as some other automakers, were lying through their teeth. Fraudulently reported emissions misled everyone, we all received emissions far worse than claimed, and all those government subsidies were wasted on a lie.

"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.


Adding range by charging the car

An electric car has a driving range based on its battery pack, just as a gasoline car has a range based on its fuel tank. Gasoline car owners seem to not think much about driving range since it's quick and easy to stop at a gas station to refuel. Electric car owners seem to think that stopping to recharge is onerous.

The "range extended electric vehicle" (a.k.a. Plug-in Hybrid EV) uses the ubiquitous gas stations to add range to the car. But there are other possible methods to extend an electric car range, including simply using a fast charging station.

Energy storage capacity, and therefore driving range, decreases as the car ages. We need to understand that effect, and how to minimize battery pack capacity loss.

Some cities have few charging stations presenting us with a challenge to drive to such cities with our electric car. The task is not impossible since electricity is everywhere, but it means we need to carry our own charging station.


Range Confidence is Copyright © 2016-17 by David Herron
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