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

By David Herron
The confidence to drive wherever we want on electricity means understanding how far our car can go, and the best way to use public charging stations. The solution to Range Anxiety is not an electric car with a zillion miles of range. Instead it is an electric car (or motorcycle) with enough range, along with enough fast charging stations.

Our gasoline powered brethren do not seek out cars with massive fifty gallon gasoline (or diesel) tanks. They know that gasoline stations are everywhere, and therefore rely on refueling as needed. Yes there is a time and place for a car with a massive fuel tank, like if you're driving across the Gobi Desert. But for most of us "enough" is "enough".

But the discussion about electric vehicles seems to be focusing on driving range, without understanding the systemic approach of "enough range" in the vehicle and "enough charging stations" in the public.

How far can an electric car drive?

This seems like a simple thing to answer. The EPA sticker on the car gives us three numbers (city range, highway range, and combined range). But there are many facets to how far a given car will drive, on a given day, in certain terrain, with a certain driver.

Each electric car has its own efficiency rating. For example one car might require 27 kiloWatt-hours to drive 100 miles, and another might require 28 kWh. The 27 kWh/100 miles car is more efficient because it uses less energy to go the same distance. This efficiency number is shown on the EPA label, but the official numbers do not account for the terrain, weather, or driver habits, and instead are an estimate based on averages.

  • Lead footed drivers always get less range and it doesn't matter whether they're driving on gasoline or electricity
  • Driving on snow covered roads, or in heavy rain, takes more energy than driving on a clear road

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

The "enough range" part is then clearly about the size of the battery pack, and the car's efficiency.

But - how much range is enough range for an electric car?

Four hours of driving at a stretch is surely enough. At 65 miles/hr, four hours of driving is 260 miles.

On top of that, we need a safety buffer so that we don't arrive at the charging station with zero miles remaining. Routinely running the battery pack to empty harms battery pack capacity - so DO NOT DRIVE THE PACK TO EMPTY. A good safety buffer is:

  • A minimum 10-20% state of charge arriving at the charging station
  • Additional 20-30 miles to accommodate occasional broken charging stations or other issues.

For a useable 260 mile driving range, that means at least 50 miles of safety buffer.

We extend total driving range by recharging the car

Just as with gasoline/diesel cars, we increase total driving range by charging the car. Stop at a staton, and top up.

We just estimated that a 300 mile range is "enough range". Obviously to drive a 2000 mile road trip will require a few refueling stops.

If our target is driving 260 miles between refueling stops, then the the recharging stations must be positioned at least every 260 miles along the highway. But we notice that gasoline stations are positioned closer together than that, which tells us something. What if we arrived at the highway with 100 miles remaining range, and the closest charging station was 150 miles away? Those who design the infrastructure know this risk, and instead space charging stations closer together than 260 miles apart.

In any case it seems that "enough charging stations" along the highway means to space them 50-100 miles apart.

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

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.

See: What happens if an electric vehicle runs out of power in a city with no charging stations?

How is the official range estimate calculated?

In the USA, the Environmental Protection Agency (EPA) is in charge of the information sticker on vehicles. Since fuel efficiency determines fuel consumption and therefore determines the environmental impact, the EPA tracks fuel efficiency. In other countries there is a similar information label, that is determined by the correct regulatory agency in each country.

The regulatory agencies define the testing processes and method to calculate fuel efficiency. Typically they put the car on a dynamometer along with emissions testing equipment, and operate the vehicle through a standardized set of tests. After some calculations the numbers are generated for the label.

See:

Why does the dashboard range estimate vary from the EPA estimate?

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.

First you must eliminate some variables such as lead-foot tendencies, or steep terrain, that will innately require more energy to drive. Your dashboard estimate might still be significantly lower than the EPA estimate.

The first issue is that the dashboard estimate cannot be accurate. It is always based on recent driving history. The dashboard computer doesn't know the driving you'll do over the next hour. It doesn't know if you'll be driving across town at 35 miles/hr, or driving up a mountain.

Instead the dashboard computer guesses the range based on the current energy capacity (kiloWatt-hours remaining in the car) divided by recent energy consumed per mile. That roughs out a guess that can be displayed to the driver.

See: Why doesn't the dashboard show accurate range?

The other side of this is the remaining energy capacity of the battery pack. As a battery pack ages its total energy capacity decreases.

See: Battery pack age, and capacity loss


Range Confidence is Copyright © 2016-17 by David Herron

About the Author(s)

David Herron : David Herron is a writer and software engineer focusing on the wise use of technology. He is especially interested in clean energy technologies like solar power, wind power, and electric cars. David worked for nearly 30 years in Silicon Valley on software ranging from electronic mail systems, to video streaming, to the Java programming language, and has published several books on Node.js programming and electric vehicles.
(disqus.com) comments powered by Disqus