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

By David Herron

Why doesn't the dashboard show accurate range?

A truism about electric cars is that the indicated range shown on the dashboard is often wrong. The dashboard will show a number, but trying to drive that distance might or might not be successful. Many call it the "Guess-O-Meter", or GOM, because the car seems to have made a bad guess rather than giving us a serious range estimate. Of course this is risky because an incorrect estimated range can cause us to make a bad choice of where to drive.

Some cars do a better job of estimating range than others. Ideally the car would give us an accurate remaining range estimate, because of course we need reliable information. Unfortunately it's hard to estimate remaining range because of all the variables.

Remember the range of an electric car is, roughly speaking, the remaining energy (in kiloWatt-hours) divided by energy consumption per mile. As you drive, consuming kiloWatt-hours, the range of course decreases. But energy consumption varies from moment to moment, based on driving habits, conditions, and the impossibility of predicting the future.

Cannot predict future energy consumption

What's the problem? Can't the car tell you an accurate range estimate? The problem is actual energy consumption is not straight-forward. Since the car cannot predict the future, the car cannot give an accurate range estimate. What is the future energy consumption for this drive?

You may have driven 30 miles through a city at 35-45 miles/hr to get to the highway, where you'll then speed up to 65-70 miles/hr for a 40 mile drive. Driving in the city requires less energy than driving on the highway. A remaining range estimate based on city driving in the first phase cannot know the trip will head to the highway.

Electric cars tend to estimate remaining range from recent energy consumption. Someone who drives primarily in the city, consuming less energy per mile, will tend to score a higher range estimate than those who primarily drive on the highway.

Does your car show more estimated miles than the EPA certified range for that car? If so, it means the car thinks your driving habits are highly efficient.

You might have gotten that high remaining range score by driving mostly around town. That day you head out of town for a long highway drive, expect to see your estimated remaining range plummet.

Cold weather shrinking range

Cold weather causes most electric cars to report lower driving range estimates. Folks often notice the onset of winter lowers their estimated range. There's approximately two causes of this.

Running the heater is not "free" in an electric car. Gasoline engines are extremely inefficient, produce a lot of excess heat, and that heat is useful in the winter to heat up the passenger cabin. Electric motors are extremely efficient, and do not have this excess waste heat.

Therefore heating the passenger cabin means using electricity from the battery pack to drive an actual heating unit. That's energy which cannot be used to drive the car, but instead is used to heat the passengers. Turning on the heater drives up energy consumption, reducing the driving range.

It's easy to see this. While driving, simply turn on the heater and watch the remaining range estimate. Then turn the heater off, and watch the remaining range estimate. Turning the heater on should drop the range by several miles, then turning it off the range will rise again. The same effect happens with the air conditioner.

For efficiency, most electric cars have heating/cooling units built into the seats. Seat heaters/coolers are more energy efficient. Rather than heating/cooling the entire passenger cabin, just the passenger is heated or cooled.

Cold weather also directly affects battery performance, for most lithium-ion battery chemistries. Because batteries store energy in chemical reactions, and cold temperatures tends to inhibit chemical reactions, the weather limits how much energy can be withdrawn from the battery pack.

Cold weather impact varies based on battery chemistry. Some battery systems will be affected less than others.

Some electric cars can heat the battery pack. A warm battery pack won't be affected by the cold. Make sure to only heat the battery pack while it's plugged into a charging station. This will increase the time required for charging, but the car will have more energy and therefore a longer range.


Range Confidence is Copyright © 2016-17 by David Herron

Why doesn't the dashboard show accurate range?

(disqus.com) comments powered by Disqus