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. And, the car cannot predict the future. An accurate range estimate depends on correctly predicting future energy consumption.

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. How should the car estimate remaining range at each moment during that trip? The car cannot know you'll switch to a higher speed that consumes more energy per mile.

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, consuming more energy per mile.

Generally speaking, it's a matter of efficient driving habits. Making a serious study of hypermiling techniques will lower your energy consumption and stretch the driving range. On the other hand, if you don't care about hypermiling and instead get your kicks from the full-torque-at-zero-RPM performance, your higher energy consumption will shrink the driving range.

One way to see this effect is by comparing EPA certified range to the range estimate on a fully charged car. A range estimate higher than the EPA certified range shows that your car thinks your driving habits are highly efficient.

You could have a string of highly efficient driving days, with associated longer range estimates, then need to take a long trip on the highway and see plummeting range estimates.

Cold weather shrinking range

Cold weather causes most electric cars to report lower driving range estimates. 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 to heat 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. Therefore, energy consumption is higher, when the heater is on, reducing the driving range.

You can see this effect by simply turning on the heater while driving and observing the change in estimated remaining range. It's not just the heater, but the air conditioning unit, because that also consumes energy.

It's inefficient to heat the entire passenger cabin. Heating just the seats is much more efficient. Most (or all) electric cars have seat heaters, and even steering wheel heaters. Using those for creature comfort uses less energy, with a smaller range impact.

The cold weather also directly affects battery performance, for most lithium-ion battery chemistries. Because batteries store energy in chemical reactions, cold temperatures tends to inhibit chemical reactions, limiting how much energy can be withdrawn from the battery pack. With less energy available the car has less range.

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?

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