2015 Kia Soul EV and 2014 Nissan Leaf, at Blink DC fast charger - Fife, WAEnlarge Photo
The first wave of modern electric cars still remains fairly young, but as those vehicles rack up more miles, the performance of their gradually aging battery packs is a topic of great interest.
All batteries degrade over time, losing energy-storage capacity at different rates depending on their chemistry and how they're used.
In an electric car, that means less range and--potentially--more "range anxiety," which remains a prime concern among potential buyers.
Now, though, a new study may help reduce those concerns.
Researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory found that electric cars can continue to meet owners' needs even with significant amounts of battery degradation.
Analyzing real-world driving patterns, they found cars with battery packs that have lost 20 percent of their originally-rated energy storage capacity can still meet the daily needs of more than 85 percent of U.S. drivers.
The findings were published in the online Journal of Power Sources, co-authored by Berkeley researchers Samveg Saxena and Jason MacDonald, with Caroline Le Floch and Scott Moura of UC Berkeley.
Electric-car batteries are generally considered ready for replacement when they're down to 70 or 80 percent of their original capacity--which can bring a noticeable drop in range.
It's assumed that such a drop in range would render a car useless to the average driver.
To test that premise, researchers used nearly 160,000 24-hour driving itineraries from the National Household Travel Survey conducted by the U.S. Department of Transportation.
RELATED: Electric-Car Batteries: What Happens To Them After Coming Out Of The Car? (Aug 2014)
The itineraries were compared against a theoretical electric car with a 24-kilowatt-hour battery pack--the same as a Nissan Leaf.
Many variables--including energy capacity, charging regimen, and driving conditions--were changed to make a representative sample.
Saxena noted that most people in the sample proved not to drive more than 40 miles on most days, leaving some battery capacity in reserve even after the packs start losing capacity.
(It's worth noting that these results are most apt in more temperate climates; electric-car drivers in colder states know that frigid temperatures plus the need for cabin heat can cut effective range by as much as 30 percent.)
Even a pack with only 50 percent of its original capacity remaining could potentially meet the needs of more than 80 percent of U.S. drivers, the researchers found.
At 30 percent capacity, 55 percent of drivers would still find enough energy storage to meet their needs.
As battery ages, its ability to discharge power also decreases--potentially affecting a car's ability to accelerate.
However, the researchers found that this "power fade" does not have a significant impact on performance.
The determining factor in battery-pack retirement will still be capacity degradation, they concluded.
And with the driving patterns analyzed in the study, that is apparently less of a factor than originally thought.
That's not only an encouraging conclusion for drivers of older electric cars, but also anyone advocating for them.
Range anxiety "may be an over-stated concern," the researchers said.
If degraded battery packs can still provide enough range for the average driver, then healthy packs should be able to provide more than enough, they concluded.
All that said, however, retail car buyers usually buy cars capable not only of providing their average usage but their most demanding use as well--meaning that a functioning range of 62 to 100 miles will often be perceived as inadequate.
Carmakers have learned that, and the arrival of mass-priced electric cars with ranges of 120 to 200 miles within two or three years should further reduce buyer concerns over range anxiety.
[hat tip: Alex Bernstein]