EV battery degradation concerns overstated

Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have shown that electric vehicles (EVs) will meet the daily travel needs of drivers longer than commonly assumed.

Many drivers and prior research on EV batteries have assumed that batteries will be retired after the losing 20 percent of their energy storage capacity. This study shows that the daily travel needs of drivers continue to be met well beyond these levels of degradation.

Samveg Saxena, who leads a vehicle powertrain research program at Berkeley Lab, analysed real-world driving patterns and found that batteries that have lost 20 percent of their original capacity can still meet the daily travel needs of more than 85 percent of U.S. drivers.

Saxena said: "There are two main reasons people are hesitant to buy an EV: first, they’re unsure it will satisfy their mobility needs, and second, they’re afraid the battery won’t last the whole life of the car and they’ll have to replace it for a lot of money.

"We've shown that, even after substantial battery degradation, the daily travel needs of most people are still going to be met."

To conduct the study, the researchers took nearly 160,000 actual driving itineraries from the National Household Travel Survey conducted by the Department of Transportation.

These are 24-hour travel itineraries showing when a car was parked or driving, including both weekend and weekday usage by drivers across the United States.

The researchers then assumed all itineraries were driven using a vehicle with specifications similar to a Nissan LEAF, which has about 24 kilowatt-hours of energy storage capacity, similar to many other EVs on the market.

This data was fed into the team’s unique simulation tool which quantifies second-by-second energy use while driving or charging for any number of different vehicle or charger types under varying driving conditions.

Then for each of the itineraries, they changed different variables, including not only the battery’s energy storage capacity, but also when the car was charged at home and at work, whether it was city or highway driving, whether the air conditioner was on, and whether the car was being driven uphill. More than 13 million individual daily state-of-charge profiles were computed.

Saxena added: "People have commonly thought, ‘if I buy an EV, I’ll have to replace the battery in a few years because I’ll lose the ability to satisfy my driving needs, and it’s not worth it.

"We have found that only a small fraction of drivers will no longer be able to meet their daily driving needs after having lost 20 percent of their battery’s energy storage capabilities.

"It is important to remember that the vast majority of people don’t drive more than 40 miles per day on most days, and so they have plenty of reserve available to accommodate their normal daily trips even if they lose substantial amounts of battery capacity due to degradation."

As the battery continues to degrade down to 50 percent of its original energy storage capacity, the research found that the daily travel needs of more than 80 percent of U.S. drivers can still be met, and at 30 percent capacity, 55 percent of drivers still have their daily needs met.

The researchers concluded that “range anxiety may be an over-stated concern” since EVs can meet the daily travel needs of more than 85 percent of U.S. drivers even after losing 20 percent of their originally rated battery capacity.

They also conclude that batteries can “satisfy daily mobility requirements for the full lifetime of an electric vehicle.”

Given these results, the authors propose that an EV battery’s actual retirement may be delayed to when it can no longer meet the daily travel needs of a driver, leading many EV batteries to have a longer lifetime than is commonly assumed.

Berkeley Lab

Peter Thomas

Author:Peter Thomas
Date Updated:2nd Apr 2015

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