Why Does an Electric Car’s Battery Degrade?

The large battery packs used in electric vehicles (EVs) are sturdier than the battery in your laptop, but they will degrade over time like any other lithium-ion battery. Here we’ll look at what causes the battery in an EV to degrade, and why.

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How An EV Battery Degrades Over Time

Two main factors will affect the speed at which an EV’s battery degrades: how old the battery is, and your use and environment. The second category includes things like how the electric car is driven, how the battery is charged, how the vehicle is stored, and environmental factors like climate.

Battery Age

The first category, degradation due to the battery’s age, is inevitable—all lithium-ion batteries steadily decline in effectiveness with use over time. This is also called calendar aging, and it’s a very gradual process. It also doesn’t happen at the same rate year-over-year.

According to electric vehicle startup Recurrent, which monitors and provides shoppers with reports on used electric vehicles, EV batteries see their biggest declines in capacity at the beginning and the end of their usable lifespan. There’s usually a quick drop-off at first, which levels out once the battery becomes stabilized, then another drop after a few years. These decreases are usually small, between 5-10% of overall capacity even after thousands of miles.

So why does this degradation happen? Because of the way lithium-ion batteries are built and how they work. Electric car batteries rely on chemical reactions to generate the electrons that power the car’s motor. They generate them by using what’s called an “active material” to spark the reaction. In Li-ion batteries, that material is lithium.

As the battery is used over and over again, some of that lithium is used up permanently. When that happens for long enough, you see a decrease in the overall charge that the battery can hold because there’s just not enough active material to generate the same amount of energy. This type of degradation is also called capacity fade.

Environmental and Use Factors

Environmental factors, especially temperature, are proven to affect how well an EV’s battery works. In very cold weather, for example, the liquid inside an electric car’s battery becomes more viscous, slowing the reactions necessary to generate electricity. That means less electricity is available for the motor to use, so you have less power to accelerate. This is called, unsurprisingly, power fade. EVs are built with temperature regulation systems to help keep this from happening, but in extreme weather, it’ll still be an issue to some degree. Corrosion or buildup inside the battery over time can also result in power fade.

And it isn’t just cold weather. Heat degrades capacity temporarily and can contribute to accelerated capacity loss overall, but the difference in capacity lost between temperate and hot climates is small for modern EVs. As Cars.com puts it:

“According to Geotab data, after four years, an EV in a temperate climate shows less battery degradation than one in a hot climate, but the difference is less than a quarter of a percent…A comparison of one 2015 model EV without active thermal management, the Nissan Leaf, with another 2015 that has it, the Tesla Model S, might be the most useful: Geotab reports the Leaf’s average degeneration rate at 4.2% and the Model S’ at 2.3%.”

Relying heavily on DC fast charging (DCFC) stations will also degrade an EV’s battery life. Automaker Kia attributes 10% capacity loss over a battery’s lifetime to excessive DCFC use. DCFC puts a heavy load on the battery pack in order to funnel all that power in so quickly, the same as constantly using fast charging on your cell phone will shorten its battery life. That’s because the more power you put into the battery, the more forcefully electrons and ions get moved around the interior of the battery cells. That can cause micro-damage and extra stress on the battery components, eventually wearing down capacity when done too often.

Most electric car manufacturers agree fast charging should be used “sparingly,” but as of this writing, there’s no set definition of what sparingly means. A good rule to go by is to fast charge as little as possible, when it can’t be avoided or on long trips, and stick to regular level 1 or 2 charging most of the time.

Should You Be Worried About Battery Degradation?

As long as you’re aware the battery will decrease in capacity, slowly, over time, you shouldn’t need to worry about degradation much. You will want to plan for that decrease ahead of time, though, and factor it into your calculations of how far you’ll be able to drive the vehicle a few years down the line. As far as complete failure is concerned, total battery failure is very rare and usually covered by the manufacturer’s warranty.

Some degradation is inevitable, but if you take good care of the battery, chances are it’ll keep most of its range well into the car’s lifespan. Avoid too much DCFC charging. Precondition the battery in extreme heat or cold before charging. Leave the vehicle plugged into a charger when you can to conserve power. Consider the climate where you live and how it could affect the battery before you buy. As technology improves, many of these constraints on EV battery power and life could lessen or disappear entirely. That’s especially good news if you know the cost of replacing an EV battery.

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