Why does the battery of an electric car degrade?

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The large batteries used in electric vehicles (EVs) are more robust than your laptop battery, but they will degrade over time like any other lithium-ion battery. Here we will see what causes the degradation of the battery of an electric vehicle and why.

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How an EV battery degrades over time

Two main factors will affect how quickly an electric vehicle battery degrades: the age of the battery, your usage, and your 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 the climate.

Battery age

The first category, degradation due to battery age, is unavoidable: all lithium-ion batteries steadily lose efficiency over time. This is also called calendar aging, and it is a very gradual process. It also doesn’t happen at the same pace from year to year.

According to electric vehicle startup Recurrent, which monitors and provides buyers with reports on used electric vehicles, EV batteries experience their biggest drops in capacity at the start and end of their useful life. There’s usually a quick dip at first, which stabilizes once the battery stabilizes, and then another dip after a few years. These decreases are generally small, between 5 and 10% of the overall capacity, even after thousands of kilometres.

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

As the battery is used over and over again, some of this lithium is constantly consumed. When this happens long enough, you see a decrease in the overall charge the battery can hold because there simply isn’t enough active material to generate the same amount of power. This type of degradation is also called capacitance fading.

Environmental and Usage Factors

Environmental factors, especially temperature, have been proven to affect the operation of an electric vehicle battery. In very cold weather, for example, the liquid inside an electric car’s battery becomes more viscous, slowing down the reactions needed to generate electricity. This means less electricity is available for the motor to use, so you have less power to accelerate. This is called, unsurprisingly, power fading. Electric vehicles are built with climate control systems to prevent this from happening, but in extreme weather conditions this will still be a problem to some extent. Corrosion or buildup inside the battery over time can also cause loss of power.

And it’s not just the cold. Heat temporarily degrades capacity and can help accelerate overall capacity loss, but the difference in lost capacity between temperate and warm climates is small for modern electric vehicles. As Cars.com says:

“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 a 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 that of the Model S at 2.3%.

Relying heavily on DC fast charging stations (DCFC) will also degrade the battery life of an electric vehicle. Automaker Kia attributes a 10% capacity loss over the life of a battery to excessive use of DCFC. DCFC puts a heavy load on the battery in order to channel all that power so quickly, the same way constantly using fast charging on your cell phone will shorten its battery life. Indeed, the more power you put in the battery, the more the electrons and the ions move with force inside the cells of the battery. This can cause micro-damage and additional stress on battery components which eventually drains capacity if done too often.

Most electric car manufacturers agree that fast charging should be used “sparingly”, but at the time of this writing there is no precise definition of what sparingly means. A good rule of thumb is to charge as slowly as possible, when it can’t be avoided or on long journeys, and stick to a regular Level 1 or 2 charge most of the time.

Should you be worried about battery degradation?

As long as you are aware that the battery capacity will decrease, slowly, over time, you shouldn’t have to worry much about degradation. However, you will want to plan for this decrease in advance and factor it into your calculations of how far you can drive the vehicle in a few years. As for complete failure, 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, it’s likely to retain most of its range for the life of the car. Avoid too much DCFC load. Precondition the battery in extremely hot or cold conditions before recharging. Leave the vehicle plugged into a charger when you can to save energy. Consider the climate you live in and how it might affect the battery before purchasing. As technology improves, many of these constraints on electric vehicle battery power and life may decrease or disappear altogether. This is especially good news if you know the cost of replacing an EV battery.

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