Battery life is a big question as people shop for a used electric car. A battery is a lithium ion black box that can make up 50-70% of an electric car’s value. The reality is that if the battery dies, so does the car.
Some newer EVs have not been on the road long enough to fully understand how long the batteries last. But a lot of information can be gleaned from studies on older models of Nissan LEAF and Tesla Model S, both of which have been on the road for almost a decade.
The study findings are encouraging: it looks like EV batteries have a lot of life in them.
Studies of Older Electric Car Batteries
Lithium ion batteries start to degrade as soon as they are made. It’s an unavoidable part of battery science that you might have noticed in your cell phone, your laptop or maybe even your car. Even if you never use them, they slowly lose power and efficiency as they sit on the shelf.
The good news is that your EV is far more complex and sophisticated than other batteries in day-to-day life! EV batteries degrade slowly and most estimates suggest that they should last at least 10 years, if not longer than the car itself.
The best indication we have about the life of lithium ion batteries is looking at how the oldest electric cars on the road perform today. Nissan’s LEAF first hit the road in 2011, meaning we have a decade of data, and many of those batteries are still going strong. Francisco Carranza, Managing Director of Renault-Nissan Energy Services, points to data collected from 400,000 European LEAF cars to suggest that even the oldest batteries should last 22 years.
Another EV that’s been on the road many years is the Tesla Model S. It launched in 2012 so battery scientists have a lot of data. Maarten Steinbuch, who shares community-sourced Tesla data, suggested in 2020 that drivers can expect faster decay in the first 25,000 mile with a much slower decay to 175,000 miles.
This is consistent with what Recurrent sees in its own data of over 800 Model S vehicles. The chart below shows a scatter plot of range estimates for various Model S trims as a function of odometer. Each dot is an individual car tracked in the Recurrent fleet. After an initial drop off in range (easiest to see in the green 100 kWh plot), degradation begins to level off. Analyzing the data points for our 2012-2015 85 kWh Model S's, we estimate around a 5% decrease in range from 50,000 to 200,000 miles. Of course, how you store and charge your battery will have an effect on its long-term range loss. Newer battery chemistries may also behave differently than the older models used for this data.
Recurrent's data confirms the laboratory-based understanding of how batteries should decay: an initial, sharp decline, followed by a slow, linear capacity fade. Assuming linear degradation for most of the battery lift, Steinbuch extrapolates an average Tesla Model S lifetime of around 500,000 miles. This is a far longer lifetime than most people expect from an ICE car, and long enough that an EV can have several owners.
Other data from high mileage Teslas suggest modest degradation with odometer, and although these cases remain anecdotal, the industry is optimistic about long battery lives.
What Affects Battery Life
Lithium ion batteries have been studied a lot. While most of the scientific foundations for this understanding comes from laboratory experiments on battery cells, the principles generally hold for the larger packs used in cars.
Batteries generate energy via chemical reaction. Since the chemical reactions happen in a physical cell, unwanted or waste reactions also occur. This is unavoidable and it doesn’t mean there’s anything wrong with the battery. Some of these side reactions can even help the battery last longer, if other conditions are favorable. The best practices to avoid battery degradation are things that either reduce the physical stress of the chemical reactions on the battery or avoid things that speed up the chemical reactions.
Early LEAF batteries taught us something important about the longevity of lithium batteries -- they do not like the heat. The earliest LEAF batteries had no active coolant, meaning that EVs in hot climates degraded more quickly than expected and many did need to have a battery replacement. Although many automakers learned from Nissan’s early misstep and installed thermal management in their battery packs, it is still true that batteries last longer if they are kept as cool as possible. Since not all thermal management systems operate the same, this means parking in the shade, cooling your car before charging, and considering more active cooling if you live in a hot climate.
Charging your battery is a physical process that moves lithium ions and electrons around in the cells. The higher voltage that you use, the more forcefully the physical process happens, and the more physical stress or micro-damage occurs to the battery materials. With very high voltage charging, such as DC fast charging, a lot of heat is also generated, which is not ideal for battery longevity. DC fast charging is the double bacon cheeseburger of charging: great on a road trip but best to avoid everyday. Most batteries are built for regular Level 2, or 220V charging.
Depth of Discharge
Depth of discharge refers to how much battery you use in between charges. For instance, if you have a 100 kWh battery, a 80 kWh depth of discharge is 80% of the battery capacity. Laboratory studies show that battery cells last much longer if the depth of discharge is small, and conventional wisdom suggests keeping charge in the band around 50%, where the battery is chemically most stable. In other words, rather than using 50% of your battery before recharging, you might use 20% of the battery, recharge, and then use another 30%.
What Warranties Say About Batteries
Another way we can understand the expected lifespan on major EV batteries is by seeing how long the manufacturers guarantee them. EV warranties are much like other car warranties, except for the part about the battery! And since the battery is so important, make sure you read your specific warranty carefully. For used cars that are out of warranty, supplemental protection may be available from the manufacturer or from third parties.
The standard warranty in the US is 8 years or 100,000 miles, but manufacturers can decide what percentage of original battery capacity is ensured over that time. Getting at the actual battery capacity is not always easy, and manufacturers often want their service centers to verify faulty packs. However, these warranties set the expectation that a battery should retain a certain amount of energy over a fixed period of time.
Sample manufacturer warranties
- Tesla battery warranties vary a bit between the Model 3 Standard Range, Long Range and the Model S/X. The Model 3 Standard Range battery is guaranteed to stay at 70% original capacity for 100,000 miles or 8 years, whichever happens first. Other trim levels of the Model 3, and Tesla’s Model Y have a 70% guarantee to 120,000 miles or 8 years. The Model S and X have the same 8 years but 150,000 miles.
- Chevy (GM) has a standard 8 year, 100,000 mile warranty for “electric propulsion components” of the Bolt, Volt, and Malibu. It acknowledges degradation of 10-40% is possible over the warranty period.
- Hyundai boasted “America’s Best Warranty” between 2012 and 2019 with a lifetime, albeit non transferable, battery guarantee. The warranty is against failure, which is largely unspecified in documentation. From 2020 onwards, the battery is guaranteed to maintain 70% of capacity for 10 years or 100,000 miles.
- BMW guaranteed 70% original capacity for 8 years or 100,000 miles, transferable to new owners.
- VW guaranteed 70% original capacity for 8 years or 100,000 miles.
These warranties should reinforce that EV batteries last for quite a while, and as technology improves, we should expect to see batteries that will show only slight degradation over decades or hundreds of thousands of miles. Maybe the million mile battery is out there!
Battery Second Life
The big question that drops out of the conversation about battery life is what comes next for all these lithium ion batteries. The material and labor that goes into fabricating them is not insignificant, and many of the chemicals are expensive, toxic, and difficult to extract and reuse.
What can car manufacturers do with old EV batteries, and what will happen to the mountains of batteries that are used in future EVs?
One consideration is that as batteries degrade, they reach a point where they are no longer useful for cars, but they still can store and generate electricity for other uses that don’t require the same power. Nissan has projects to use old LEAF batteries to power streetlights, and GM uses batteries to back up data centers in Michigan. There is hope that linking together older EV batteries will provide storage for renewable energy, support grid resilience, or serve as emergency backup power. As vehicle manufacturers devise plans to repossess and reuse batteries, they may find themselves expanding into other industries and working to electrify more than just vehicles.
Do You Have an Older EV?
Older electric cars provide a lot of insight on the past, present and future of EV batteries. If you have one, please consider contributing some of that valuable information to the Recurrent driver community!