The low battery indicator on my phone seems to be an increasingly frequent source of stress. It turns out I’m not alone as there is real social anxiety associated with a dead cell phone battery. As batteries age, less of the stored energy becomes accessible, so less time for sending email and playing Candy Crush away from the plug. For an electric vehicle (EV), low battery fear can contribute to what’s known as ‘range anxiety’, an often cited barrier to wide-spread EV adoption.
As the market for EVs continues to develop and technology improves, the lithium batteries in electric vehicles are going further for far longer. But unfortunately, batteries don’t last forever. Because battery performance inevitably degrades, batteries will eventually become wastes and need to be recycled.
As the Hitz Climate Fellow investigating strategies and policy for sustainable management of retired electric vehicle batteries, I’ll unpack the lifetime of lithium batteries for electric vehicles, provide some tips on how to maximize battery life, and explain why the battery of an EV may well outlive the car in this post.
Why do batteries wear out?
In general, there are three primary pathways for degradation in a lithium ion battery: temperature, cycles, and time. Repeated utilization of the maximum storage potential of the battery, rapid charge and discharge cycles, and exposure to high temperatures are all likely to reduce battery performance. While the causes of battery degradation are complex, poor thermal and charge management will quickly accelerate performance losses.
And a battery does not necessarily need to be used to degrade performance; storing a lithium battery for long periods at or near its full capacity or close to empty will also gradually reduce its maximum storage potential; a process referred to as calendar degradation. Given the fact cars are stationary most of the time, calendar degradation and thermal conditions are likely the main determinants of battery longevity.
How many times a battery can deliver its stored energy at a specific rate is a function of degradation. When the battery performance degrades significantly, the battery may need to be replaced or the device (vehicle) retired. While some degradation is inevitable, depending on the application and design, reductions in capacity may not have the same impact on the performance.
How far for how long?
The real question for the range-anxious is for how many times will the battery and vehicle system deliver the maximum (or at least a reasonable) driving range on a single charge?
The original Nissan Leaf from 2012 had an estimate range of just 82 miles when new, and lost as much as 20% of its effective range over 5 years of use. The Leaf relied on a small battery pack compared to current models, and the battery lost potential more rapidly due to a lack of thermal management.
Eight years later, current electric vehicles have three times the battery storage potential and driving ranges approaching 300 miles. The durability of the battery has also improved, where current vehicles could go over 120,000 miles with just 10% reduction in maximum range.
To illustrate, let’s look at how battery degradation and EV driving range might change over the life of a vehicle. Over the average lifetime (~12 years), passenger vehicles in the US travel approximately 155,000 miles, or a little less than 13,000 miles per year. Due to degradation, the battery system is likely to lose between ~11 to 17 kWh of effective capacity over that period. This translates to anywhere from 12% to 50% of the initial battery system size for these vehicles.
A small EV, like the original 2012 Nissan LEAF with a 24 kWh battery, could have realized over 1000 equivalent cycles within the first 5 years of ownership. Compare this to an EV with a 75 kWh battery, where both the rate of range reduction, cycle accumulation, and relative loss in range is less significant.
While the range of an EV is going to decrease as batteries age, the reduction in vehicle range is going to have less of an impact on travel decisions in today’s EVs. The average person in the US travels less than 40 miles per day, and a used EV with 200 to 250 miles of range on a single charge could be a great fit for most travel.
Another upshot is that given the way private vehicles are operated, the lifetime of larger battery packs may be a better match for the average life of vehicles. Longer-range EVs have a better value proposition for used EV buyers, which could also help to decrease the costs of EV adoption.
How can I make my battery last longer?
Cycling is not the only thing that ages a battery. While calendar aging is likely to be the dominant degradation mechanism for EV batteries, exposure to extreme temperatures and the state of charge of the battery during vehicle storage can also impact battery performance.
Exposure to heat will stress the battery and can result in premature degradation; freezing or extreme cold temperatures can also negatively impact battery performance, reducing the effective storage capacity of the battery and increasing loads. That’s a main reason most current EVs rely on active liquid cooling systems to distribute heat evenly across the pack, and ensure a stable range of temperatures during operation.
While time and climate are big factors for battery lifetime, it doesn’t mean you’re powerless to improve the performance of your batteries. Keeping the battery system below its maximum voltage, and above the minimum, decreases the rate of battery degradation. In other words, charging and discharging a battery completely, from 0% to 100%, can degrade the battery performance orders of magnitude more rapidly than small depth of discharge cycles.
EVs already have a battery management system on-board that prevents the vehicle from charging or discharging at the very top and bottom of the potential. But you may still be able to improve your battery life by managing your charging. For example, Professor Jeff Dahn recommends daily charging to less than 75% to maximize battery life. High power charging also stresses the battery, so keep charging slow and low when you aren’t on an EV roadtrip.
Vehicles parked or stored with an empty or full battery will also experience higher levels of degradation than a system with at ~75% charge. One strategy is to charge the battery to slightly lower levels, especially when the vehicle might be stored for an extended period of time. So while it might be reassuring to leave your vehicle at 100% while parked at the airport, the battery will be struggling to maintain that state of charge while you are gone.
Three tips for maximizing the life of an EV battery:
- Limit DC fast charging, particularly in cold weather.
- Don’t drain the battery down to zero, and try to store the vehicle at an average charge level between 25% and 75%.
- While we still can’t control the weather, avoid parking the car for long periods in extreme heat.
Daily or even heavy use (i.e. cycling) doesn’t necessarily translate to accelerated degradation, so drive on! And don’t worry – most batteries will probably outlast the life of the vehicle.
Will I need to replace my EV battery?
While cell phone batteries might deliver a year or two of daily use, EV battery packs are likely to last a long time. Phone batteries lack thermal management, and the number of equivalent cycles per day are typically lower for vehicles. Given the range of EV batteries and the improvements in performance, it seems likely that EV batteries’ lifetimes will be similar to the life of average vehicles.
Batteries may also be retired due to defects in manufacturing or contamination of the cathode material, but well under 1% of all lithium batteries are likely to experience this type of premature failure. And almost half of vehicles are retired before 12 years on the road due to accidents and the subsequent cost of repairs, another reason batteries might outlive the vehicle.
A new EV with a 250 mile range is expected to have 150 to 200 miles of range after 12 years, which is still sufficient to make it useful to the owner or valuable on the used market. Some batteries might be replaced, but many EVs will simply be sold as used vehicles.
Used EV batteries can still be useful. After 8 to 12 years in a vehicle, EV batteries could be removed and have a second-life as solar home power systems or replacements for diesel back-up generators. In these lower power, less demanding applications, batteries could provide an additional 6 to 10 years of service if properly managed.
Used EV batteries could also be reused to support the electricity grid, helping to shift renewable generation and shave peaks in demand. Automakers are also testing second-life batteries for supporting vehicle charging infrastructure, all of which could help to decrease the costs of batteries and provide a better value to consumers. Second-life batteries could also be used to support community-led energy solutions to reduce emissions and increase resilience, or even manufacturing new batteries using renewable energy.
Maximizing the number of cycles a battery can deliver in it’s first or even second application is also a great way to reduce the environmental impacts of new battery production. Because keeping batteries in service longer means we have to build fewer new batteries and decreases the need for increased extraction of raw materials.