Electric cars have not been around long enough for us to tell. The best we can do is observe the apparent degradation in those cars on the road. Even that observation can prove a challenge, though, since most EVs have been on the road well under six years, with almost 30% sold in 2022. So, we still have very little sense of how they degrade over their lifetime - which car makers say should be as long as 20 years. So far, it seems that EV batteries have much longer lifespans than anyone imagined, since very few of them have been replaced, even once the 8-year, 100,000 mile warranty period ends.
Why do we have to worry about battery degradation in EVs?
Most electric cars rely on a high voltage lithium ion battery, which is the same sort of battery found in other household devices. Lithium ion batteries start to degrade as soon as they are made - it’s an unavoidable part of battery science that you probably noticed with your cell phone and your laptop. Even if you never use lithium ion batteries, they slowly lose power and efficiency over time. The good news is that your EV battery is far more complex and sophisticated than other lithium ion batteries in your life and is built to ensure its lifetime exceeds its warranty - and more.
Coming up with an exact answer to what a battery lifetime is complicated by two things:
Batteries are complicated systems
Just because you turn on a battery powered device and it works, it doesn’t mean it's a small engineering feat. Batteries are carefully calibrated chemical, physical, and electrical systems that harness some of the most fundamental principles of science to produce electricity. However, it can be hard to tell what’s going on inside of them.
Some of the limitations of the standard battery are:
- You can’t see how much energy the battery has remaining
- You can’t see how much power the battery has remaining
- The total available capacity of the battery can only be estimated, not directly observed.
The solution is a battery management system (BMS) that communicates between the battery, the electric car, and the driver. The BMS is a computer that gets data from the battery, and extrapolates and computes information. It also runs software to protect the battery from dangerous voltages, extreme temperatures, and extreme state of charge during charging and use.
We know more about battery cells than battery packs
There have been decades of research into battery cells of all chemistries, including lithium ion. This research helps us understand what causes battery cells to degrade more quickly:
- High heat - high temperatures accelerate the chemical and physical reactions that break down battery materials
- High voltage - high voltage, which often leads to high heat, can also accelerate reactions
- Extreme state of charge and depth of discharge - lithium ion batteries are most stable when their state of charge is near 50%. High state of charge, low state of charge, or using a lot of the battery before recharging can all accelerate degradation
However, an individual cell, which is made up of a positive and negative electrode, and an electrolyte between them, is much more vulnerable and delicate than a battery pack, which includes a BMS to help regulate the factors above. And, not all battery packs are created equally. A pack can link together thousands of cells in different configurations and with different wiring. Plus, the software and hardware that is used in battery management are all different and proprietary.
For instance, Nissan was famous for using passive air cooling in its LEAF batteries. This left early EV batteries more vulnerable to heat than in cars that used liquid cooling to manage battery temperatures. The way Tesla calculates battery state of charge is different from the way Chevy does. And so on.
Put all together, this means that scientific wisdom about battery cells does not necessarily translate to real world advice about battery packs. Instead, we have to use observations. Until more EVs have been on the road for more than ten years, we rely largely on the oldest models: The Nissan LEAF and Tesla Model S.
What we do know: the shape of degradation
Lithium ion battery cells all go through degradation in a similar way.
- When they are first used, there is a brief period of pretty rapid range loss as the battery builds a protective layer around one of the nodes. This protective layer is made up of lithium, so some of the energy source is used up. In some cars, this first phase seems to last 20-30K miles, and in other cars, we have not seen a very distinct first phase at all. Note that almost all battery manufacturers also pre-condition batteries so they have some protective layer already built before they are used in a car.
- After this layer is created, the battery health starts to level off and decline in a slow, linear fashion - kind of like how people age. We don’t notice major changes in energy levels or agility over a few weeks or months, but when we look back over years of our life, we do. This is where EV batteries spend most of their lives, at least so far as we’ve seen.
- Finally, batteries reach the end of life where the battery capacity rapidly declines again. No one is really sure when EV batteries reach this point, because it has not really happened in cars with healthy lithium ion batteries.
What we do know: most batteries are still in cars
There have been two major battery recalls in recent years, both related to similar packs in the Chevrolet Bolt EV and EUV and the Hyundai Kona Electric. The remedy for both of these recalls was a sweeping battery replacement program covered by the manufacturers. Other than these two recalls, though, battery replacements in the Recurrent community remain rare. In the charts below, we show the number of owner-reported battery replacements in our community of around 15,000 drivers:
The LEAF and the Model S unsurprisingly have the highest percentage of battery replacements, due to the fact that they are some of the oldest models in our fleet. Below, we look at the same battery replacements by model year.
The peak in 2017 battery replacements is due to the Chevrolet Bolt recall, in which the battery replacement priority was given to 2017 models.
We saw increased numbers of non-recall battery replacements in:
- 2013 Tesla Model S (8.5%)
- 2014 Tesla Model S (7.3%)
- 2015 Tesla Model S (3.5%)
- 2011 Nissan LEAF (8.3%)
- 2012 Nissan LEAF (3.5%)
Read more about what to expect from certain makes and models here.