How much do electric car batteries degrade?

It’s always the battery in my mobile phone that gives up on me first. After just a few years, it can barely make it through the day without getting another charge. Most electric cars have the same types of batteries — usually lithium-ion — so the assumption is that they degrade just as quickly. This is a fairly common fear for people considering a new EV: “Won’t the battery need to be replaced after a few years?”. And I think it’s even more prominent in the second-hand market: “Oh, I’d never buy a second-hand battery!”. But the types and structures of electric car and mobile phone batteries are not the same. Car batteries are designed to last far longer. A few months ago, I wrote about the fact that the electric versions of many cars were now cheaper than their petrol equivalents in the second-hand market. Most of the responses to that article suggested that battery degradation is the reason why. But looking at the data, I’d say that it’s more likely to be the perception of battery degradation that pushes the value down, not the actual degradation in reality. Pessimism about battery longevity is giving us all cheaper second-hand EVs, which is a nice perk for now, but not great if we want to see a widespread shift from petrol to electric. This is not the only reason I think used EVs are now undercutting the price of petrol cars; the fact that so many newer (and better) models are coming on the market means that many models — even just a few years old — lose some of their comparative value. Again, I think this is a pretty good thing (at least if you’re a prospective buyer). Anyway, let’s take a look at battery degradation: why it happens, how much EV batteries degrade, and how to reduce it. Why do batteries degrade, and how do rates of degradation vary over their life? Two types of degradation happen in an electric car battery: First, calendar aging, which is when the battery loses capacity over time, even when the car isn’t being used. So if you were to have an electric car, and not touch it for a year, the battery would still experience small amounts of degradation. Why does this happen? Lithium-ion batteries have a thin layer called the SEI (Solid Electrolyte Interphase) that forms on the anode surface. This slowly grows thicker over time, and as it thickens, it uses lithium and reduces usable capacity. Calendar aging tends to be small — typically around 1% to 2% per year — but can be higher in very hot climates. Second, we have cyclical aging, which is the degradation that happens when batteries charge and discharge. Every time a battery recharges or discharges, lithium ions move in and out of the electrodes. The mechanical stress of this process gradually creates structural changes in the electrodes, which reduce their capacity. This charge cycling can also grow the SEI layer (which I mentioned above), reducing usable capacity. Before we quantify how big this effect is, it’s interesting to look at how these processes work over the life of a battery. In the chart below, you can see battery retention measured across a large cohort of Teslas up to 200,000 miles (that’s already telling us something about how big the effect is). But what’s interesting is that degradation tends to happen quickest in the first 20,000 miles or so. This is because initial lithium salts react with other materials and start building that SEI layer we discussed earlier. After this initial drop, degradation is fairly slow and linear. Of course, this fact might be one of the explanations why even fairly low-mileage electric cars quickly lose a lot of value once they’ve been driven. As soon as you get on the road, you’re entering the steepest part of the decline. What’s missing, though, is the context that the overall drop in capacity is still small — probably around 3% to 5% within 25,000 miles — and degradation won’t continue at this rate. So if you buy a second-hand electric car that’s done 20,000 miles, it’s not going to degrade at the same pace that it was. Most electric cars still have more than 80% capacity after 200,000 miles We’ve now had enough electric cars on the road - and for long enough - to have a good idea of how the battery holds up over time. Here we’ll focus on a metric used to capture the battery’s “State of Health” (SoH). It’s what percentage of a battery’s initial capacity is still usable after a given number of miles or years. Let’s start with the results of the huge Tesla cohort that we looked at above. In its 2023 Impact Report, Tesla reported that after 200,000 miles of use, the batteries in a Model 3 and Model Y had lost just 15% of their capacity, on average. For the Model S and X, it was just 12%. That’s not bad, given that most cars are scrapped somewhere in the 150,000 to 200,000 miles range. At that point, a Tesla will have more than 80% of its initial capacity, and in some cases, even more. So people will probably give up their car, well, well before the battery gets close to becoming a burden. What about other car models? The very early Nissan LEAFs — one of the first electric cars to break through — did have real degradation problems, especially in hotter climates. They used a passive thermal management system — in other words, there was no active cooling of the battery — which led to faster degradation. Many of these batteries would need to be replaced. But the early Nissan LEAFs were a vital lesson. Most manufacturers do not experience the same issues today. Manufacturers such as Tesla, GM, Kia and Volkswagen using liquid cooling systems to prevent this. A large study of 7,000 cars by AVILOO — some of which had done as much as 300,000 kilometres (almost 200,000 miles) — found that the majority still had more than 80% of battery capacity, even at these high-mileage levels. In another study across 15,000 cars — which had collectively clocked up 250 million miles — just 1.5% had needed a battery replacement for any reason, so the share that needed one due to degradation was probably even lower. I would expect that many cars with far more than 200,000 miles would still have a fairly healthy battery left. But not many cars get to this driving distance, and I’d be a bit cautious about survivorship bias if we had a very small sample size. This is also something to be aware of, even when talking about 200,000-mile vehicles, although here the sample sizes are not that small. Laboratory models tend to overestimate rates of degradation When looking at degradation rates, I’d recommend looking at real-world data rather than some of the earlier models. A common model — the P3 SoH — tends to overestimate degradation rates and is, therefore, too pessimistic about how long EV batteries last. It’s based on battery cell data generated from laboratory tests (without a battery management system), but these tend not to be a great match for real driving conditions. In the chart, you can see the P3 SoH predicted line in red, and the Aviloo trend line - based on real car data - in blue. Some cars do degrade as quickly as the P3 model would suggest after 200,000+ miles, but these tend to be the poorer-performing outliers, rather than the typical experience. A study published in Nature Energy also found that under “real” driving conditions, batteries lasted around 38% longer compared to laboratory tests. If you want to understand car batteries, we now have more than enough data from actual drivers and experiences on the road. Just look at that. Many manufacturers provide long warranties for their batteries The final reason to have confidence in the performance of batteries over time is that manufacturers clearly have confidence. Most now offer battery warranties: if your battery degrades more than this within a given mileage or timescale, then they’ll repair or replace it for you. Most manufacturers offer a warranty somewhere in the range of 8 to 10 years, and 100,000 miles. That usually means that if your battery is below 70% health within either 8 years or 100,000 miles, they’ll replace it for you. Some are going even further. Some Mercedes models offer over 150,000 miles and 10 years. The Lexus UX300e offers over 600,000 miles. They clearly have a huge amount of confidence that by the end of your car’s life (which is going to be at far less than 600,000 miles), the battery will still have well over 70% of capacity. How to protect the health of an electric car battery The point is not that battery degradation is not an issue at all. Knowing that your car’s capacity could drop by 10% to 20% over its lifetime is important and useful to know. I said the same in my article on how a car’s range changes in cold temperatures: they do lose a bit and I think it’s important for buyers and drivers to know that up-front. The point is that for most drivers, it’s not a dealbreaker. Most are going to manage fine, even with this drop. But it’s also important to know what things we can do to protect the battery, and slow this degradation as much as possible. Some of these things will not be new to most of you. But here’s a relatively uncontroversial list of things that are recommended: Avoid extremely high or low temperatures . This tends to increase both cyclical and calendar aging. Try to keep it out of direct heat if you can. If you can find a model with a heat pump, this is useful to reduce degradation from cold charging. Avoid extreme “state-of-charge” . Leaving the battery sitting with more than 80% or less than 10% of charge can accelerate calendar aging. Don’t fast charge all the time. Fast charging can increase degradation rates, so only use it when necessary. There are examples of EVs in taxi fleets — which relied heavily on fast-charging — where batteries needed to be replaced. Finally, it’s worth noting that battery designs and chemistries are getting better every day. From here on out, this is as bad as things are going to get. The longevity of the batteries that went into cars a decade ago — and are now reaching 200,000 miles or the end of their lives — is worse than that of the ones going into cars today. As I’ve previously said about the emissions associated with an electric car, things are only going to get better.