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.
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