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TIL: Regenerative braking in the cold

TIL: Regenerative braking in the cold

Today I learned that regenerative braking in an electric car doesn't work at all well in the cold.

(And by "cold" I mean -1 – -5 ° C – cold in Scotland, in other words.)

I discovered this when an unfamiliar warning light came on, that I had the look up the meaning of: "Regenerative braking efficiency may be degraded". This turned out to be an understatement.

Electric cars can have several driving that control how the accelerator works. Many drivers, including me, use what's sometimes called the "one pedal" mode. When you press the accelerator, you accelerate; when you reduce pressure, you brake, and the energy is used to regenerate the charge in the battery (hence the term). You don't get a huge amount of power back from this, but it's actually a comfortable driving mode anyway: it feels like the "engine braking" you get in an internal-combustion car with manual transmission, but is somewhat stronger and doesn't require gear changes.

So I took my foot off when driving along and … nothing happened. We kept cruising along at exactly the same speed, with no braking at all. The normal brake still works, of course, but it was a weird change of handling and one that requires quite a lot of care once you're used to one-pedal driving. I think I used the brake explicitly more in going 10km than I'd used it in the entire previous year! After about 15km the handling came back, the warning light went out, and everything went back to normal. That's good, because regenerative braking is so gentle that it's ideal for driving on snow and ice – far better than normal braking, which triggers the traction control if done too harshly.

I have a basic, theoretical knowledge of how regenerative braking works, and I was familiar with it not working well when the battery's full ("because it's harder to push the electrons in" – one of those mental models that's pretty much entirely wrong but still gets you the right answers). It seems this is also the case for cold weather, as cold batteries don't accept charge easily (see here). So the battery has to "warm up", literally. This is a good thing, I think, because it means that the car will eventually perform regardless of the ambient temperature, even if you leave it standing for half an hour.

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