Having studied battery lifetimes in an engineering context for a significant amount of time I've regularly wondered how much of the slow battery degradation in these car battery packs is "cheating".

That is how much of the battery capacity is hidden by the battery management system when the car is new and then slowly doled out as the battery ages to make for the appearance of very slow degradation even though the individual raw cells would be wearing out quite a bit faster? If this were true what you would see is after this excess capacity was exhausted would be battery capacity falling off a cliff eventually, though this data seems to show a couple hundred thousand miles of consistent capacity with no cliff.

SSDs do a similar thing for capacity and wear with a sizable proportion of capacity reserved to replace bad blocks as the SSD ages.

Whenever I make this comment almost everyone responding is just guessing about how I'm wrong and new chemistries are so much better, etc.

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barbegal 3 days ago | parent | next [-]

Yeah as far as I can see all the companies that study EV batteries and provide degradation reports etc. all do so by using the data from the manufacturer. I would trust data about battery degradation a lot more if the data came from an independent data logger, logging voltage and current.

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thinkharderdev 3 days ago | parent | prev | next [-]

> I've regularly wondered how much of the slow battery degradation in these car battery packs is "cheating".

Using the word "cheating" has a very negative valence, but it's not exactly a secret that EV batteries are not designed to use their full "raw" capacity. The manufacturer is quite clear that you should avoid charging to more than 80% on a regular basis as it will degrade the battery faster. What matters is not that the batteries are capable of some theoretical "raw" capacity but that the advertised capacity is correct, just like with SSDs. It doesn't strike me as cheating that SSDs have more capacity than what is advertised on the (proverbial) box.

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colechristensen 3 days ago | parent [-]

I don't know the right word, scare quotes were to accommodate for that. If not cheating then at least misleading or avoiding disclosing the actual mechanics and degradation of the battery. To the tune of it might be possible a new car would actually have 50% more than the range it allows you to use to make it seem like the batteries degrade much slower than they do.

>The manufacturer is quite clear that you should avoid charging to more than 80% on a regular basis as it will degrade the battery faster.

This is one of the things that doesn't add up. If the article says you can drive a tesla 200,000 miles and still have a mid-80s percent of total battery capacity left, why are car manufacturers being so clear about charging patterns to "save" the battery? With the std deviation bars in the graph showing a pretty small distribution, it would seem charging behavior doesn't matter (of course there will be people who don't follow the guidelines and if so there should be an expected much wider distribution)

The facts from studying the mechanics of raw cells of earlier lithium chemistries, the advice from the vehicle manufacturers, and the data in this article do not add up.

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colechristensen 3 days ago | parent [-]

And also we have things like this, openly demonstrating much larger capacities

>Tesla extended the range of some Florida vehicles for drivers to escape Hurricane Irma

In this case a selection of Tesla vehicles were temporarily "upgraded" from 60 kWh to 75 kWh, that's 25%!

https://www.theverge.com/2017/9/10/16283330/tesla-hurricane-...

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mikestew 3 days ago | parent [-]

That’s because 60/70/75kWh cars all had the same battery pack, with lower end models software-locked, not because of any “cheating” that you alledge.

	▲	colechristensen 3 days ago \| parent [-]
		Ok the value judgement is up to you but selling a bigger battery pack as a small one will result in exactly the kind of artificial longevity I'm talking about.

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neogodless 3 days ago | parent | prev [-]

I'm a bit confused as you're saying this article refutes your hypothesis, right?

I'll also offer up an example. The Polestar 2 (prior to 2024) has an advertised 78 kWh battery, but also clearly only 75 kWh available for use. That's about 96% right from the factory. So presumably it's doing what you're saying, but it's also not a secret. It's also a way to prevent regular 100% charges from happening, which have proven to accelerate degradation.

	▲	colechristensen 3 days ago \| parent [-]
		Their data fit on the extracted (supposedly real) data between 100k and 300k km suggests that you could drive around the planet 5 times while losing only a few percent of total battery capacity and I don't believe that raw cells behave that way regardless of recent improvements.