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Solar modules deployed in France in 1992 still provide 75.9% of original output power
(www.pv-magazine.com)
This is a most excellent place for technology news and articles.
Similar holds for EV or phone batteries. They usually don’t suddenly die, but lose more and more health over time. Realistically, you have to set a threshold, where you call it no longer useful.
If the life expectancy was 80%, then we’ve passed it and they are due for replacement. If it was 70% the they still have years of useful life.
It’s probably one of those two. For phones, I replace batteries when health drops to 80%, because I spend too much of my life online. Also, I’m probably giving my phone to my kid about then, so they deserve a fresh battery. I have kept phone batteries down to about 70% life, but then it usually doesn’t last the day and I’m carrying portable chargers everywhere
I haven’t had an EV long enough but I believe the typical battery warranty is defined like that: not just that it’ll work for 10 years, but that it will still be at least 70% health after ten years