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