I have a K750, the battery is about 15 years old with daily use. The original battery was dead when I opened it, I just bought one when it was new, and it’s still going.
In another thread some days ago others shared similar experiences as you, so maybe I’m the lucky one.
I live in a bright flat, and my computer was always near to the window, it’s charge never went below 80-90%, so maybe that’s the reason for its longevity.
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https://www.amputatorbot.com/