Why is Lenovo battery for T440S lasts only one year? After the warranty ended, the battery that as new lasted more than 8 hour

Petertal

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Aug 31, 2015
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I have a one year and one month ThinkPad T440S. When it was new, the batteries lasted more than 8 hours. A year after - the computer cannot run more than 1/5 hour on the same batteries. Why?