Earlier quoted context omitted.
Your battery is likely worn from keeping it charged at 100% constantly, lenovo have an option to limit max charge to something like 80% which will greatly reduce battery wear, you just have to turn it off if you are actually planning to run it without AC for as long as possible.
It is terrible to limit max battery charge, so max battery charge doesn't decline!!!
Its the same with batteries, you keep them at 100% constantly they will wear out. Its just a limitation of the technology.
Some devices now try and use "machine learning" to determine max charge, for example iphones will maintain a medium charge throughout the night then do a final top up charge to reach 100% just before you wake up.