uncontrollable shutdowns

The problem exists since today (or eventually yesterday).
This morning, I had the feeling that the displayed battery charge percentage was unusual low.
During commute, it suddenly shutdown, restarted several times, broke down several times.

What do I know?
* It seems as if the displayed percentage is not correct. Before one shutdown, it showed 12%. When I connected the charger, it immediately went to 88%.
* Breakdowns happen at irregular charge percentages, 15%, 55%, 68%, ...
* No breakdowns when using during the charger is connected, only when battery only.

So, what do you think is happening here? My device with the first battery is nearly three years old.

Thank you.

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