Battery percentage drops with restart and rises while charging

Since some time while charging my phone the battery percentage rises slowly and then suddenly says 100%.
At the same time the percentage plummets sometimes - more often after a restart than while using.

I realy don’t get it.
More over since my battery sometimes lasts for 2 or more days and whith no specific things going different, only one day.

I really would like some advice/help!

A post was merged into an existing topic: Battery problems (wrong charging status / battery might be dying)