The status applet sometimes goes down way low, then up again (e.g. from 5% back to 25%) - seemingly random.

But this is consistent:
every time, it goes down to 0% but I can still use the phone for at least 2h. And I mean really use it, not just standby. Screen on, screen off, SSH over WiFi, un/installing stuff...
I installed bnf now which seems to prove that the battery is good, but the applet is wrong:
I have checked several times and the SoC (as well as the charge) goes down gently, percent by percent.

When I charge the phone up, the applet shows that, and after unplugging when full it shows something around 95%, but after a few hours it starts going haywire again.

Is this something to worry about? The applet must be getting its values from somewhere, and it can't be that buggy?

The only thing that really annoys me are the "Battery low" notifications I keep getting

Here are two screenshots taken just seconds apart, and some more info:



edit:
I did my due diligence and trawled the forums for at least an hour before I wrote this, going back to 2010 (threads with "battery" in title); but nothing seems to fit here.