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.

Your battery is good, but it is calibrated wrong. That's why the applet thinks the battery is empty when there's still some juice left. You should try to calibrate your battery, eg. This post (Estel), or just use the search for n900 battery calibration: http://talk.maemo.org/showthread.php?t=92576

^ ah thanks, not sure how I missed this one. It's "only" 6 years old...

An involved process, have to plan ahead a little. Can't start tonight.
I will report back evtl.

It seems to have worked.
bnf still shows slightly different values, but at least the standard battery indicator is making sense now (currently 66% from bnf and 58% from indicator).

It should slowly calibrate itself to be more accurate in some time.

My battery has started to go back and forth. Showing red and then suddenly showing almost 30 percent.

I did the whole process a second time (and both times the phone died before I could switch it off) and it's still like that: better than it was but not perfectly accurate.

Doing now second time.I think it will die until i can do it. New battery, maybe some other thing is wrong.

^ There's no new battery afaik.
It's what the device came with when I bought it (used), an original BL-5J.
Right now, after a full overnight charge, bnf shows 98%, battery status icon shows 88%. Not so good.

Strangely, I just noticed that lsmod|grep batt does not return anything!
Thoroughly confused now.
What's going on?
I thought one of these modules is needed for battery status to work properly, and should be autostarted at boot time?

Probably you are using kernel-cssu which is based on kernel-power53 but has some bleeding-edge features disabled.
So just use kernel-power. It has rx51_battery module.

And yes, I know this is old thread