I'll try removing battery-related software first. Device is only 3 weeks old and I haven't reflashed it (yet).
I'll let you know how it goes.

Your signature says Mugen battery. Is this accurate?

Also, you can "d*mm well see" [sic] what you see, the N900 is designed to stop charges over a certain level. If yours doesn't, you have a problem, because once the battery stops charging if the phone starts charging it again it will exhaust its cycle count fairly quickly.

Either you have a non-standard battery, you messed with settings or something is plain broken. It should not charge already charged batteries.

In fact, I'm testing now.

mine stops at about 95%, and if i plug it in at 80% it still charges, not sure when it is over 90% which i agree it shouldn't charge if it is over 90%

my signature says HOMEMADE mugen battery, but apparently this is confusing so now it's is going to say XL battery

I thought that was what I said. Oh well.

At 90%, I get battery full from both wall wart and USB cable. From memory, it charges at 80, but not at eighty-something. It's hard to determine, because the percentage keeps going up and down on the same charge level, depending on usage.

I think that at one time I managed to do it at 85%, with a very high load (I started everything). Pretty sure it refuses at 90%.

Yes lets agree that it is about 90%.

I was thinking of trying to make my meter as actuate as possible, but then im afraid that it will use power doing it.
The way to make it more accurate is to make data points and then compare and make an estimate on power left

The "bad" thing with my meter is that i reacts to the voltage so it can fluctuate a bit, even on my 3000+ battery it can fluctuate by 10%

Thanks for your replies. I see some interesting general conversation on battery going on.
I was so used to the Advanced Power Monitor on status bar that I forgot it was an app (showing the percentages and time left). And for this reason didn't mention that I got that installed. Of course, I cannot be sure if that was causing the problem, but I'm getting the feeling it was.
Please keep on talking if you like to. I'll let you know if the problem pops up again.

In general, the builtin meter in maemo, as well as any meter that takes data from the same source (advanced power included) is very inaccurate.

Also, the "Battery Full" notification runs independently of whatever the charging chip is doing.