Battery meters on 6510 - unlinear!

Hello :)
My 6510, version 04.06, NPM-9, is almost perfect. It never hangs, crashes, self-resets or does other strange things. I use all of it's functions inclusive radio and wallet. But there is ONE thing that bothers me from beginning:
It got 7 segmentys for the battery meter. After charging, it shows seven. Then i use it. And use. And use... after three days still show seven. On day four, it drops to six. Then, within a 15 minute call, it goes to one and starts beeping. It is as sure as the daily sunrise, when in the morning after powering up the phone there are six segments, it won't do longer rthan five hours anymore, but before three days with a "full" battery.
Thge same Behavior is when i use a BLB-3 battery from a 8855, which fits "just so" in it when i use a 8310 cover. Just with that one it stands a day longer... but same unlinear. And also now, as i am using a flashing battery with only 600 mAh, it stays on for a bit more than two days (i am heavy user) and also unlinear, i.e first day (approx. 1 1/2 hour talked) and still seven segments, next day empty after additional 20 minutes. Is there any chance to change that? I don't want to mess with the software as everything else is really perfect. I heared of other versions that hang or turn themself off and on again....
Does anyone have the same problem?