@illemann - I think we'll have to see the bigger image, I myself can't understand anything from it...
Anyways - its good that you upped the thread again, as I completely forgot to update it, so here goes:

As I wrote earlier, I bought a brand new original BL-5J battery and put it in my N900, right from the package and without even recharging it, and let it run idle for a while. This is what BatteryGraph recorded from that event:



As you can see - it behaved quite fine and the battery could've last for more than a day this way... That is, until I decided to recharge the battery (with my phone) - this is how it looked afterwards:



So it seems that my battery empties "as normal" (e.g - in 7 hours), so the problem is most probably in my phone, though I have no idea what is causing this, AGAIN
Something more interesting - I noticed that when my battery gets completely drained so my phone is turned off, if I try to plug it into the charger, the led immediately lights in green (which means "battery full") and the phone won't charge it.
This should not happen, but maybe it has something to do with my initial problem...