Ah yea, sorry for not updating the thread.
Flashing without the microSD card didn't help

BUT! I conducted a little experiment - I switched my battery with my brother's girlfriend battery (she has a Nokia 5800) for a couple of hours, and waited BatteryGraph to log at least two hours of usage with that battery, and even though her battery was almost drained - look at that:



(Surrounded with a red box)
This led me to believe that my batteries are busted (maybe because of the excessive use of a stand-alone battery charger?), so I bought a new battery from EBay and it should arrive to me in this week or the next... I will update this thread as soon as I'll get it and use it for a while