It took so long, because after sending the battery to the dealer and receiving it back, it stopped working before I could even check if the problem was solved, because water came inside when using in rain... So I had to send it back once again. Didn't know that the battery almost isn't water resistant at all, unlike other bike accessories using electricity.
And then winter came with temperatures below 0, so I didn't want to risk damaging the battery and sending it once again...
At last I managed to discharge the battery and it looks like everything is rather OK - it cuts out at far below 50 V (don't remember exactly), the lights on the throttle show "half" and "empty" before the battery is completely discharged, and charging takes a few hours. Thanks again for your help, at least everything works now