Battery suddenly dropped to 4%. BME restarted. But after BME was restarted, battery level was 5%. So pretty much immediately it dropped to 4% and another restart was issued. After that battery level was over 10%. Then all kinds of funny stuff happened. It gradually dropped to 6%, from where it started to grow and quite soon reached 12% (I guess in under 5 minutes). There the battery level was stable, and I got a few hours of normal usage of phone (ie. mostly idle).