After extensive testing (taking into account that the N900 is confused by the unknown battery) I decided that this battery is f***ed. Sometimes holds a charge, sometimes doesn't, and worst of all: it has some sort of loose contact so whenever I twist the phone slightly, it switches off. 100% reproducible, does not happen with my other battery.

This battery was sold to me as "brand new and unused" (and without a picture), ergo the seller is a liar (see image - clearly taken from a trash phone). I did get my money back though.