Here is the theory that I tend to go along with (for what it is worth!) One of the properties of a semi conducting material is that the resistance of the material drops as it's temp increases (As opposed to any other material where the resistance increases with temperature).. .Therefore when you charge the battery too fast, it heats up and it's resistance drops and so a larger current flows. The heat produced in the battery is given by Power = Voltage * Current or more importantly Power = Resistance * (Current Squared) so although the drop in resistance would reduce the heating effect this is more than countered by the effect of the increased current (because it is dependent on the current squared).. .As the Semi-conductor heats up further, so the resistance drops - more current - more heat - less resistance - more current and so on ad-infinitum until it goes bang or you disconnect the battery. (In retrospect can you go on ad-infinitum until something happens?)