Commercial Li-ion batteries have been submitted to different stage of overcharge by a "soft" overcharging technique. This technique gives us flexibility to investigate each step of the overcharging process. DSC and SEM tests of the anode, cathode and separator recovered from overcharged cells have been performed. It was shown that the thermal runaway will happen when the heat generation is not equal to the heat dissipation during the battery operated at an overcharge condition. The high overcharging rate shows short responding time for the cell thermal runaway and large amount of heat generation. A kinetic mechanism proposed in this study for interpreting the observation of species growth on the surface of separator in the anode side and causing soft shorting of the cell. (C) 2008 Published by Elsevier B.V.