The overcharge behavior of prismatic lithium ion batteries was studied under abusive conditions. Experimental cells were constructed with a systematic variation in cell balance and overcharge tested to the point of failure. This test demonstrated that the point of cell rupture tracked the amount of cathode in the cell, independent of the amount of anode material. The rate of charge was found to be an important parameter, as cells overcharged at low charge rates remained hermetic while high charge rates (C/2 and above) resulted in cell rupture. The internal temperature of the cells monitored during overcharge was found to be as high as 199°C, which was 93°C higher than the external skin temperature of the cell. Coin cell tests identified the cathode as the source of heat produced in the cell, and Rdc measurements identified a large increase in cell resistance past the full delithiation point of Li x CoO2.