The short-circuit and overcharge behavior of prismatic lithium-ion batteries containing LiCoO2 cathodes and graphite anodes were studied in detail. Internal thermocouples were used to characterize the thermal profiles of the cells under abusive conditions. Differences between the internal and surface temperatures of the cells during the safety tests highlighted the importance of the internal measurement for obtaining more meaningful data. Under short-circuit conditions the cells remained hermetically scaled, reached an internal temperature of 132 degreesC (the shutdown temperature of the separator), and then slowly cooled to ambient temperature. However, on extreme overcharge testing different results were obtained depending on the current used to charge the battery. At low currents (less than or equal toC/5) the cells remained hermetic, but swelled significantly. When higher currents were used, the cells ruptured during overcharge. Experimental cells were constructed with a systematic variation in cell balance and the point of cell rupture tracked to the amount of cathode in the cell, independent of the amount of anode material. The internal dc resistance of the cell was also measured during the overcharge reaction and remained low throughout most of the test, although a large increase was observed at the end of the test due to the melting of the shutdown separator. The cells overcharged with high currents all reached high temperatures (greater than or equal to 195 degreesC) immediately prior to rupturing, which suggests that the melting of lithium is a key underlying factor leading to the rupture of the cells. To test this proposal, cells were assembled with lithium removed from the LiCoO2 cathode, so that lithium metal would not plate on the anode during the overcharge test. These cells reached a significantly higher temperature (similar to 280 degreesC) prior to rupture.