In this work, the rate of heat generation in the overcharge period for 103450 prismatic lithium ion batteries (LIBs) of the LiCoO2–graphite jellyroll type with a basic electrolyte consisting of 1M LiPF6–PC/EC/EMC (1/3/5 in weight ratio) has been found to be more important than the gas evolution which was traditionally considered as the main reason in the overcharge protection mechanism. The cell voltage, charge current, and skin temperature were monitored during the charge process. For a single battery or batteries in parallel, LIBs without any additives is an acceptable design if the cell voltage is not charged above 4.55V under the common charge program. The rate of heat generation from the polymerization of 3wt% cyclohexyl benzene (CHB) is high enough to cause the explosion or thermal runaway of a battery, which is not found for an LIB containing 2wt% CHB+1wt% tert-amyl benzene (TAB). In the 12V overcharge test at 1C, the thermal fuse was broken by the high skin temperature (ca. 80°C) due to the polymerization of 3wt% CHB, which was also the case for LIBs containing 2wt% CHB+1wt% TAB. The disconnection of the thermal fuse, however, did not interrupt the thermal runaway of LIBs without any additives because the battery voltage was too high (ca. 4.9V). The influence of specific surface area of active materials in the anode on the polymerization kinetics of additives has to be carefully considered in order to add correct amount of overcharge protection agents.