The safety characteristics of lithium-ion batteries for mobile phones and the nail penetration test

The lithium-ion batteries, which are widely used in mobile phones, portable PCs, and other various portable devices, have the danger which causes thermal runaway essentially, and in the worst case, could ignite or explode, when they are overcharged or overheated. Most lithium-ion batteries therefore use protection ICs and PTCs. However, these protection circuits cannot prevent internal short circuits. It is thus very important to examine the thermal stability of lithium-ion batteries when an internal short circuit occurs. A calculation result using a simple heat transfer model with thermal insulation shows that the local heat generation by short resistance becomes the maximum and the battery will be in the most dangerous state when the internal short circuit resistance becomes equal to internal resistance of a battery. Moreover, the merits of the nail penetration test and its limitations are discussed.