Abstract Effects of internal short circuit (ISCr) on thermal stability of Li-ion cells of various sizes (130–1100 mAh) are investigated using a combination of experimental methods and thermal modeling. Three experimental methods were evaluated: small nail penetration, small indentation, and cell pinch test. The small nail penetration and indentation tests created significant heat sinking to the cell-can or to the nail. Only the cell pinch test provided a reasonable approximation of a high risk ISCr event. ISCr location plays a critical role in the consequences of an ISCr event. ISCr at the edge of the electrode is the worst case because of its limited heat conduction to the cell-can. The effects of cell capacity and state of charge on ISCr are also evaluated through tests and thermal modeling.