A new controlled test method has been developed to simulate the occurrence of internal short in Li-ion cells. Two different internal short kinds namely aluminum shorting to anode and cathode shorting to anode has been studied with this test method at several states of charge. Infra-red imaging technique has been adopted to analyze the thermal propagation for both the short kinds. As a comparison, the most commonly adopted nail penetration test was also conducted and analyzed using IR-imaging. The instantaneous rise in temperature referred as temperature spike upon incurring internal short was able to be captured using the IR imaging for the anode–aluminum short kind and the magnitude of such temperature spike was found to be proportional to SOC of the cell.