Thermal diffusion determination method and thermal diffusion detection system for battery pack

Provided is a method for determining thermal diffusion of a battery pack provided with a plurality of cells inside the battery pack, a pressure acting member that acts on a change in pressure inside the battery pack, at least one pressure sensor that detects the pressure inside the battery pack, and at least one voltage sensor that detects the voltage of the cells. In the thermal diffusion determination method, the pressure in the battery pack is detected by the pressure sensor, and the voltage of the cell is detected by the voltage sensor. When the detected amount of change or rate of change in the pressure in the battery pack is greater than a predetermined value, it is determined that the pressure in the battery pack is abnormal, and when the detected amount of change or rate of change in the voltage of the cell is greater than a predetermined value, it is determined that the voltage of the cell is abnormal. Furthermore, when both the pressure in the battery pack and the voltage of the cells are abnormal,