Provided is a method for designing a battery module including a cell assembly in which a plurality of battery cells are stacked and a module case having an internal space in which the cell assembly is accommodated, the method including: a) setting a lowermost portion of the module case with respect to a direction of gravity as a design side, and setting a region between the design side and the cell assembly as a design space; and b) obtaining a relative permittivity εr of the design space satisfying Equation 1 εrε0A0/T1≤PCy by assuming that a shape of the design space is a form of a film having an area of A0 and a thickness of T0.