This disclosure relates to a battery assembly of an electrified vehicle, and in particular to a battery pack with a hinged flap configured to release vent gas. In some aspects, the techniques described herein relate to a battery assembly for an electrified vehicle, including: a battery array including a plurality of battery cells; an enclosure assembly surrounding the battery array; and a frame within the enclosure assembly, wherein the frame includes a flap, wherein the flap includes a perimeter having a first portion along which a first gap is present between the flap and the frame throughout an entirety of a thickness of the frame, and a second portion configured as a hinge of the flap and along which a second gap is present only partially through the thickness, wherein the second gap is formed in a side of the frame facing the plurality of battery cells.