NOVELTY - The apparatus has a unitary disk shaped rupture diaphragm (4) comprising an outer annular portion (43) placed around periphery of an inner portion. A circular rupture line is defined at a boundary between the inner and outer portions, where the rupture diaphragm ruptures partially along the rupture line when fluid pressure exerted on a first major face is exceeded by fluid pressure exerted on a second major face by a threshold value i.e. 1.0 bar. A cover portion (5) applies clamping force to the outer portion of the rupture diaphragm to hermetically seal a vent aperture (23).    USE - Vent apparatus for a battery casing of a battery e.g. high voltage traction battery such as lithium-ion battery, for a vehicle (all claimed) i.e. hybrid electric vehicle (HEV). Can also be used for a nickel metal hydride battery, and for a battery electric vehicle (BEV).    ADVANTAGE - The diaphragm ruptures along the rupture line when the pressure inside a battery cavity increases to a point at which the pressure difference on opposite sides of the diaphragm exceeds the threshold value such that pressurized gas in the cavity can flow out of the battery casing through a throat of a body portion and out through apertures in the cover portion, so that rapid venting of gas in the cavity can be achieved when the diaphragm ruptures if compared to amount of gas, which can be vented through a breather port alone, thus mitigating effects of pressure increase, which can lead to damage to the battery casing and can impair performance of the battery. The diaphragm can be replaced by removing the apparatus from the casing and replacing the apparatus with a new-apparatus in a simple manner without directly attaching the diaphragm to the casing by an adhesive when the diaphragm is ruptured inadvertently with no battery fault.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a vent apparatus.    Unitary disk shaped rupture diaphragm (4)    Cover portion (5)    Vent aperture (23)    Outer annular portion (43)    Planar cover plate (51)