Pressure Equalising Device and Test Method

The application discloses a pressure equalizing device (, ) for a battery box (). The pressure equalizing device comprises: a cage (, ) for attachment to a valve opening () in the battery box, the cage defining a fluid passage (, ) for movement of gas between an inside of the battery box and an outside of the battery box; a valve element (, ) covering the fluid passage, and a spring () arranged to urge the valve element against the cage so as to close the fluid passage and such that the valve element is moveable to an open configuration when an internal pressure inside the battery box exceeds a threshold pressure. The valve element comprises a breathing opening (, ) and a breathable membrane (, ) covering the breathing opening. A test method and apparatus is also presented.