A stopper (1) for closing and sealing an opening (120) in a housing (110) of an energy storage system (100), in particular for closing and sealing an opening (120) in a housing (110) of a lead acid storage battery, wherein the stopper (1) comprises a connecting region (20), which is configured to engage with the opening (120) in the energy storage system (100), and a sealing region (30), which is disposed above the connecting region (20) in the axial direction (A) of the stopper (1), wherein the sealing region (30) is configured to accommodate a sealing element (32) which is designed to be deformed by a force acting at least partially, in particular completely, in the axial direction (A) of the stopper (1) when the stopper (1) is in the installed state.