Stopper for Closing and Sealing an Opening in a Housing of An Energy Storage System and Energy Storage System

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