Plug for closing off and sealing off an opening in a housing of an energy-storage system , in particular for closing off and sealing off an opening in a housing of a lead-acid storage battery, wherein the plug has the following: - a connecting region , which is configured to come into engagement with the opening of the energy-storage system ; and - a sealing region , which is arranged above the connecting region in an axial direction (A) of the plug , wherein the sealing region is configured to receive a sealing element which is designed to be deformed, in the fitted state of the plug , by a force acting entirely in the axial direction (A) of the plug , wherein a cross section of the sealing element has at least substantially an L shape, in particular already in a non-fitted state of the plug , wherein the sealing element has a first region on a side of the sealing element that is directed towards the connecting region , and has a second region on a side of the sealing element that is directed away from the connecting region , wherein the second region of the sealing element has a larger outer diameter than the first region already in a non-fitted state of the plug , wherein the sealing region is configured to receive the sealing element in such a way that a sealing action of the sealing element is generated by the force acting entirely in the axial direction (A) of the plug , and wherein the sealing element is of fully encircling and rotationally symmetrical form. - a connecting region , which is configured to come into engagement with the opening of the energy-storage system ; and - a sealing region , which is arranged above the connecting region in an axial direction (A) of the plug , wherein the sealing region is configured to receive a sealing element which is designed to be deformed, in the fitted state of the plug , by a force acting entirely in the axial direction (A) of the plug , wherein a cross section of the sealing element has at least substantially an L shape, in particular already in a non-fitted state of the plug , wherein the sealing element has a first region on a side of the sealing element that is directed towards the connecting region , and has a second region on a side of the sealing element that is directed away from the connecting region , wherein the second region of the sealing element has a larger outer diameter than the first region already in a non-fitted state of the plug , wherein the sealing region is configured to receive the sealing element in such a way that a sealing action of the sealing element is generated by the force acting entirely in the axial direction (A) of the plug , and wherein the sealing element is of fully encircling and rotationally symmetrical form. wherein the sealing region is configured to receive a sealing element which is designed to be deformed, in the fitted state of the plug , by a force acting entirely in the axial direction (A) of the plug , wherein a cross section of the sealing element has at least substantially an L shape, in particular already in a non-fitted state of the plug , wherein the sealing element has a first region on a side of the sealing element that is directed towards the connecting region , and has a second region on a side of the sealing element that is directed away from the connecting region , wherein the second region of the sealing element has a larger outer diameter than the first region already in a non-fitted state of the plug , wherein the sealing region is configured to receive the sealing element in such a way that a sealing action of the sealing element is generated by the force acting entirely in the axial direction (A) of the plug , and wherein the sealing element is of fully encircling and rotationally symmetrical form. <EOS>