A battery module comprising: a plurality of aligned battery cells each comprising a terminal (11, 12); and a casing for housing the battery cells, the casing comprising: a pair of end plates each configured to face a wide surface of a battery cell at a respective outer end of the plurality of battery cells; and side plates configured to support side surfaces of the battery cells at respective sides of the plurality of battery cells; and a cover configured to cover the battery cells, wherein at least one of the side plates comprises: a wall portion arranged closest to the battery cells to face the side surfaces of the battery cells, a seating portion directly connected to and extending perpendicular to the wall portion, an extension portion extending from and perpendicular to the seating portion and offset from the wall portion inwardly, wherein the seating portion is arranged to receive an end portion of the cover, and wherein the cover includes a first partition wall and a second partition wall inwardly spaced apart from the first partition wall, and wherein the extension portion is inserted between the first and second partition walls. a plurality of aligned battery cells each comprising a terminal (11, 12); and a casing for housing the battery cells, the casing comprising: a pair of end plates each configured to face a wide surface of a battery cell at a respective outer end of the plurality of battery cells; and side plates configured to support side surfaces of the battery cells at respective sides of the plurality of battery cells; and a cover configured to cover the battery cells, wherein at least one of the side plates comprises: a wall portion arranged closest to the battery cells to face the side surfaces of the battery cells, a seating portion directly connected to and extending perpendicular to the wall portion, an extension portion extending from and perpendicular to the seating portion and offset from the wall portion inwardly, wherein the seating portion is arranged to receive an end portion of the cover, and wherein the cover includes a first partition wall and a second partition wall inwardly spaced apart from the first partition wall, and wherein the extension portion is inserted between the first and second partition walls. a pair of end plates each configured to face a wide surface of a battery cell at a respective outer end of the plurality of battery cells; and side plates configured to support side surfaces of the battery cells at respective sides of the plurality of battery cells; and a cover configured to cover the battery cells, wherein at least one of the side plates comprises: a wall portion arranged closest to the battery cells to face the side surfaces of the battery cells, a seating portion directly connected to and extending perpendicular to the wall portion, an extension portion extending from and perpendicular to the seating portion and offset from the wall portion inwardly, a wall portion arranged closest to the battery cells to face the side surfaces of the battery cells, a seating portion directly connected to and extending perpendicular to the wall portion, an extension portion extending from and perpendicular to the seating portion and offset from the wall portion inwardly, wherein the seating portion is arranged to receive an end portion of the cover, and wherein the cover includes a first partition wall and a second partition wall inwardly spaced apart from the first partition wall, and wherein the extension portion is inserted between the first and second partition walls.