A battery assembly (802, 1702) comprising: a battery housing forming a battery compartment (906, 1002) having a width and length greater than a height; a battery array comprising a plurality of cylindrical battery cells (1202, 1506) oriented with an axis perpendicular to the height of the battery compartment, wherein the plurality of cells are organized into a plurality of rows within the battery compartment in which each row comprises a width corresponding to a length of one battery cell and a height corresponding to a height of stacked battery cells; and characterised by a plurality of cooling plates oriented perpendicular to the axis of the battery cells and positioned proximal to an end of battery cells within a row of the plurality of rows, wherein a first cooling plate is positioned between a first row of cylindrical battery cells and a second row of cylindrical battery cells and a second cooling plate is positioned between the second row of cylindrical battery cells and a third row of cylindrical battery cells; wherein the cooling plates are held in place by end plates, and a coolant pump is provided to pump fluid coolant, which comprises water and/or a refrigerant, through the cooling plates and end plates; a first bus bar (1402, 1502), positioned between the first row of cylindrical battery cells and the first cooling plate, said bus bar comprising a conductive sheet with a plurality of battery contact portions, each battery contact portion being electrically coupled to an electrode of a cylindrical battery cell in the first row of cylindrical battery cells; and an electrically insulating, thermally conductive layer positioned between the first bus bar and the first cooling plate. a battery housing forming a battery compartment (906, 1002) having a width and length greater than a height; a battery array comprising a plurality of cylindrical battery cells (1202, 1506) oriented with an axis perpendicular to the height of the battery compartment, wherein the plurality of cells are organized into a plurality of rows within the battery compartment in which each row comprises a width corresponding to a length of one battery cell and a height corresponding to a height of stacked battery cells; and characterised by a plurality of cooling plates oriented perpendicular to the axis of the battery cells and positioned proximal to an end of battery cells within a row of the plurality of rows, wherein a first cooling plate is positioned between a first row of cylindrical battery cells and a second row of cylindrical battery cells and a second cooling plate is positioned between the second row of cylindrical battery cells and a third row of cylindrical battery cells; wherein the cooling plates are held in place by end plates, and a coolant pump is provided to pump fluid coolant, which comprises water and/or a refrigerant, through the cooling plates and end plates; a first bus bar (1402, 1502), positioned between the first row of cylindrical battery cells and the first cooling plate, said bus bar comprising a conductive sheet with a plurality of battery contact portions, each battery contact portion being electrically coupled to an electrode of a cylindrical battery cell in the first row of cylindrical battery cells; and an electrically insulating, thermally conductive layer positioned between the first bus bar and the first cooling plate.