A power supply assembly includes: a casing having a base and at least two side walls; and a set of battery modules. Each of the modules includes a battery having two opposing ends, and two opposing flanges covering the ends. The modules are installed on the base in a plurality of rows of modules arranged in line with one another. The rows are arranged such that the flanges are aligned edge to edge. The aligned flanges are joined to the base of the casing so as to form rigid cross members inside the casing.