A battery assembly (10) has a series of rectangular cells (20) and a square-U shaped tray (14) to support the cells (20) in series, with bottoms (24) coplanar and adjacent sides parallel. The tray (14) has spaced arms (43) for opposing the endmost ones of the series of cells (12) interconnected by a rigid web (41) that provides a generally flat seating surface for the cell bottoms (24). The web (41) has longitudinal tracks (49) which raise above the seating surface of the wen (41) and which extend beneath all the cell bottoms (24). The cell bottoms (24) have counterpart longitudinal channels (52) for self-assembling with the tracks (49) of the web (41) and provide properly aligned seating of the cell bottoms (24) by disallowing lateral displacement but allowing longitudinal sliding order to form as dense as a single-file line-up (12) as possible. The tracks (49) also serve to stiffen the web (41) against sagging or twisting.