It is provided an assembly having a plurality of elongated battery cells (6). Each of the battery cells (6) has a top section (61 ), a longitudinal side (62) and a bottom section (63). Either the positive pole or the negative pole of the battery cell (6) is provided on the top section and the other pole is provided on the bottom section of the battery cell (6). Moreover, the battery cells each have an opening (64) on the top section through which the battery cell is able to degas in the event of a fault. A first conductor (17) is provided to electrically connect the pole provided on the top section of at least one of the battery cells. A first electric mat (4) having an upper side and an underside also forms part of the assembly. The first mat (4) has holes (140) extending from the upper side through to the under side. The top section sections (61 ) of the battery cells are positioned on the upper side (41 ) of the first mat (4). A casting compound (17) fills the holes in the first mat and encompasses at least part of the longitudinal sides of the battery cells (6).