Battery pack antipropagation techniques

A battery module 12 for a battery pack (10, Figure 1) includes a plurality of stacked battery cell units (24, Figures 2-4). At least one side of the module is provided with a sheet of flame-retardant material 60. The stacked cells may be held together by module bands 28 and the sheet of flame-retardant material may be provided under at least one of the bands. This may restrict or prevent propagation of a thermal runaway event. The module may have end plates 26, which may act as anti-propagation barriers, and a cover 30. The sheet of flame-retardant material may be provided on sides of a battery module which are not provided with an end plate or a cover. A battery pack has a plurality of a plurality of clamping plates (52, Figure 8) for clamping a plurality of battery modules. An antipropagation sheet (62, Figure 8) is provided on the clamping plates. Alternatively, or additionally a battery pack has rows of battery modules and an antipropagation sheet (80, Figures 10-12) attached to a cross-member (15, Figures 10 and 12) situated between two rows of battery modules.