Abstract: A battery pack includes: a plurality of cylindrical battery cells, each one of the plurality of cells having a cylindrical outer cell wall that extends in a z-direction between opposed first and second ends of the cell; a carrier structure that locates the battery cells relative to one another and arranges them in an array in an x-y plane perpendicular to the z-direction; and a plurality of barriers for preventing propagation of thermal runaway between cells of the array, each one of the plurality of barriers forming a barrier wall that surrounds a group of one or more of the cells. A gap is defined between the barrier wall of each respective barrier and the outer cell wall of each of the respective one or more cells enclosed by the respective barrier.