A multi-module battery pack includes a battery tray defining multiple battery tray compartments, battery modules each disposed within a corresponding one of the compartments, and a thermal barrier arranged in a predefined heat transfer path through the tray between an adjacent pair of the battery modules. The thermal barrier has a thickness of at least about 1 mm and a thermal conductivity of less than about 4 W/m-K, such that the thermal barrier blocks the heat transfer path to mitigate a thermal runaway event of one of the adjacent pair of battery modules. An electric powertrain system includes a rotary electric machine having phase leads and an output member, a driven load coupled to the output member, and the multi-module battery pack. A method includes providing the tray, identifying the heat transfer path, and arranging the thermal barrier in the heat transfer path.