A battery pack including a battery pack housing, and a battery module disposed in the battery pack housing, Tire pack housing is sealed and flooded with a dielectric fluid. The battery module includes a module housing that is fluid permeable and includes a fluid passageway, and electrochemical cells disposed in the module housing in such a way that terminals of the cells are exposed to fluid disposed in the fluid passageway. The battery pack includes a thermal management system having an inlet plenum assembly disposed at a first end of the battery module, an outlet plenum assembly disposed at a second end of the battery module, and a fluid pump that directs fluid to the inlet plenum assembly via a fluid delivery line and receives fluid from the outlet plenum assembly via a fluid return line.