Provided are cooling subsystems for a vehicle energy-storage system comprising a heat pipe disposed between two battery modules, the heat pipe being thermally coupled to each of a plurality of cells of the two battery modules at an end of each cell. The heat pipe comprises an envelope and a working fluid, the heat pipe transferring heat from the plurality of cells. Optionally, the cooling subsystem further includes a heat exchanger thermally coupled to the heat pipe, the heat exchanger receiving heat from the heat pipe.