Cooling air intake port (52), cooling air exhaust port (55), and securing walls (86, 87), which contact and secure the side surfaces of one or more battery cells (72), may be defined within two battery pack housing halves (50, 80). When battery pack (99) is assembled, at least one cooling air passage (91, 92) is defined by the side surfaces of the battery cells, the interior surface of the battery pack housing, and the securing walls. The cooling air passage connects the cooling air intake port to the cooling air exhaust port. By forcing cooling air through the cooling air passage, the battery cells can be effectively and efficiently cooled.