A battery pack apparatus has a plurality of battery cell units that are stacked together in generally parallel relation. The battery cell units are configured to define converging air flow spaces therebetween. An air inlet header provides a converging air inlet plenum that is situated adjacent one side of the battery cell units and an air outlet header provides a diverging air outlet plenum that is situated adjacent an opposite side of the battery cell units. A blower or fan forces air into the air inlet plenum. The air flows through the air flow spaces between the battery cell units to cool the battery cell units. The speed of the air increases as it advances through the air inlet plenum and the plurality of air flow spaces.