Vehicle power source device wherein cooling air is introduced into battery casing through opening formed through vehicle floor

An electric power source device including a plurality of battery cells (22) accommodated within a battery casing (30) installed on an automotive vehicle (12), such that the battery cells are cooled air introduced from a compartment of the vehicle into the battery casing, wherein the battery casing (30) is disposed adjacent to the vehicle compartment and is generally isolated from the compartment by a floor or covering member (16, 144) which partially defines the compartment or covers the battery casing, and the floor or covering member has an inlet (96; 114; 156) of an air intake passage (32, 34, 92, 94, 96; 31, 32, 106, 108, 112, 114; 31; 32; 108, 112, 156, 158), through which the air is introduced into the battery casing, for cooling the battery cells (22).