Power supply device including a plurality of battery cells arranged side by side

A power supply device includes rectangular battery cells 1, resin separators 2, end spacers 17, thick metal end plates 10, and coupling members 11. The separator 2 is inserted between the cells 1 to insulate adjacent cells 1 from each other, and in thermal contact with the cells 1. The end spacers 17 cover end battery cells 1 on the opposed end surfaces of a battery block composed of the cells 1 and the separators 2 alternately arranged. The end plates 10 cover the surfaces of the end spacers 17. The coupling members 11 couple the end plates 10 to each other. The separators 2 form gaps 4 for flowing air along the surfaces of the cells 1 in contact with the separators 2. The end spacers 17 have hollow layers 18 on their surfaces in contact with the cells 1, and define closed chambers.