The assembled battery system according to the present invention includes at least two non-aqueous electrolyte secondary cells, each including a positive electrode that occludes and emits lithium ions, a negative electrode that occludes and emits lithium ions, and a non-aqueous electrolyte comprising an electrolyte dissolved in a non-aqueous solvent, all received in a parallelepiped cell case. These cells are arranged so that each larger area side of an adjacent pair of the parallelepiped cell case faces in parallel one another. A cooling member with a cooling medium flow conduit is provided between each opposing pair of larger area cell case sides, and cooling medium flowing in this conduit directly contacts the sides of the parallelopiped cell cases that define the two opposite sides of the cooling conduit.