NOVELTY - The unit (100) has a blower (30) placed opposite to a side wall (2b) of a battery housing (2), where the side wall extends along a stacking direction without facing a set of surfaces of a module assembly (1). The blower includes a rotation shaft (32) that is located between a set of planes, where the planes are set with the surfaces of the module assembly, respectively. Fins (51a-51d) are arranged along electrode portions (4), and include a set of surface areas that increase toward a downstream location with respect to flow of cooling air.    USE - Battery unit for use in a hybrid vehicle.    ADVANTAGE - The air is supplied toward the surface of the module assembly over a substantially entire range of the module assembly with respect to the stacking direction, and substantially equally to battery modules with respect to the stacking direction while reducing resistance to flow of the air, thus reducing noise while reducing the size of the battery unit. The fin has surface area greater than surface area of another fin, so that the amount of heat transfer is substantially equal between the fins, thus reducing unevenness of heat transfer capability with respect to flow direction of the air, and reducing unevenness of temperature between the battery modules, and hence achieving stable performance of the battery unit. The air flows along outer surfaces of the battery modules, thus improving cooling performance of the battery modules. The heated air is applied to the fins when the temperature of the battery modules is lower than a predetermined temperature, so that the battery modules are heated, thus stably exhibiting battery performance. The unit restricts unexpected short circuits between the battery modules in a simple manner, and efficiently cools the battery modules. The unit efficiently heats the battery modules in a simple manner.    DESCRIPTION OF DRAWING(S) - The drawing shows a side cross-sectional view of a battery unit.    Module assembly (1)    Battery housing (2)    Side wall (2b)    Electrode portions (4)    Blower (30)    Rotation shaft (32)    Fins (51a-51d)    Battery unit (100)