A fan blows air across a battery package, which includes battery modules in a holder. The holder includes fins, configured such that air contact areas of battery modules at an upstream part are smaller than those at a downstream part. The fins at the upstream part may be longer than those at the downstream part. Alternatively, fins at a first side of the battery modules may be longest at the upstream part. Fins at a second side may be shortest at the upstream part. An air channel between the fins and the battery modules narrows from the upstream part to the downstream part. The fins at the upstream part may be closer to corresponding battery modules than those at the downstream part. Each fin may define a substantially rectangular cross-section. Alternatively, each fin may have an inclined surface on an upstream side and/or a declined surface on a downstream side.