A battery unit for a vehicle is provided. The battery unit includes a lower case having two battery compartments arranged in a direction toward opposite sides of the vehicle, respectively, and a connecting portion bent to be convex upwardly between the two battery compartments. A reinforcing structure is disposed on the connecting portion. Two battery modules are installed in the two battery compartments, respectively and a power wire is electrically connected to at least one of the two battery modules and extends from one of the two battery compartments to the other one of the two battery compartments through the connecting portion.