A battery unit is constituted of a battery assembly, coolant for cooling the battery assembly, a battery case containing the battery assembly and the coolant, an electric resistivity detector for detecting the electric resistivity of the coolant, and a battery ECU that determines whether the electric resistivity detected by the electric resistivity detector is equal to or lower than 102 Ωm. If the electric resistivity is equal to or lower than 102 Ωm, the battery ECU commands a battery protection circuit to interrupt the current output from the battery assembly.