A battery unit is provided which includes a control board, a storage case, a water detector, and an electrical conductor. The storage case is made of an assembly of a first and a second casing member. The first casing member has an upright wall which extends from the bottom thereof and surrounds the battery. The second casing member also has an extension which extends toward the bottom of the first casing member. The water detector is located close to the lower end of the extension of the second casing member. The electric conductor extends upward from the upper end of the extension and are electrically joined to the water detector. The electric conductor is also mechanically and electrically joined to the control board. This structure facilitates the ease with which the water detector is installed electrically and mechanically in the battery unit.