A battery unit includes: a battery having cell units connected in parallel, in which one or a plurality of battery cells is connected in series; an external terminal provided for the battery; a switching element provided between each internal terminal and the external terminal of the cell unit; a protection circuit monitoring whether or not a fault occurs in each of the plurality of cell units and cutting off, through the switching element, a connection between the internal terminal and the external terminal of the cell unit with the fault detected; and an informing signal output terminal notifying an external device that the connection with the external terminal with respect to at least one of the cell units is cut off.