A battery system for a vehicle is provided with discharge circuits (R1, 129A through 129D, 128A through 128D) that discharge battery cells (BC1 through BC4) via measurement lines of those battery cells (BC1 through BC4). A control circuit transmits to an integrated circuit (3A) a first discharge command that causes discharge of the odd numbered battery cells (BC1 and BC3) of a cell group (GB1), a first transmission command that causes transmission to the control circuit of the terminal voltages of only the odd numbered battery cells (BC1 and BC3) measured during execution of the first discharge command, a second discharge command that causes discharge of the even numbered battery cells (BC2 and BC4) of the cell group (GB1), and a second transmission command that causes transmission to the control circuit of the terminal voltages of only the even numbered battery cells (BC2 and BC4) measured during execution of the second discharge command; and, based on the these various terminal voltages transmitted from the integrated circuit (3A), the control circuit diagnoses abnormalities in the system that includes the battery cells, the measurement lines, and the discharge circuits.