This battery system has: a first battery cell; a second battery cell; a power load that receives current supply from the first battery cell and the second battery cell; wiring that electrically connects together the negative electrode terminal of the first battery cell and the positive electrode terminal of the second battery cell; wiring that electrically connects together the first negative electrode terminal and the second positive electrode terminal; a cell-balancing circuit that makes the open circuit voltage of the first battery cell and that of the second battery cell substantially equal to each other; and a controller. The controller determines loosening of the wiring by making the current supply to the power load possible after having the open circuit voltage of the first battery cell and that of the second battery cell substantially equal to each other.