This battery system has: secondary batteries (Ca-Ch); temperature sensors (Ta-Th) for measuring the temperature of the secondary batteries (Ca-Ch), and outputting the measured temperature as temperature information; current sensors (Ia, Ib) for measuring the current of the secondary batteries (Ca-Ch), and outputting the measured current as current information; voltage sensors (Va-Vh) for measuring the voltage of the secondary batteries (Ca-Ch) and outputting the measured voltage as voltage information; and a control device (102) into which the temperature information, current information and voltage information is inputted. The control device (102): calculates the charge/discharge power efficiency of the secondary batteries (Ca-Ch) using the current information and the voltage information; calculates the charging rate of the secondary batteries (Ca-Ch) using the temperature information, the current information and the voltage information; determines, using the current information, whether or not the electric charge discharged by the secondary batteries (Ca-Ch) and the electric charge charged by the secondary batteries (Ca-Ch) are substantially the same; and when determined to be the same, determines the degradation of the secondary batteries (Ca-Ch) by comparing the threshold set for the charging rate and the charge/discharge power efficiency.