A state determination device that improves accuracy of measuring deterioration in a secondary battery having a positive electrode and a negative electrode, the state determination device comprising:
- a current sensor that measures a current of the secondary battery;
- a controller communicatively coupled to the current sensor, wherein the controller;
- receives a current value A from the current sensor,
- retrieves, from a memory of the controller, charge and discharge characteristics per reference quantity of the positive electrode and the negative electrode,
- determines a capacity reduction parameter group A based on the charge and discharge characteristics and the current value A,
- receives a current value B from the current sensor, and
- determines a resistance increase parameter group B based on the charge and discharge characteristics, the capacity reduction parameter group A, and the current value B, wherein the current value B is larger than the current value A.