Provided is a method for determining a battery state of a lithium ion secondary battery that allows for understand a deterioration state of the battery without performing full charging of the battery.
To estimate an entire image of a charge-discharge curve on the basis of a fragmentary charge-discharge curve by analyzing a relaxation process after the electric current interruption to grasp the resistance of a battery and taking into consideration a resulting resistance value.
Specifically, a method for determining a battery state of a lithium ion secondary battery includes: a partial charge-discharge curve calculating step of acquiring, by partial charge or partial discharge, a partial charge-discharge curve which is a part of a charge-discharge curve, and calculating a positive electrode partial charge-discharge curve and a negative electrode partial charge-discharge curve; a resistance grasping step of grasping a resistance value of the lithium ion secondary battery on a basis of a relaxation process of a terminal voltage after interrupting a charging current during charging or a discharge current during discharging; and a charge-discharge curve calculating step of taking into consideration the resistance value thus acquired, and acquiring a calculation charge-discharge curve prepared by calculating an entire image on a basis of the partial charge-discharge curve.