SAFE STATE DETECTION METHOD FOR LITHIUM-ION SECONDARY BATTERY, SAFE STATE DETECTION APPARATUS, POWER STORAGE DEVICE, SAFE STATE DETECTION SYSTEM, AND RECORDING MEDIUM

A safe state detection method for a lithium-ion secondary battery, includes: calculating an absolute value of a differential coefficient of a discharge curve from which a voltage equivalent to a voltage drop of the lithium-ion secondary battery is removed; determining a first battery voltage when the degree of increase in the absolute value of the differential coefficient is greater than a threshold value; determining a second battery voltage at a start of an increase in oxidation heat in response to the first battery voltage, the oxidation heat being heat generated inside the lithium-ion secondary battery when the lithium-ion secondary battery is overcharged; and detecting a safe state of the lithium-ion secondary battery, based on the determined second battery voltage.