A power storage system having excellent characteristics is provided. A power storage system having high safety is provided. A power storage system with little degradation is provided. A storage battery having excellent characteristics is provided. A method of operating a power storage system including a storage battery, a first circuit having a function of measuring an impedance, and a neural network includes a first step of stopping charging or discharging of the storage battery, a second step of measuring an open circuit voltage of the storage battery, a third step of measuring an impedance of the storage battery, a fourth step of inputting the open circuit voltage and the impedance that are measured to the input layer, a fifth step of outputting a first signal from the output layer, a sixth step of changing a condition of charging or discharging of the storage battery in accordance with the first signal, and a seventh step of starting charging or discharging of the storage battery; the first signal corresponds to the estimated value of the discharge capacity of the storage battery.