The present invention provides methods for detecting a working condition of a non-aqueous electrolyte secondary battery which allow easy and accurate determination of the degree of degradation and remaining capacity of the non-aqueous electrolyte secondary battery by a simple test irrespective of the past charging and discharging history of the battery. In the methods of the present invention, the degree of degradation of battery is quantitatively determined on the basis of the voltage value in charging or discharging at a constant current, or from an equation with that voltage value as variable.