Data indicating a relationship of life of a battery to a value of load power applied to the battery in discharge and environmental temperature of a place where the battery is installed are prepared beforehand. Next, the load power and the environmental temperature when the battery is discharged are measured, and then a life value corresponding to these measured values is selected from the data so as to be set as an expected life value. Next, a first life reduction amount is calculated from a natural logarithmic function with the number-of-discharges as a variable, and the difference between the expected life value and the first life reduction amount is set to a remaining life value, on the basis of which the life of the nickel-hydride battery is determined. By this method, the life of the nickel-hydride battery as a backup power source can be accurately determined, while correction based on phenomena unique to the nickel-hydride battery is performed.