NOVELTY - The deterioration rate calculation method involves measuring the amount of discharge, the discharge rate and discharge temperature of a secondary battery. The amount of accumulating discharge in a predetermined time, an accumulating average discharge rate and accumulating average discharge temperature is computed based on the history of the measured amount of discharge, a discharge rate and discharge temperature. The leaving time, charge condition and the leaving to-stand temperature is measured at the time of leaving-to-stand of the secondary battery.    USE - Deterioration rate calculation method for a secondary battery for use in electric power plants, such as photovoltaic power generation plant, wind power generation plant, a building and a residence.    ADVANTAGE - The deterioration rate calculation method involves measuring the amount of discharge, the discharge rate and discharge temperature of a secondary battery, and thus enables correctly grasping deterioration state of secondary battery and correctly estimating lifetime of secondary battery.    DETAILED DESCRIPTION - The accumulating leaving time is computed in the predetermined time. The deterioration quantity of the secondary battery is computed in the predetermined time by using the computed amount of accumulating discharge, accumulating average discharge rate, accumulating average discharge temperature, accumulating leaving time, accumulating average charge condition and accumulating average leaving-to-stand temperature. INDEPENDENT CLAIMS are included for the following:    (1) a life prediction method for a secondary battery;    (2) a deterioration rate calculation system for a secondary battery; and    (3) a life expectancy predicting system for a secondary battery.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a battery life prediction system. (Drawing includes non-English language text).    Deterioration calculation part (3)    Deterioration changes estimation part (4)    Lifetime calculation part (5)    Battery pack (10)    Cycle history part (21)