NOVELTY - The apparatus has a memory (41) e.g. ROM, storing a terminal voltage of a non-degraded secondary battery (11), before the battery is discharged. A CPU (4) calculates the voltage difference between another terminal voltage measured by a voltage measuring portion (2) before a secondary battery i.e. nickel- hydrogen secondary battery, to be measured discharges after fully charged and the former voltage stored in the memory. The CPU estimates the residual life of the battery to be measured based on the life-estimation data map stored in the memory using the voltage difference.    USE - Apparatus for estimating residual life of a secondary battery i.e. nickel- hydrogen secondary battery, for use as a solar light power generator i.e. power supply system, that is driven by natural energy such as wind power and hydraulic power or artificial mechanical power such as an internal combustion engine, of a hybrid electric vehicle.    ADVANTAGE - The apparatus estimates the life of the secondary battery in a simple manner. The apparatus effectively uses the night time power. The apparatus balances the resistance relative to the battery temperature, thus producing the battery temperature for minimizing the voltage difference, which is set in advance as the above reference temperature. The apparatus suppresses the impact of the voltage drop caused by the internal resistance of the secondary battery, so that the secondary-battery module can be fully charged accurately. The voltage measuring portion measures the terminal voltage when the terminal voltage of the secondary battery to be measured stabilizes, thus measuring the terminal voltage more precisely, so that the life of the battery can be more accurately estimated.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for estimating secondary-battery life.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a power supply system.    Power supply system (1, 100)    Voltage measuring portion (2)    Temperature measuring portion (3)    CPU (4)    Communication portion (6)    Host system (7)    Control unit (8)    Secondary-battery module (10)    Secondary battery (11)    Power generator (20)    Load apparatus (30)    Charge-and-discharge controller (40)    Memory (41)    Timer circuit (42)