NOVELTY - Secondary battery system comprises: (a) a battery controller which controls charge and discharge of a secondary battery; (b) a total controller which controls the entire system; (c) an ammeter which detects a charge current and a discharge current of the secondary battery; (d) a voltmeter which detects a voltage of the secondary battery; and (e) determination means for obtaining a direct current resistance of the secondary battery at the time of charge and at the time of discharge on the basis of a current value and a voltage value detected by the ammeter and the voltmeter.    USE - The secondary battery system is used in a hybrid system, a wind power generating system, and a photovoltaic power generating system.    ADVANTAGE - The direct current resistance of the secondary battery system that transitionally (temporarily) increases is simply detected on the basis of the usage situation of the secondary battery, thus accurately calculating the deterioration state of the secondary battery and enabling an appropriate coping process, so that the life of the secondary battery can be extended.    DETAILED DESCRIPTION - Secondary battery system comprises: (a) a battery controller which controls charge and discharge of a secondary battery; (b) a total controller which controls the entire system; (c) an ammeter which detects a charge current and a discharge current of the secondary battery; (d) a voltmeter which detects a voltage of the secondary battery; and (e) determination means for obtaining a direct current resistance of the secondary battery at the time of charge and a direct current resistance of the secondary battery at the time of discharge on the basis of a current value and a voltage value detected by the ammeter and the voltmeter, to determine a temporary increase in the direct current resistance of the secondary battery caused by charge and discharge with a large current on the basis of a relation between the two obtained direct current resistances.    DESCRIPTION OF DRAWING(S) - The drawing shows a configuration of the hybrid system.    Total controller (101)    Engine (102)    Electric power generator (103)    Inverter (104)    Motor (105)    Secondary battery (110)    Voltmeter (111)    Ammeter (112)    Battery controller (120)