A battery control system for a vehicle including a vehicle body, an engine and a motor ( 41, 42) installed on the vehicle body, a secondary battery used as a driving power source of the motor ( 41, 42), and external charging means for externally charging the secondary battery, using an external power supply located outside the vehicle body, said battery control system being adapted to control the external charging means, the battery control system further comprising: degradation detecting means for detecting degradation of the secondary battery during external charging of the second battery by the external charging means; and external charging current varying means for sequentially varying a magnitude of external charging current with which the external charging means externally charges the secondary battery, using the external power supply, among a plurality of predetermined levels of external current values ( C1, C2, C3), wherein the degradation detecting means comprises current variation type degradation detecting means for detecting degradation of the secondary battery, using the plurality of external current values ( C1, C2, C3) and values of battery voltages respectively developed in the secondary battery when the secondary battery is externally charged at the plurality of external current values ( C1, C2, C3), the battery control system further comprising: state-of-charge detecting means for detecting a state of charge of the secondary battery; and degradation detection executing means for causing the degradation detecting means to execute detection of degradation of the secondary battery, when the state of charge of the secondary battery becomes a specified level of state of charge while the external charging means externally charges the secondary battery, using the external power supply, wherein the degradation detection executing means is multiple level degradation detection executing means for causing detection of degradation of the secondary battery, when the state of charge of the secondary battery becomes one of a plurality of specified levels of states of charge. degradation detecting means for detecting degradation of the secondary battery during external charging of the second battery by the external charging means; and external charging current varying means for sequentially varying a magnitude of external charging current with which the external charging means externally charges the secondary battery, using the external power supply, among a plurality of predetermined levels of external current values ( C1, C2, C3), wherein the degradation detecting means comprises current variation type degradation detecting means for detecting degradation of the secondary battery, using the plurality of external current values ( C1, C2, C3) and values of battery voltages respectively developed in the secondary battery when the secondary battery is externally charged at the plurality of external current values ( C1, C2, C3), the battery control system further comprising: state-of-charge detecting means for detecting a state of charge of the secondary battery; and degradation detection executing means for causing the degradation detecting means to execute detection of degradation of the secondary battery, when the state of charge of the secondary battery becomes a specified level of state of charge while the external charging means externally charges the secondary battery, using the external power supply, wherein the degradation detection executing means is multiple level degradation detection executing means for causing detection of degradation of the secondary battery, when the state of charge of the secondary battery becomes one of a plurality of specified levels of states of charge. state-of-charge detecting means for detecting a state of charge of the secondary battery; and degradation detection executing means for causing the degradation detecting means to execute detection of degradation of the secondary battery, when the state of charge of the secondary battery becomes a specified level of state of charge while the external charging means externally charges the secondary battery, using the external power supply, wherein the degradation detection executing means is multiple level degradation detection executing means for causing detection of degradation of the secondary battery, when the state of charge of the secondary battery becomes one of a plurality of specified levels of states of charge.