Battery pack and method for detecting degradation of battery

A battery pack comprising:

• one or two or more secondary batteries;
• a charge control switch that turns on/off a charging current to said secondary battery;
• a discharge control switch that turns on/off a discharging current from said secondary battery;
• a current-detecting element for detecting said charging current and said discharging current;
• a voltage measuring part that measures the voltage of said secondary battery;
• a control unit that controls said charge control switch and said discharge control unit;
• and
• a storage unit that stores an initial internal resistance of said secondary battery, wherein
• said control unit measures a closed circuit voltage and a charging current during charging, and a first closed circuit voltage after a first waiting time and a second closed circuit voltage after a second waiting time elapsed from turning off said discharge control switch immediately following the measurement of said closed circuit voltage, where said second waiting time is longer than said first waiting time,
• said control unit calculates a DC resistance component from said charging current and a voltage difference between said closed circuit voltage and said first closed circuit voltage,
• said control unit calculates a polarization resistance component from said charging current and a voltage difference between said first open closed circuit voltage and said second open closed-voltage circuit, and
• said control unit further calculates a degradation degree of said secondary battery by subtracting said initial internal resistance from the sum of said DC current resistance component and said polarization resistance component.