Remaining capacity calculation method for secondary battery, and battery pack

A battery pack of a secondary battery, comprising a measurement unit for measuring voltage and current of the secondary battery, and a remaining capacity calculation unit;

• wherein the remaining capacity calculation unit measures the voltage and current when using the secondary battery at preset time interval, and integrates the used electric power by way of integration of the voltage and the current;
• calculates an available dischargeable electric power by subtracting an energy loss and unavailable energy from a discharge electric power of the secondary battery;
• and calculates a remaining capacity rate from a ratio between the integrated electric power and the dischargeable electric power, wherein the dischargeable electric power is calculated as an area based on an intersection of a first straight line and an intersection of a second straight line, an abscissa-axis and an ordinate-axis, where the first straight line is a line obtained by approximation of discharge voltage associated with no load to a first termination of discharge value of the secondary battery and the second straight line is a line obtained by the calculation of discharge voltage associated with a load to a second termination of discharge value of the secondary battery, and wherein the abscissa-axis represents discharge capacity and the ordinate-axis represents voltage, wherein a table of correspondence relationship between the remaining capacity rate and a DC impedance factor is prepared, and a discharge termination voltage in a discharge termination section is obtained taking into consideration an increase in the DC impedance, wherein the unavailable energy comprises a first electric power unavailable for use because of heat generated by polarization due to load current, a second electric power unavailable for use because of the load or the environmental conditions, and a third electric power unavailable for use because of the degradation of the battery.