A method of changing a secondary battery, a method of calculating a
remaining capacity rate of the secondary battery, and a battery pack are
provided.The method of charging a secondary battery includes the steps
of: performing a detection of a charging rate of the secondary battery
using an integrating method in which a battery capacity is calculated by
integrating a current value or a power value of the secondary battery for
a certain period of time, and a detection of the charging rate of the
secondary battery using a voltage method in which a voltage value of the
secondary battery is measured and the charging rate is calculated based
on a correlation between the voltage value and the charging rate; and
performing a weighted addition of the charging rate detected by the
integrating method and the charging rate detected by the voltage method
in accordance with the charging rate of the secondary battery, whereby
detecting a final charging rate.