A method of charging a secondary battery and a charging device that can improve stability and extend the life span of the battery. When the secondary battery includes a plurality of cells, the charging method is changed when a voltage imbalance from 100 mV to 300 mV occurs among the cells. In that range, the charging method changes from a constant current-constant voltage charging method to a pulse-charging method. When the voltage imbalance is 300 mV or more, the electricity path is blocked, shutting down the battery. When the voltage imbalance is 100 mV or less, the constant current-constant voltage charging method is maintained. The method and device also stop charging when the battery reaches full charge.