A circuit for charging a battery pack includes a transformer and a charger controller. The transformer is operable for receiving an input power and for providing a charging power according to a driving signal to charge the battery pack. The charger controller is operable for monitoring a status of the battery pack, for selecting a charging mode from multiple charging modes according to the status, and for generating the driving signal according to the charging mode to adjust the charging power. The charger controller further executes a state machine that stores data indicating predetermined battery statuses that cause saturation of the transformer, and that selects a protection mode in which said charging power is restricted if the status of the battery pack matches to one of the predetermined battery statuses.