A charging method includes detecting whether two battery modules are balanced, simultaneously charging the two battery modules by a first charging current when the two battery modules are balanced, and executing a specific charging procedure when the two battery modules are not balanced. The step of executing the specific charging procedure includes charging by the first charging current a first battery module between the two battery modules that is not balanced and disconnecting the other between the two battery modules, and simultaneously charging the two battery modules by a second charging current upon the first battery module reaching balance, wherein the second charging current is less than the first charging current.