A battery device includes at least a battery cell, a management chip and a bus. The management chip is coupled to the battery cell for detecting voltage or remaining capacity of the battery cell and managing an operation state of the battery device according to the voltage or the remaining capacity of the battery cell. The bus is coupled to the management chip. The management chip communicates with a host device via the bus. The management chip further determines whether the remaining capacity of the battery cell is not increasing while in a charging state. When the remaining capacity of the battery cell is not increasing while in the charging state, the management chip activates a protection mechanism to make the battery device exit the charging state.