A battery characteristic determining device includes a control module and a processing module. The control module outputs a control signal when a battery module is in a charging state and an SOC (state of charge) of the battery module reaches a predetermined target value, so as to result in change of a current of the battery module to a predetermined test value for a predetermined test time period. The predetermined test value is such that the change of the current causes a voltage of the battery module to decrease in the predetermined test time period. The processing module obtains a voltage variation of the battery module in the predetermined test time period, and a C rate of the battery module with reference to the voltage variation.