A system of testing electricity flow of a battery, the battery comprising a used battery, the system comprising: a test item manager operating in response to an output of a user input unit and configured to manage registration, modification, deletion, and display of a set value, a measured value, and a diagnostic value for each test item for the battery; a set value storage configured to match the set value for each test item registered by the test item manager with a type of the battery and store the set value; an AC impedance meter configured to vary an input frequency, sequentially input the varied input frequency to the battery, and output an output frequency for each sequentially inputted input frequency; an impedance identifier configured to operate the AC impedance meter according to an instruction of the test item manager, identify a phase difference between the input frequency input to the battery and an output frequency corresponding to the input frequency to determine the highest frequency among at least one resonant frequency, and calculating an AC impedance corresponding to the determined highest frequency; and an impedance diagnoser configured to compare the AC impedance calculated by the impedance identifier with an electricity flow upper limit and diagnose that it is normal when the calculated AC impedance is lower than the electricity flow upper limit and diagnose that it is abnormal when the calculated AC impedance is higher than the electricity flow upper limit, and wherein the impedance identifier comprising, a measurement controller operates the AC impedance meter according to an instruction of the test item manager, a phase difference identifier receives an input frequency input to the used battery and an output frequency output from the used battery to identify a phase difference between the two frequencies, and an impedance calculator identifies a point in time when a value of the phase difference continuously received from the phase difference identifier is '0', and calculates the AC impedance by using an amplitude, which is the current value and the voltage value at the input frequency and the output frequency at the point in time when the value of the phase difference is '0', wherein the phase difference identifier identifies a high-frequency resonant frequency at the point in time when the value of the phase difference is '0', and that the impedance calculator calculates the AC impedance of the highest point corresponding to the identified high-frequency resonance frequency. a test item manager operating in response to an output of a user input unit and configured to manage registration, modification, deletion, and display of a set value, a measured value, and a diagnostic value for each test item for the battery; a set value storage configured to match the set value for each test item registered by the test item manager with a type of the battery and store the set value; an AC impedance meter configured to vary an input frequency, sequentially input the varied input frequency to the battery, and output an output frequency for each sequentially inputted input frequency; an impedance identifier configured to operate the AC impedance meter according to an instruction of the test item manager, identify a phase difference between the input frequency input to the battery and an output frequency corresponding to the input frequency to determine the highest frequency among at least one resonant frequency, and calculating an AC impedance corresponding to the determined highest frequency; and an impedance diagnoser configured to compare the AC impedance calculated by the impedance identifier with an electricity flow upper limit and diagnose that it is normal when the calculated AC impedance is lower than the electricity flow upper limit and diagnose that it is abnormal when the calculated AC impedance is higher than the electricity flow upper limit, and wherein the impedance identifier comprising, a measurement controller operates the AC impedance meter according to an instruction of the test item manager, a phase difference identifier receives an input frequency input to the used battery and an output frequency output from the used battery to identify a phase difference between the two frequencies, and an impedance calculator identifies a point in time when a value of the phase difference continuously received from the phase difference identifier is '0', and calculates the AC impedance by using an amplitude, which is the current value and the voltage value at the input frequency and the output frequency at the point in time when the value of the phase difference is '0', wherein the phase difference identifier identifies a high-frequency resonant frequency at the point in time when the value of the phase difference is '0', and that the impedance calculator calculates the AC impedance of the highest point corresponding to the identified high-frequency resonance frequency.