I. DC internal resistance test method

The DC internal resistance test method given in the standard needs to give the battery a pulse of high current. The accuracy of this test method is not only related to the accuracy of the used charging and discharging equipment and the sensor's detection device, but also to the internal resistance of the battery itself, which will affect the error.

(1) fill the lithium battery pack with constant current 40A voltage limiting 4.2V

(2) discharge 10% DOD (depth of discharge) with 100A current, and the SOC of the battery is 90%

(3) static for 1 hour

(4) carry out one test according to the pulse power test diagram below.

 (5) repeat the tests of (1) - (3), increase the depth of each discharge by 10%, until 90% DOD is released for the final test.

 (6) discharge 100% DOD of the battery

2. According to the physical formula r = u / I, the test equipment forces the lithium battery to pass a large constant DC current in a short time (generally 2-3 seconds) (at present, 40a-80a current is generally used), measures the voltage at both ends of the lithium battery, and calculates the current internal resistance of the lithium battery pack according to the formula.

The accuracy of this method is high. If it is properly controlled, the measurement accuracy error can be controlled within 0.1%.

III. measurement of internal resistance of AC voltage drop

Because the lithium battery pack is actually equivalent to an active resistance, we apply a fixed frequency and a fixed current to the lithium battery (at present, we usually use a frequency of 1kHz and a small current of 50mA). Then we sample its voltage, and calculate the internal resistance of the lithium battery through the operational amplifier circuit after a series of processing such as rectification and filtering. The measurement time of lithium battery by AC voltage drop internal resistance method is very short, generally about 100 milliseconds.

The accuracy of this method is also good, and the error of measurement accuracy is generally between 1% and 2%.