The determination method of direct current internal resistance of lithium ion batteries

Resistance is to evaluate one of the important indicators of battery performance. Internal resistance test includes the AC resistance and DC resistance. For single cells, generally evaluate for AC resistance, which is often called the ohm internal resistance.

But for large battery applications, such as electric vehicle power supply systems, due to the limitation of test equipment and so on, cannot or will not convenient to communicate direct resistance test, usually by dc resistance to evaluate the characteristics of the battery pack. In practice, also use dc resistance for evaluating the health of the battery, life prediction, and systematically estimate SOC, output/input ability, etc.

In production, it can be used to detect the fault battery such as micro short circuit phenomenon.

Dc resistance test principle is applying larger electric current through the battery or the battery pack (charging or discharging), short duration, inside the battery has not reach full polarization, according to the applied current and battery voltage change before and after applying the current, the calculation of the dc resistance of the battery.

Dc resistance test must choose good four parameters: current ratio (or use), pulse time, charged state of (SOC), the test environment temperature. The change of these parameters has larger influence on dc resistance.

Dc resistance includes not only the ohm internal resistance part of the battery pack (ac internal resistance), also includes some of the polarization resistance of the battery pack. And the polarization of the battery is influenced by current, time and so on. The current commonly used dc resistance test method has the following three:

(1) the United States in the freedom CAR battery test handbook HPPC test method, test duration of 10 s, exert the discharge current is 5 c or higher, charging current for the discharge current of 0.75.The choice of specific current formulated according to the characteristics of the battery.

(2) the Japanese JEVSD7132003 test method, the original mainly for Ni/MH battery, after also applied to lithium ion batteries, first of all, establish a 0 ~ 100% under the battery SOC current voltage characteristic curve, with 1 c, 2 c and 5 c respectively, 10 c under the current set of SOC of the battery to charge or discharge, alternately charging or discharging time 10 s, respectively, to calculate the dc resistance of the battery.

(3) "863" plan in our country the electric car major projects of the HEV with high power lithium ion power battery performance test code for the proposed test method, test duration for 5 s, charging test current for 3 c, discharge test current for 9 c.

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