How to Calculate the DC Internal Resistance of a Battery?

DCIR: Refers to the internal resistance of the electrode materials, electrolyte, separator, and contact components, as well as the resistance caused by polarization during the electrochemical reaction.

Testing Method: A short pulse is applied to the battery cell, and the difference between its terminal voltage and open-circuit voltage is measured. This difference is divided by the test current to obtain the cell's DC internal resistance.

Formula: DCIR = ΔV/ΔI = (V1-V2)/(I1-I2)

Testing the DC internal resistance of the battery cell allows for the screening of substandard cells.

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During DCIR testing, to ensure data accuracy, several key parameters need to be controlled: (equipment precision, accuracy, discharge process, ambient temperature, and cell SOC).

3.1 The Influence of SOC on DCIR: DCIR is significantly affected by SOC and temperature. Different SOC states result in different polarization resistances; the polarization resistance is lowest between 55% and 70% during charging, therefore this range is chosen as the test SOC.

3.2 The Influence of Temperature on DCIR: At the same SOC, the discharge DCIR decreases with increasing temperature. Therefore, the influence of temperature should be minimized when measuring DCIR in practice.

Here, we only introduce the influence of temperature and SOC, and these influences are relative; the degree of influence varies in different systems.

Furthermore, many other factors influence the DCIR of lithium batteries, which comprehensively reflects the internal resistance of the battery. For example, adding carbon nanotubes to the positive electrode slurry can significantly reduce the DCIR. However, a lithium battery cell is a complex system; a low DCIR value does not necessarily guarantee good performance.

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