Measurement method for internal resistance of lithium batteries

　　1. DC internal resistance measurement method: Principle of short-circuit current method: Short circuit the battery, measure the maximum current at the moment of short circuit and the voltage at both ends of the battery, and calculate the internal resistance according to Ohm's law. Advantages: Simple operation and fast measurement speed. Disadvantages: High short-circuit current may cause damage to the battery, and measurement results are greatly affected by contact resistance.

　　2. Principle of pulse current method: Apply a short-term high current pulse to the battery, measure the voltage change and current of the battery before and after the pulse, and calculate the internal resistance according to Ohm's law. Advantages: Less damage to the battery and more accurate measurement results. Disadvantage: Requires high-precision measuring equipment and complex control circuits.

　　3. AC internal resistance measurement method: Principle of AC injection method: Inject a small amplitude AC current signal into the battery, measure the AC voltage response at both ends of the battery, and obtain the internal resistance by calculating the ratio of voltage and current. Advantages: High measurement accuracy and minimal impact on the battery. Disadvantages: The measuring equipment is complex and the cost is high.

　　4. Principle of Electrochemical Impedance Spectroscopy (EIS): Within the operating frequency range of the battery, a series of small AC signals of different frequencies are applied to measure the impedance response of the battery, and the internal resistance is obtained by analyzing the impedance spectrum; Advantages: It can provide rich internal information of the battery, such as electrode reaction kinetics, charge transfer resistance, etc. Disadvantages: Long measurement time and complex data analysis.

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