In the research and application of lithium batteries, internal resistance is an important parameter that reflects the energy loss and electrochemical reaction state inside the battery. Internal resistance is mainly divided into DC resistance and AC impedance. This article will provide a detailed introduction to the difference between AC impedance and DC internal resistance of lithium batteries.

1.Definition and measurement methods

① DC internal resistance (DCR): DC internal resistance refers to the ratio of the voltage change of a battery to the corresponding discharge current change under working conditions. It is usually expressed in ohms (Ω) and can be measured using a dedicated battery internal resistance tester or multimeter with current and voltage settings.

② AC Impedance: AC impedance is derived by injecting sine wave current signals into the positive and negative terminals of a battery, and detecting the sine wave voltage signals at the other two terminals.

2. Characteristics

① Characteristics of DC internal resistance measurement: Suitable for large capacity batteries: This method requires applying a large test current (40-80A, 2-3 seconds), which small capacity batteries may not be able to withstand. Short testing time: High current can cause polarization resistance, so the testing time must be short to avoid excessive impact on the battery. There may be losses: High current testing may cause certain losses to the battery.

② Characteristics of AC internal resistance measurement: Short testing time: AC internal resistance measurement only requires the application of a fixed frequency and current (currently 1KHz, 50mA), sampling voltage, rectification and filtering processing, and calculation of resistance value through an arithmetic circuit. The entire process only takes 0.1 seconds. Suitable for various battery types: Due to the small testing current, it causes almost no damage to the battery, making it suitable for almost all battery types. Relatively low accuracy: The accuracy of the AC internal resistance testing method is 1% -2%, which is not as accurate as the DC internal resistance testing method.

3. Reflected information

① DC internal resistance: DC internal resistance more directly reflects the Ohmic impedance inside the battery, including the resistance of electrode materials, electrolyte, separators, and other parts. It can provide information about the internal conductivity of the battery.

② AC impedance: In addition to Ohmic impedance, AC impedance can also reflect the electrochemical reaction impedance inside the battery. The semi-circular part is related to the charge transfer at the interface between the electrolyte and electrode material, while the low-frequency part is related to the diffusion of lithium ions in the electrolyte and positive and negative electrode materials. AC impedance can provide more comprehensive information on battery electrochemical reactions

4. Application scenarios

① DC internal resistance: DC internal resistance is commonly used to evaluate the power performance and performance of batteries in practical use. It is of great significance for the discharge capacity and charging efficiency of batteries.

② AC impedance: AC impedance is widely used in battery research and development, quality control, and electrochemical analysis. It can help study the electrochemical reaction mechanism, interface performance, and aging process of batteries.

5. The influence of temperature, state of charge, and charge discharge state

① Temperature: Temperature has a significant impact on the internal resistance of lithium batteries. Generally speaking, as the temperature decreases, the internal resistance of the battery will increase. This is because ion transport and electrochemical reaction rates slow down at low temperatures.

② State of Charge (SOC): The state of charge of a battery also affects its internal resistance. During the discharge process, as the discharge depth increases, the internal resistance usually increases. This is due to changes in the internal electrochemical reactions of the battery and changes in the electrode structure.

③ Charging and discharging status: Charging and discharging status can also affect internal resistance. For example, during the charging process, the internal resistance of the battery may increase, while during the discharging process, the internal resistance may decrease.

6. Overall consideration

In practical applications, the measurement results of DC internal resistance and AC impedance are usually considered comprehensively to gain a more comprehensive understanding of battery performance. DC internal resistance can more accurately reflect the superposition of various resistances and capacitances inside the battery, while AC impedance provides more information about electrochemical reactions. Meanwhile, by analyzing factors such as temperature and state of charge, the health status and performance of the battery can be more accurately evaluated.

7. Summary

There are significant differences in the definition, measurement methods, characteristics, and reflected information between the AC impedance and DC internal resistance of lithium batteries. Understanding these differences helps us choose appropriate testing methods and interpret test results correctly, thereby better evaluating the performance and status of lithium batteries. In practical applications, according to specific needs and conditions, one can choose to use DC internal resistance testing or AC impedance testing, or combine the two for a more comprehensive analysis.