The internal resistance of the battery is one of the most important characteristic parameters of the battery, which is an important parameter to characterize the battery life and the operating state of the battery, and an important symbol to measure the difficulty of the transmission of electrons and ions in the electrode. The internal resistance also reflects the health of the battery. The internal resistance of the battery is very small when it leaves the factory, but after long-term charging and discharging, due to the loss of the electrolyte inside the battery and the reduction of the activity of the chemical substances inside the battery, the internal resistance will gradually increase, the electrolyte will gradually denature in multiple charging and discharging, and the internal resistance will increase until the internal resistance is large enough that the electricity inside the battery cannot be released normally, and the battery will age. The relative battery capacity also decreases.

The internal resistance of lithium-ion batteries is not a fixed value, and it is related to the working state of lithium-ion batteries, including the two parts of ohmic internal resistance and polarization internal resistance
1) Ohmic internal resistance
Ohmic internal resistance is the inherent resistance of lithium-ion batteries, that is, DC internal resistance, which can be considered to be fixed at a certain state of SOC. It is mainly composed of the electrode material, the resistance of the electrolyte, the diaphragm, and the internal resistance of other parts of the material. This diagram shows the discharge process of a lithium-ion battery in a specific SOC state. When the lithium-ion battery begins to discharge, the ohmic internal resistance will show an instantaneous voltage drop ΔU1 at both ends of the lithium-ion battery. This voltage drop lasts for a short time (less than 2ms). Therefore, more testing of the dynamic internal resistance of lithium-ion batteries is needed. Short response time. After a short time, the polarization of the lithium-ion battery will take effect. The voltage drop at both ends of the lithium-ion battery is mainly caused by the internal resistance of polarization. During the charging process, the ohmic internal resistance also causes the instantaneous voltage at both ends of the lithium-ion battery to rise. After the change, the internal resistance of polarization plays a role.
2) Internal resistance to polarization
The voltage drop of the lithium-ion battery is caused by the ohmic resistance for a short time after discharge, and the subsequent voltage drop is mainly caused by polarization. Due to the internal chemical reaction of the lithium-ion battery, the use of polarization and the change in the SOC state of the battery will cause the battery output voltage to drop, but this change is slow, which is different from the instantaneous drop in the voltage at both ends of the battery caused by the ohmic internal resistance. The internal resistance at this time is caused by the ion concentration in the chemical reaction of the lithium-ion battery, which is called the polarized internal resistance. The internal resistance varies with the reaction, and its magnitude is related to the detection time and current intensity. The voltage continues to drop slowly within 10-20s during the discharge process, and the voltage continues to rise slowly after the charging process of 20s is caused by the internal resistance of polarization.

Factors affecting internal resistance

The AC and DC internal resistance of lithium-ion batteries has a significant inverse relationship with temperature, that is, the internal resistance will increase when the temperature drops, and it shows a typical nonlinear characteristic. In practical applications, lithium-ion batteries should be prevented from charging and discharging at low temperatures, especially low temperature charging has a great impact on battery performance.

The DC internal resistance of lithium-ion batteries under different SOC states shows a trend of increasing the discharge depth and increasing the DC internal resistance. It can be seen from the figure that the AC internal resistance of lithium-ion batteries in different SOC states is very close, and it can be considered that the internal resistance of lithium-ion battery AC does not change with the change of SOC. The AC internal resistance of lithium-ion batteries under different SOC states basically does not change, so in the application process, only the DC internal resistance value under different SOC states can be studied.

Internal resistance characteristics of lithium batteries

With the use of lithium batteries, battery performance continues to decline, mainly due to the decrease in capacity, increase in internal resistance, and decrease in power. The change in the internal resistance of the battery is affected by various conditions of use such as temperature and depth of discharge. Internal resistance is one of the important indexes to evaluate the performance of lithium battery. For large lithium battery pack applications, such as power systems for electric vehicles, it is not possible or convenient to test AC internal resistance directly due to the limitations of the test equipment. Typically, the characteristics of the battery pack are assessed by the DC internal resistance. In practical applications, DC internal resistors are also used to assess the health of the battery, predict life, and evaluate the system SOC, output/input capabilities, etc.