Analysis of internal resistance standards and characteristics of lithium-ion batteries
Resistance represents the degree of obstruction of a circuit element to current transmission. The unit is ohm.
For lithium-ion batteries, the internal resistance of lithium-ion batteries is divided into ohmic internal resistance and polarization internal resistance. The ohmic internal resistance is composed of electrode material, electrolyte, diaphragm resistance and the contact resistance of each part. The internal polarization resistance refers to the resistance caused by polarization during an electrochemical reaction, including the resistance caused by electrochemical polarization and concentration polarization.
The ohmic resistance is mainly composed of the electrode material, electrolyte, diaphragm resistance and the touch resistance of the current collector, the connection of the tabs and other parts. It is related to the size, structure, and connection method of the battery.
Polarization resistance, the resistance that appears at the moment when current is applied, is the sum of various trends in the battery that prevent charged ions from reaching their destination. Polarization resistance can be divided into two parts: electrochemical polarization and concentration polarization.
The accurate calculation of the internal resistance of a lithium-ion battery is quite complicated, and it will continue to change during the use of the battery. According to relevant relevant experience statement, the larger the volume of the lithium-ion battery, the smaller the internal resistance; and vice versa.
The internal resistance of the 18650 lithium-ion battery that performs well at present is about 12 milliohms, and generally about 13-15 milliohms; because the impedance will affect the performance of the battery, generally 50 milliohms is normal, 50-100 It can be used, but the performance begins to decay. It must be used in parallel when it is above 100, and it cannot be used when it is above 200.
Internal resistance is an important parameter for weighing the power performance of lithium-ion batteries and evaluating the life of lithium-ion batteries. The initial internal resistance of lithium-ion batteries is critically determined by the battery's structural design, raw material performance, and process technology. In actual use, the change of battery internal resistance is also affected by various factors such as temperature and SOC.
Lithium-ion battery internal resistance characteristics
With the use of lithium-ion batteries, battery performance continues to decay, which is mainly manifested as capacity decay, increase in internal resistance, and power drop. The change in battery internal resistance is affected by various use conditions such as temperature and depth of discharge.
Internal resistance is one of the important indicators for evaluating the performance of lithium-ion batteries. Regarding the use of large-scale lithium-ion battery packs, such as power systems for electric vehicles, due to the limitations of detection equipment, it is impossible or inconvenient to directly detect AC internal resistance. The characteristics of the battery pack are generally evaluated by DC internal resistance. In actual use, DC internal resistance is also used to evaluate the health of the battery, to predict the life, and to estimate the system SOC, output/input capabilities, etc.
The internal resistance of the battery produced by each lithium-ion battery manufacturer for the same model varies greatly because of the different manufacturing processes. Even if the battery is processed by the same manufacturer, the same material, the same process, the same material, and the same batch, the internal resistance of the battery is also very different.