The internal resistance of a battery refers to the resistance encountered when current flows through the battery's interior during operation. It includes ohmic internal resistance and polarization internal resistance, with polarization internal resistance further divided into electrochemical polarization internal resistance and concentration polarization internal resistance. Resistance represents the degree to which a circuit element impedes the transfer of current, and its unit is ohms.
For lithium-ion batteries, the internal resistance is divided into ohmic internal resistance and polarization internal resistance. Ohmic internal resistance is composed of the electrode material, electrolyte, separator resistance, and contact resistance of various components. Polarization internal resistance refers to the resistance caused by polarization during the electrochemical reaction, including the resistance caused by electrochemical polarization and concentration polarization.
Lithium-ion battery internal resistance standard
The ohmic resistance is mainly composed of the contact resistance of various parts such as electrode materials, electrolyte, separator resistance, current collector, and tab connection, and is related to the size, structure, and connection method of the battery.
Polarization resistance is the resistance that appears only at the instant current is applied. It is the sum of various forces within the battery that prevent charged ions from reaching their destination. Polarization resistance can be divided into two parts: electrochemical polarization and concentration polarization.
Precise calculation of the internal resistance of lithium-ion batteries is quite complex, and it changes continuously during battery use. According to relevant experience, the larger the volume of a lithium-ion battery, the lower its internal resistance; conversely, the smaller the volume, the lower the internal resistance.
The internal resistance of the best-performing 18650 lithium-ion batteries is around 12 milliohms, while it is generally around 13-15 milliohms. Since impedance affects battery performance, 50 milliohms is generally normal, 50-100 milliohms is usable but performance begins to degrade, above 100 milliohms it needs to be used in parallel, and above 200 milliohms it is basically unusable.
Internal resistance is a crucial parameter for evaluating the power performance and lifespan of lithium-ion batteries. The initial internal resistance of a lithium-ion battery is primarily determined by its structural design, raw material properties, and manufacturing process. In actual use, the battery's internal resistance is also affected by various factors such as temperature and state of charge (SOC).
Lithium-ion battery internal resistance characteristics
With the use of lithium-ion batteries, battery performance continuously degrades, mainly manifested in capacity decay, increased internal resistance, and decreased power. The change in battery internal resistance is affected by various usage conditions such as temperature and depth of discharge.
Internal resistance is one of the key indicators for evaluating the performance of lithium-ion batteries. For large lithium-ion battery packs, such as power systems for electric vehicles, limitations in testing equipment make it difficult or inconvenient to directly measure AC internal resistance. Therefore, DC internal resistance is generally used to evaluate the battery pack's characteristics. In practical applications, DC internal resistance is also frequently used to evaluate battery health, predict battery life, and estimate system state of charge (SOC), output/input capabilities, etc.
Even for the same model, the internal resistance of batteries produced by different lithium-ion battery manufacturers varies greatly due to different manufacturing processes. Even batteries from the same manufacturer, using the same materials, processes, and batches can have significant differences in internal resistance.
