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What is the internal resistance of lithium iron phosphate batteries? What factors affect the internal resistance of lithium batteries? With the use of lithium iron phosphate batteries, battery performance continues to deteriorate, mainly manifested as capacity degradation, internal resistance increase, power decrease, etc. The changes in battery internal resistance are influenced by various usage conditions such as temperature and discharge depth. Resistance is the resistance that a lithium battery experiences when the current flows through the interior of the battery during operation. Usually, the internal resistance of lithium batteries is divided into ohmic internal resistance and polarization internal resistance.

What is the internal resistance of lithium iron phosphate batteries?

The internal resistance of lithium iron phosphate batteries mainly includes two parts: ohmic resistance and polarization resistance. Under constant temperature conditions, the ohmic resistance remains stable, while the polarization resistance changes with the factors that affect the polarization level. The internal resistance, static internal resistance, and working internal resistance of lithium iron phosphate batteries often differ, and the internal resistance also varies under different environments and temperatures.

Lithium iron phosphate battery refers to a lithium-ion battery that uses lithium iron phosphate as the positive electrode material. The positive electrode materials of lithium-ion batteries mainly include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, ternary materials, lithium iron phosphate, etc. Lithium cobalt oxide is currently the cathode material used by the vast majority of lithium-ion batteries. The internal resistance of a battery refers to the resistance experienced by the current flowing through the battery during operation. It includes Ohmic resistance and Polarization resistance, which in turn include Electrochemical polarization resistance and Concentration polarization resistance.

Ordinary 18650 lithium batteries, such as Samsung 18650 lithium batteries, generally have an internal resistance of less than 80 milliohms, but the actual resistance is less than 65 milliohms. The internal resistance of the 18650 lithium battery is generally lower due to its high discharge current. For example, the Samsung 18650-2200-5C power has an internal resistance of less than 35 milliohms.

The common 18650 batteries are divided into lithium-ion batteries and lithium iron phosphate batteries. The nominal voltage of lithium-ion batteries is 3.7V, and the charging cutoff voltage is 4.2V. The nominal voltage of lithium iron phosphate batteries is 3.2V, and the charging cutoff voltage is 3.6V. The capacity is usually 1200mAh to 3000mAh, and the common capacity is 2200mAh to 2600mAh.

What factors affect the internal resistance of lithium batteries?

1. Temperature

The internal resistance of lithium iron phosphate batteries, as the main parameter to measure the difficulty of conducting ions and electron transfer inside the battery, directly determines the power characteristics of the battery, and thus has an important impact on the power performance of electric vehicles. The actual operating temperature range of electric vehicles is wide, and the internal resistance of the battery varies under different temperatures and operating conditions. Master the inherent relationship between battery internal resistance, temperature, SOC, and charge discharge rate. As the temperature increases, the Ohmic resistance and Polarization resistance of lithium iron phosphate batteries during charging and discharging both decrease, and the rate of change in Ohmic resistance is higher than that in Polarization resistance at different temperatures. The rate of change in Ohmic resistance at low temperatures is greater than that at high temperatures.

2. The influence of membrane on ion impedance

The main factors affecting ion impedance of a diaphragm include the distribution of electrolyte in the diaphragm, diaphragm area, thickness, pore size, porosity, and tortuosity coefficient. For ceramic diaphragms, it is also necessary to prevent ceramic particles from blocking the pores of the diaphragm and hindering the passage of ions. While ensuring that the electrolyte fully infiltrates the membrane, there should be no remaining electrolyte residue, which reduces the efficiency of electrolyte usage.

3. Structural design impact

In battery structure design, in addition to riveting and welding of battery structural components themselves, the number, size, position, and other factors of battery pole ears directly affect the internal resistance of lithium iron phosphate batteries. To a certain extent, increasing the number of pole ears can effectively reduce the internal resistance of the battery. The position of the pole ear also affects the internal resistance of the battery. The winding battery with the pole ear located at the positive and negative pole tips has the highest internal resistance, and compared to the winding battery, the stacked battery is equivalent to dozens of small batteries in parallel, with lower internal resistance.

4. Battery's own factors

① Electrolyte inside the diaphragm

Influencing factors: electrolyte conductivity, diaphragm area, thickness, porosity, and tortuosity coefficient (Gurley)

② The electrolyte inside the positive electrode

Influencing factors: electrolyte conductivity, positive electrode thickness, thickness, porosity, and tortuosity coefficient

③ The electrolyte inside the negative electrode

Influencing factors: electrolyte conductivity, positive electrode thickness, thickness, porosity, and tortuosity coefficient

④ The level of craftsmanship, the manufacturing process of the electrode, whether the coating is uniform, and the compaction density, as well as the level of craftsmanship during the processing of these battery cells, can directly affect the polarization resistance.

The above are the factors that affect the internal resistance of lithium iron phosphate batteries. The high internal resistance of lithium batteries cannot discharge large currents. The lower the theoretical internal resistance, the better. There is a direct relationship between internal resistance and capacity. A typical lithium battery assumes an internal resistance of approximately 30 to 80 milliamperes per hour, while a good power lithium battery can reach below 15 milliamperes.

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