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What is the internal resistance of 18650 lithium-ion batteries?

For lithium-ion batteries, the internal resistance of the battery is divided into ohmic internal resistance and polarization internal resistance. Ohmic internal resistance is composed of electrode material, electrolyte, diaphragm resistance, and contact resistance of various parts. Polarization resistance refers to the resistance caused by polarization during electrochemical reactions, including resistance caused by electrochemical polarization and concentration polarization.

The internal resistance of the 18650 lithium-ion 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. A regular 18650, such as a Samsung 18650, usually has an internal resistance of less than 80 milliohms, but in reality, it is less than 65 milliohms.

The internal resistance of the currently performing 18650 lithium-ion battery is approximately 12 milliohms, while it is generally around 13-15 milliohms; Due to the impact of impedance on battery performance, 50 milliohms is generally normal. It can be used between 50 and 100, but performance begins to decline. When it reaches 100 or above, it needs to be used in parallel, and when it exceeds 200, it cannot be used.

If the 18650 lithium-ion battery is used for too long, the internal resistance of the lithium-ion battery will increase, and the output voltage will decrease. The internal resistance, static internal resistance, and operational internal resistance of lithium-ion batteries often differ, and the internal resistance also varies under different environments and temperatures.

How to measure the internal resistance of 18650 batteries?

18650 lithium-ion batteries require specialized internal resistance testers and capacity testers to measure capacity and resistance. The internal resistance of 18650 is generally between 20-65 milliohms, and a multimeter can only measure voltage.

● AC voltage drop internal resistance measurement method

Because the 18650 lithium-ion battery is actually equivalent to an active resistor, we apply a fixed frequency and fixed current to the lithium-ion battery (currently generally using a 1kHz frequency and 50mA small current), then sample its voltage, undergo a series of processes such as rectification and filtering, and calculate the internal resistance value of the lithium-ion battery through an operational amplifier circuit. The measurement time of lithium-ion batteries using the AC voltage drop internal resistance measurement method is extremely short, usually around 100 milliseconds.

The accuracy of this measurement method is also good, and the measurement accuracy error is generally between 1% and 2%.

According to the physical formula R=U/I, the testing equipment forces the lithium-ion battery to pass through a large constant DC current in a short period of time (usually 2-3 seconds) (currently using a large current of 40A to 80A), measures the voltage at both ends of the lithium-ion battery, and calculates the current internal resistance of the lithium-ion battery according to the formula. This measurement method has high accuracy, and if controlled properly, the measurement accuracy error can be controlled within 0.1%.

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