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What is the internal resistance of 18650 lithium-ion battery? How to measure the internal resistance of 18650 batteries? The accurate calculation of the internal resistance of 18650 lithium-ion batteries is quite complex and will constantly change during battery use. According to relevant experience, the larger the volume of lithium-ion batteries, the lower their internal resistance; vice versa. The internal resistance of 18650 is generally between 20-65 milliohms, and a multimeter can only measure voltage.

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

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

The 18650 lithium-ion battery has a lower internal resistance due to its relatively 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 the Samsung 18650, typically has an internal resistance of less than 80 milliohms, but in reality, it is less than 65 milliohms.

The internal resistance of 18650 lithium-ion batteries, which currently perform well, is about 12 milliohms, while it is generally around 13-15 milliohms; Due to the impact of impedance on battery performance, 50 milliohms is generally considered normal and can be used between 50-100 ohms, but performance begins to decline and needs to be used in parallel above 100 ohms. It is generally not suitable for use above 200 ohms.

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 resistance, and operational resistance of lithium-ion batteries are often different, and the internal resistance also varies under different environments and temperatures.

How to measure the internal resistance of 18650 batteries?

To measure the capacity and internal resistance of 18650 lithium-ion batteries, specialized internal resistance testers and capacity testers are required. The internal resistance of 18650 batteries 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 essentially equivalent to an active resistor, we apply a fixed frequency and current to the lithium-ion battery (currently generally using a frequency of 1kHz and a small current of 50mA), and then sample its voltage. After a series of processes such as rectification and filtering, the internal resistance value of the lithium-ion battery is calculated 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, with a measurement accuracy error 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 for a short period of time (usually 2-3 seconds) (currently using high currents of 40A to 80A), measures the voltage at both ends of the lithium-ion battery at this time, 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%.

The above is an introduction to the internal resistance of 18650 lithium-ion batteries. Resistance is the resistance that a lithium-ion battery experiences when current flows through its interior during operation. In battery structure design, in addition to the riveting and welding of the battery structural components themselves, the number, size, and position of battery terminals directly affect the internal resistance of the battery.

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