The internal resistance value is one of the important indicators for evaluating the performance of lithium batteries.

　　The internal resistance of lithium batteries is mainly composed of ohmic internal resistance and polarization internal resistance. Under the condition of constant temperature, the ohmic resistance is basically stable, while the polarization resistance will vary with the remaining capacity.

　　The internal resistance test methods of lithium batteries are mainly AC internal resistance test and DC internal resistance test.

　　Method 1: AC internal resistance measurement method

　　l This method is suitable for testing batteries with low capacity, such as the common 18650 battery cell.

　　l Use a fixed frequency of 1KHZ and a fixed current of 50mA for the lithium battery.

　　l Sampling the voltage of the lithium battery and calculating the internal resistance value after processing by professional equipment.

　　l The measurement time is extremely short, generally about 100 milliseconds.

　　l The measurement accuracy error can be controlled within 1%-2%.

　　Method 2: DC internal resistance test method

　　l This method is suitable for testing batteries with high capacity, such as batteries with a square aluminum shell over 50Ah.

　　l The lithium battery is forced to pass a constant DC current of 3-10 times in a short period of time (usually 2 to 3 seconds).

　　l Sampling and recording the voltage of the lithium battery during discharge.

　　l Calculate the DC internal resistance of the lithium battery according to the physical formula R=U/I.

　　l The measurement accuracy error can be controlled within 0.1%-0.2%.

　　Measuring the internal resistance of a lithium battery is a complicated process, and the above are just a brief description.

　　Lithium-ion battery (LIB) has become the main energy storage solution in modern social life. Among them, lithium iron phosphate batteries are a perfect replacement for lead-acid batteries, and they are the first choice for grid-connected peak shaving, off-grid energy storage, photovoltaic energy storage, UPS, data center and other industries.