Methods for measuring the internal resistance of lithium-ion batteries
1. DC internal resistance measurement method
Using a current source, a short pulse is applied to the lithium iron phosphate battery, and the difference between its terminal voltage and open-circuit voltage is measured. This difference is divided by the test current to obtain the battery's DC internal resistance. The polarization internal resistance of a lithium-ion battery is affected by the magnitude of the applied current. To minimize this factor, the DC internal resistance measurement method uses a relatively short current-carrying time and a relatively large applied current.
In theory, a smaller measuring current will minimize polarization reactions and reduce interference from polarization resistance. However, because the internal resistance of lithium iron phosphate batteries is very small, typically on the order of milliohms, excessively small currents can lead to measurement errors in voltage detection instruments, which, limited by their measurement accuracy, cannot eliminate the influence of measurement errors on the results. Therefore, a balance must be struck between instrument accuracy and the impact of polarization resistance to find a suitable measuring current value.
For ordinary battery cells, the measured current is generally around 5C-10C, which is quite high. As the cell capacity increases, or multiple cells are connected in parallel, their internal resistance decreases. Therefore, without improvements in instrument accuracy, it is difficult to reduce the measured current.
2. AC Internal Resistance Measurement Method
A small-amplitude AC input is applied as an excitation to a lithium iron phosphate battery, and its terminal voltage response is monitored. A specific program is used to analyze the data to determine the battery's AC internal resistance. The resistance value obtained is only related to the battery's inherent characteristics and is independent of the magnitude of the excitation signal used.
Due to the characteristics of battery capacitance, the measured resistance value will be different depending on the frequency of the excitation signal.
