How to measure the internal resistance and voltage of a lithium battery?

Internal resistance is one of the important indicators to evaluate the performance of lithium battery. The internal resistance test of lithium battery includes AC internal resistance and DC internal resistance. For single battery cell, the internal resistance of the AC (ACIR) is generally used for evaluation, which is usually called the ohmic internal resistance.

Currently, the battery internal resistance tester is generally used to measure the battery internal resistance. It is to apply a 1KHz AC signal to the measured object and obtain its internal resistance by measuring its AC voltage drop.

Unlike the principle of measuring resistance with a multimeter, the value measured by the battery internal resistance tester is milliohms, while the value measured by the multimeter is ohms.

The system of the battery internal resistance tester consists of an output input circuit, an input conversion circuit, a sampling circuit, a low noise preamplifier, a square wave conversion circuit, a multiplier circuit, an integrator circuit, an AC constant current signal generation circuit, a microcontroller control system, and a display Circuits, interface circuits and computers.

The output AC constant current signal is connected to both ends of the battery, and then the voltage signal generated by the internal resistance of the battery is directly connected from both ends of the battery to the input transfer switch circuit. The injection current loop and the signal measurement loop are separated to reduce the influence of the wire impedance on the internal resistance of the battery and realize the four-terminal lead connection.

The input conversion switch is controlled by the single-chip microcomputer. First, the sampling circuit is turned on to detect the current value injected into the battery circuit; then the two ends of the battery are turned on to detect the voltage signal generated on the internal resistance, thereby calculating the battery according to the internal resistance calculation formula Internal resistance and display. At the same time, relevant information can be sent to the PC computer through the interface circuit, relevant data can be stored, and the charging and discharging characteristic curve can be automatically drawn.

Typical industrial application case - Battery Cell Sorting Machine

The automatic battery cell sorting machine adopts HIOKI 3561 battery tester to execute voltage and internal resistance measurement by four-terminal lead system method, which is high in precision and consistency. The repeated test error of voltage is within ±0.3mV and that of internal resistance is within ±0.3 mΩ. The resolution of voltage is as low as 0.1mV and that of internal resistance is as low as 0.1 mΩ. There are 8 channels for OK battery cells and 1 for NG battery cells.