Understanding Lithium Battery Internal Resistance Testing

The internal resistance testing is actually quite similar to the AC impedance testing method. The principle is essentially based on the volt-ampere method, which means calculating the impedance by dividing the voltage by the current. The internal resistance of a lithium battery is also approximated using this method to determine the equivalent ohmic resistance. However, to achieve practical results with a milliohm-level impedance meter, several common issues in measurement instruments must be addressed, such as calibration, noise, and measurement accuracy.

Key Challenges in Lithium Battery Internal Resistance Testing

In the test circuit, many factors are difficult to quantify precisely, such as the phase change of the excitation signal in the test loop, test fixtures, resistance, and distributed inductance of the wiring, capacitance, and so on. Other factors include the actual circuit gain and drift in the AC signal amplification circuit, as well as phase changes in voltage and current signals during amplification. All of these factors can impact the measurement results, making the testing process more complex.

Standard Calibration Methods to Overcome Measurement Errors

To eliminate the effects of these variables, standard calibration methods are typically employed. These include short-circuit zero calibration, open-circuit calibration, and standard resistor calibration. These adjustments are usually done in software, and once the correction data is obtained, data fitting is performed to eliminate errors. In more complex cases, the influence of the above factors is non-linear, and non-linear fitting can be more complicated. To simplify, piecewise linear fitting can often be used as an alternative.

Addressing Test Noise in Lithium Battery Internal Resistance Measurement

Regarding the issue of test noise, there are already measures in place in the circuit, but here we focus on the software solution. When measuring tiny resistances, the current and voltage signals are often drowned out by noise. How can we extract the desired signal? One advantage in the lithium battery internal resistance meter solution is that the excitation signal frequency is known. The sine wave signal generated by the microcontroller using SPWM can have its frequency and phase precisely controlled. If the time base comes from a crystal oscillator, its frequency and phase will be quite stable. This condition allows us to use a lock-in amplifier to eliminate noise and extract useful signal information.

The Principle of Lock-in Amplification

The principle is to multiply the detected voltage and current signals by a sine wave of the same frequency as the excitation signal, then use a low-pass filter to obtain the DC component, which gives the amplitude of the measured signal. However, in practice, even with a lock-in amplifier, the data still fluctuates. This is due to the many factors that influence the results, such as temperature changes, electromagnetic interference, and external factors. Even the battery itself may be changing, especially after just being charged.

Achieving Stable and Accurate Results

To obtain stable test values, some data smoothing methods, such as moving averages or Kalman filtering, may be necessary. Achieving accurate and stable measurement results requires a lot of effort in the details. Factors like temperature stability, electromagnetic shielding, and signal conditioning need to be carefully controlled to minimize fluctuations. In particular, newly charged batteries may behave differently, requiring extra care in measurement to avoid errors.

Conclusion

Achieving high-precision internal resistance measurements in lithium batteries involves addressing various technical challenges, from circuit design to calibration and noise management. By understanding these factors and utilizing advanced measurement techniques like lock-in amplification and data smoothing, reliable and accurate results can be obtained. This meticulous process is what makes high-quality internal resistance meters stand out, ensuring precision and stability in the measurement of milliohm-level resistances.