# This article explains how to measure the internal resistance of the battery?How to measure the internal resistance of lithium batteries

According to the physical formula R=U/I, the test equipment forces the lithium battery to pass a large constant DC current in a short period of time (usually 2 to 3 seconds) (currently, a large current of 40A to 80A is generally used). The voltage at both ends of the lithium battery, and the current internal resistance of the lithium battery is calculated according to the formula.

According to the physical formula R=U/I, the test equipment forces the lithium battery to pass a large constant DC current in a short time (usually 2 to 3 seconds) (currently a large current of 40A to 80A is generally used), and the measurement is performed at this time. The voltage at both ends of the lithium battery, and the current internal resistance of the lithium battery is calculated according to the formula.

The accuracy of this measurement method is high, and if it is properly controlled, the measurement accuracy error can be controlled within 0.1%.

But this method has obvious shortcomings:

(1) Only large-capacity lithium batteries or lithium-storage batteries can be measured, and small-capacity lithium batteries cannot load a large current of 40A-80A within 2-3 seconds;

(2) When the lithium battery passes a large current, the electrodes inside the lithium battery will be polarized, resulting in polarization internal resistance. Therefore, the measurement time must be very short, otherwise the measured internal resistance value has a large error;

(3) The large current passing through the lithium battery will damage the electrodes inside the lithium battery to a certain extent.

AC voltage drop internal resistance measurement method: Because the lithium battery is actually equivalent to an active resistance, we apply a fixed frequency and a fixed current to the lithium battery (currently, a frequency of 1kHz and a small current of 50mA are generally used), and then the voltage is measured. Sampling, after a series of processing such as rectification and filtering, the internal resistance value of the lithium battery is calculated through the operational amplifier circuit. The measurement time of the lithium battery by the AC voltage drop internal resistance measurement method is extremely short, generally around 100 milliseconds.

The accuracy of this measurement method is also good, and the measurement accuracy error is generally between 1% and 2%.

(1) Using the AC voltage drop internal resistance measurement method can measure almost all lithium batteries, including small-capacity lithium batteries. This method is generally used for the internal resistance measurement of notebook lithium battery cells.

(2) The measurement accuracy of the AC voltage drop measurement method is likely to be affected by the ripple current, as well as the possibility of harmonic current interference.This is a test for the anti-interference ability in the circuit of the measuring instrument

(3) Measurement by this method will not cause much damage to the lithium battery itself.

(4) The measurement accuracy of the AC voltage drop measurement method is not as good as that of the DC discharge internal resistance measurement method.

Battery internal resistance measurement method

At present, the common methods for measuring the internal resistance of batteries are as follows:

1. Density method

The density method mainly estimates the internal resistance of the battery by measuring the density of the battery electrolyte. It is often used for the internal resistance measurement of open lead-acid batteries and is not suitable for the internal resistance measurement of sealed lead-acid batteries. The scope of application of this method is narrow.

2. Open circuit voltage method

The open circuit voltage method is to estimate the internal resistance of the battery by measuring the terminal voltage of the battery, and the accuracy is very poor, and even wrong conclusions are drawn. Because even a battery whose capacity has become very small, its terminal voltage may still behave normally in the re-floating state.

3. DC discharge method

The DC discharge method is to measure the instantaneous voltage drop on the battery by instantaneously discharging a large current of the battery, and calculate the internal resistance of the battery through Ohm’s law. Although this method has also been widely used in practice, it also has some disadvantages. If this method is used to detect the internal resistance of the battery, it must be carried out in a static or offline state, and online measurement cannot be achieved. Moreover, high current discharge will cause greater damage to the battery, thereby affecting the capacity and life of the battery.

4. AC injection method

The AC method determines the internal resistance R of the battery by injecting a constant AC current signal IS into the battery, measuring the voltage response signal Vo at both ends of the battery, and the phase difference impedance formula between the two. The method does not need to discharge the battery, and can realize the safe online detection of the internal resistance of the battery, so it will not affect the performance of the battery. However, this method needs to measure the AC current signal Is, the voltage response signal Vo, and the phase difference between the voltage and current. It can be seen that this method not only has many interference factors, but also increases the complexity of the system and affects the measurement accuracy. . In order to solve the defects of the above methods, the four-terminal measurement method is adopted in this paper, and the voltage response signal at both ends of the battery is multiplied by the AC differential circuit and the sinusoidal signal that generates a constant AC source by an analog multiplier, and then the analog multiplier’s voltage is multiplied. The output voltage signal passes through the filter circuit to convert the AC signal into a DC signal, the DC signal is amplified by the DC amplifier and then converted into analog-to-digital, and the converted value is sent to the single-chip microcomputer for simple processing.

5. Internal resistance tester

The internal resistance tester is an instrument used to measure the internal impedance of the battery and the degree of damage to the acidified film of the battery, hereinafter referred to as the instrument. It applies a 1KHz AC signal to the measured object, and obtains its internal resistance by measuring its AC voltage drop. (It is different from the principle of measuring resistance by a multimeter. The value it measures is milliohms, while the value measured by a multimeter is ohms; and the multimeter can only measure the resistance of objects without power supply, while the internal resistance meter can measure the resistance of objects without power. The resistance value of the power supply object can also be measured with the resistance value of the power supply object, so the two should not be equal.) Use the resistance value of the internal resistance to judge the deterioration state of the battery. (Generally speaking) the smaller the resistance value, the better the performance of the battery. good. Therefore, the method of testing the battery by measuring the internal resistance is a good method with high speed and high reliability.