Research on the method of accurately measuring the internal resistance of batteries-EEWORLD

Collect

1. Introduction

As a backup power source for power outages, batteries have been widely used in industrial production, transportation, communications and other industries. If the battery fails or has insufficient capacity, it may cause a major accident, so the operating parameters of the battery must be fully monitored online. One of the important indicators of the battery status is its internal resistance. Whether the battery is about to fail, has insufficient capacity or is improperly charged or discharged, it can be reflected in the change of its internal resistance. Therefore, the working status of the battery can be evaluated by measuring the internal resistance of the battery. At present, the common methods for measuring the internal resistance of the battery are:
(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 to measure the internal resistance of open lead-acid batteries, but not suitable for measuring the internal resistance of sealed lead-acid batteries. The scope of application of this method is narrow.
(2) Open circuit voltage method
The open circuit voltage method estimates the internal resistance of the battery by measuring the terminal voltage of the battery. The accuracy is very poor and may even lead to wrong conclusions. Because even if the capacity of a battery has become very small, its terminal voltage may still be normal when it is floating charged.
(3) DC discharge method
The DC discharge method is to discharge the battery with a large current instantaneously, measure the instantaneous voltage drop on the battery, and calculate the internal resistance of the battery by Ohm's law. Although this method has been widely used in practice, it also has some disadvantages. For example, this method must be used to detect the internal resistance of the battery in a static or offline state, and online measurement cannot be achieved. In addition, large current discharge will cause great damage to the battery, thereby affecting the capacity and life of the battery.
(4) AC injection method
The AC method injects a constant AC current signal IS into the battery, measures the voltage response signal Vo at both ends of the battery, and the phase difference between the two

, according to the impedance formula

To determine the internal resistance R of the battery. This method does not require discharging the battery and can achieve safe online detection of the battery internal resistance, so it will not affect the performance of the battery. However, this method requires measuring 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 also affects the measurement accuracy.

In order to solve the defects of the above methods, this paper adopts a four-terminal measurement method. The voltage response signal at both ends of the battery is multiplied by the sinusoidal signal that generates a constant AC source through an AC differential circuit and then passed through an analog multiplier. The output voltage signal of the analog multiplier is then passed through a filter circuit to convert the AC signal into a DC signal. The DC signal is amplified by a DC amplifier and then converted to an analog-to-digital signal. The converted value is sent to the microcontroller for simple processing.

[page] 2. Battery internal resistance detection principle

Since the battery internal resistance is in the milliohm level, the conventional two-terminal measurement method has a large measurement error. Here, a four-terminal measurement method is used. During measurement, a frequency of

The other two terminals are used for measurement. The measurement working principle diagram is shown in Figure 1. The response signal refers to the AC voltage signal measured at both ends of the battery after the battery is injected into the AC constant current source. The sinusoidal signal is the input signal of the voltage-controlled constant current source generated by the D/A.

Assume the sinusoidal signal is:

(1)
The response voltage signal at both ends of the battery is:

(2)

It is the phase difference between the AC current injected into the battery and the voltage signal at both ends.
After passing through the analog multiplier, we have:

K is the gain factor of the analog multiplier.
After low-pass filtering and filtering out the AC component, we get:

(3)
According to the principle of measuring internal resistance by AC method:

(5)
Where I is the maximum value of the AC constant current source signal. Comparing (4) and (5), we can get:

In the above formula, K, A, and I are all known quantities, and u is the sampled value sent to the microcontroller for processing after A/D sampling, so a simple division operation in the microcontroller can get the internal resistance of the battery.

3. Design of AC constant current source

The premise for successfully detecting the battery status is to provide the required AC constant current source. The constant current source is a power supply device that can provide a constant current to the load. It is a power supply with a very large internal resistance. In order to ensure that the internal resistance has a high measurement accuracy and good reproducibility, the constant current source is required to have sufficient stability and low waveform distortion. The AC signal amplitude required here is 40mV and the frequency is 1KHZ.
However, there are many shortcomings in the design of traditional low-frequency AC signal generators: the application of general circuits, many components, especially the large size of capacitors, and the poor stability and large distortion of the waveform, and the adjustment is also extremely inconvenient; the application of special circuits, such as ICL8038, MAX038, etc., has significantly improved distortion and stability, but it is not suitable for low-frequency applications, inconvenient to adjust, and the cost is also high.

[page]3.1 Design Principle

This paper adopts a digital signal generator to generate a standard sine wave and a current negative feedback method to generate an accurate AC constant current source. The implementation principle of the AC constant current source is shown in Figure 2.

The circuit block diagram is shown in Figure 2: This is a closed-loop control system, a current negative feedback circuit. The standard sine wave generates a 1KHz sine wave signal with stable frequency, symmetry and low distortion. The drive circuit amplifies the sine wave to drive the power amplifier circuit to obtain a sinusoidal AC current output. The constant current control circuit obtains the signal from the power amplifier output and adjusts the signal of the drive circuit by comparing it with the given signal, so that the output current remains stable.

3.2 Principle of Standard Sine Wave Generation

The standard sine wave signal is generated by a digital signal generator. First, the sine table data is stored in the sine signal memory as shown in Figure 3. The crystal oscillator generates an oscillation frequency f, which is converted into a complete square wave frequency through a shaping circuit, and then passes through the R frequency division circuit to obtain a frequency of f/R. After the phase detector FD and the loop filter LF circuit are phase-locked and divided, the sine value stored in the sine signal memory is read, and the standard sine wave required in Figure 2 is generated through the D/A conversion circuit and the low-pass active filter circuit.

Figure 3 Principle block diagram of standard sine wave signal source

4. Conclusions

Compared with the existing technology, this processing method has a wide range of applications, high measurement accuracy, and little damage to the battery. It can be used to safely monitor and manage the battery online. At the same time, the internal resistance of the battery can be obtained without AC sampling and solving cos. This simplifies the complexity of the software and hardware required to measure the AC voltage and phase difference at both ends of the battery in the AC injection method. This method can meet the requirements of battery testing, achieve good practical results, and complete the performance testing and fault diagnosis of lead-acid batteries. It provides a practical method for online battery testing.

References:

[1] Li Liwei, Zou Jiyan. Research on battery internal resistance measurement device. Power Supply Technology, 2003(1)
[2] Xia Yong, Application of phase-locked amplifier technology in battery internal resistance detection. Electronic Application Technology, 2004(3)
[3] Markle GJ. AC Impedance testing for valve regulated cells[J]. Conf. Proc. INTELEC, 1992

Reference address：Research on the method of accurately measuring the internal resistance of batteries

Previous article：Optimization Design Ideas for Substation Automation Protection
Next article：Monitoring technology and application of battery online internal resistance in DC power supply system

Popular Resources
Popular amplifiers