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hello everyone! I'm glad to meet you again. This article is the second in the series of [BMS algorithm design]. This issue mainly introduces the first method of battery SOC estimation - direct estimation method. Let's learn together!

In fact, various experimental methods, models and algorithms for estimating battery SOC have been proposed and developed. Each method has its own advantages and disadvantages. The following figure is a summary of SOC estimation methods, and is also the algorithm that will be discussed in this series of articles one after another.

Several typical SOC estimation methods:

In the direct measurement method, physical measurements are used to estimate SOC, such as battery voltage and impedance. The most commonly used direct measurement methods are open-circuit voltage method, terminal voltage method, impedance method and spectrum method.

Open Circuit Voltage method (OCV)

OCV is the thermodynamic potential of the battery under no-load condition, and has a nonlinear relationship with the SOC of the battery. OCV is usually obtained through offline OCV test at specific ambient temperature and aging stage. Although the OCV method is relatively accurate, it needs a period of rest time to estimate the SOC, so it is difficult to use it directly in practical applications (usually combined with other algorithms). OCV appears in the form of ideal variable voltage source in the equivalent circuit model, and its overvoltage is increased by the remaining resistance and capacitance elements in the equivalent circuit. In addition, the OCV-SOC curve relationship between batteries is also different. Therefore, the SOC estimation algorithm may produce an unacceptable error result by directly using the changed OCV-SOC curve data. The traditional OCV-SOC curve is obtained by measuring OCV at each SOC stage. This relationship varies with the change of battery capacity, and even for batteries with the same structure and material, the results are different. However, it is a time-consuming process to estimate the OCV of each battery on each SOC to determine the effectiveness of the estimation process.

The hysteresis of OCV has a great impact on the estimation of SOC. Hysteresis can be defined as the difference between OCV charging and discharging. Therefore, we can say that the pure OCV information cannot fully determine the SOC, and we should also take the historical charge and discharge data into account.

More importantly, the hysteresis of different types of lithium ion battery electrodes is also different (there is hysteresis phenomenon in the electrode with lithium iron phosphate as the active material). In order to accurately analyze the influence of hysteresis on battery SOC estimation or equivalent circuit parameters, the influence of hysteresis should be measured according to the battery SOC value or capacity. The OCV-SOC function can be realized by analyzing and expressing or looking up tables. Among them, the analysis method has many advantages, including the efficiency of data processing.

Terminal Voltage method

It can be said that only some studies can show that the terminal voltage method of lithium ion battery can be used to determine its SOC. This method is based on the fact that when the battery is discharged due to internal resistance, the terminal voltage will drop and the electromotive force is equal to the terminal voltage.

Impedance method

In order to use the impedance method to calculate the SOC, we must simultaneously record the voltage and current at different excitation frequencies, because the impedance of the battery depends on the frequency. The principle includes injecting current in a certain frequency range to find the impedance. When the SOC value is very high, the impedance change can be ignored, but when the SOC reaches a certain low level, the impedance will rise rapidly. Among many methods, EIS (Electrical Impedance Spectroscopy) is regarded as an important source of information for the complex electrochemical process inside the battery. Although many methods of estimating SOC are based on EIS, the complexity of using EIS directly is very high. As a method, the impedance model is established based on EIS data. EIS data is presented in the form of Nyquist diagram, in which the measured impedance is drawn into real and imaginary parts. Nyquist impedance spectrum is divided into three parts: low frequency region, intermediate frequency region and high frequency region. Because this division simplifies the identification of parameters, the method based on ECM model can be used to estimate SOC. Note: Nyquist diagram is a graphic method to show the frequency characteristics of the system. The frequency response is represented in the polar coordinates by its amplitude frequency characteristics and phase frequency characteristics, which is called amplitude phase diagram, or Nyquist diagram. The above is the introduction of the direct measurement method in the SOC estimation algorithm in this issue. The next article will continue to introduce other estimation algorithms (counting method). See you next time! In order to prevent some small partners from not seeing the first article, the original link of the first article is attached: Introduction to Battery SOC of BMS Algorithm Design (I) If you have different views, please scan the QR code below to follow the official account, and we look forward to sharing with you. Reference materials: HowNet, Energy journals, relevant books, etc

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