2025-04-18
As an important component of modern electric vehicles and renewable energy systems, it is vital to accurately measure the State of Charge (SOC) of motive power lithium batteries, which can help optimise battery use, extend battery life and provide more reliable data to support battery management. This article will introduce some common SOC measurement methods for lithium batteries and help readers understand the principles, advantages and disadvantages of various measurement methods with concrete examples.
The voltage method is one of the simplest and most common methods of measuring SOC. For example, when we use an electric vehicle, the battery level display on the dashboard is based on the voltage method. The current SOC is calculated by measuring the open-circuit voltage (OCV) of the lithium battery and using a pre-established voltage-SOC relationship model. 3.7V or 3.2V is the majority of cells used in most electric motorcycle products today, and the SOC is 50% when the individual cell voltage is 3.7V and 20% when the individual cell voltage is 3.2V. However, the voltage method is somewhat inaccurate, especially when the battery ages or at high discharge rates, the relationship between voltage and SOC can change. When the battery is about to run out of charge, the error in the SOC measurement will be large.
Coulomb counting is a method of calculating the SOC by measuring the charge and discharge of the battery. When we use the phone, we can see the percentage of charge remaining, which is based on the coulomb counting measurement. By recording the input and output charge of the battery and combining this with the initial SOC value, the current SOC can be calculated. for example, if we start using the phone from a 100% full charge, for every 10% of the charge used, 10% of the SOC is subtracted. however, the coulomb counting method is susceptible to system errors, current measurement errors and battery capacity decay, resulting in cumulative errors in SOC estimation.
The extended Kalman filter method is a state estimation based SOC measurement method. It combines the kinetic model of the battery with measurement data and uses a filtering algorithm to estimate SOC. Based on the EKF method, the current SOC can be estimated more accurately by measuring the voltage, current and temperature of the battery in combination with the model of the battery. the EKF method is able to reduce the influence of measurement errors and systematic errors and improve the accuracy of the SOC estimation. For example, in an electric vehicle, the EKF method can be used to combine a kinetic model of the battery with real-time measured current, voltage and temperature data to accurately estimate the battery's SOC and provide the vehicle driver with reliable information about the battery's condition.
Statistical methods use historical data and battery performance models to infer the current SOC by analyzing and modeling a large amount of data. for example, by conducting a large number of charge and discharge experiments on the same type of power lithium battery and recording the relevant current, voltage, capacity and other data, a statistical model between SOC and battery characteristics can be established. The statistical model is then used to estimate the current SOC based on the current and voltage data measured in real time. this method can improve the accuracy of the SOC measurement by taking into account factors such as changes in the internal resistance of the battery, current characteristics, and temperature effects.
In practice, we often combine multiple methods or use hybrid estimation strategies to improve the accuracy and stability of SOC measurements. For example, voltage and coulomb counting methods can be combined to reduce the error of each method by calibrating and complementing each other. Meanwhile, with the continuous development of battery technology and the advancement of intelligence, more accurate and reliable SOC measurement methods will emerge in the future to provide better support for the performance optimisation and safety management of battery systems.
Accurate measurement of the SOC of power lithium batteries is essential for battery management and use. Different SOC measurement methods have their own advantages and disadvantages and should be selected according to specific application scenarios and needs. By combining the above mentioned methods, the SOC of power lithium batteries can be roughly estimated, providing important support for the optimisation and safe management of battery systems. With the advancement and innovation of technology, we can expect continuous improvement in SOC measurement methods to bring better experience to customers.
