What is SOC?
A battery's State of Charge (SOC) is the ratio of the current charge available to the total charge capacity, usually expressed as a percentage. Accurately calculating the SOC is crucial in a Battery Management System (BMS) as it helps to determine the remaining energy, manage battery usage, and control charging and discharging processes, thus extending the battery's lifespan.
The two main methods used to calculate SOC are the current integration method and the open-circuit voltage method. Both have their advantages and disadvantages, and each introduces certain errors. Therefore, in practical applications, these methods are often combined to improve accuracy.
1. Current Integration Method
The current integration method calculates the SOC by integrating the charge and discharge currents. Its advantage lies in its simplicity, not requiring calibration. The steps are as follows:
- Record the SOC at the start of charging or discharging.
- Measure the current during charging and discharging.
- Integrate the current to find the change in charge.
- Calculate the current SOC using the initial SOC and the charge change.
The formula is:
SOC=initial SOC+Q∫(I⋅dt)
where I is the current, Q is the battery capacity, and dt is the time interval.
It's important to note that due to internal resistance and other factors, the current integration method has a degree of error. Moreover, it requires longer periods of charging and discharging to achieve more accurate results.
2. Open-Circuit Voltage Method
The open-circuit voltage (OCV) method calculates the SOC by measuring the battery's voltage when there is no load. Its simplicity is its main advantage as it doesn't require current measurement. The steps are:
- Establish the relationship between SOC and OCV based on the battery model and manufacturer data.
- Measure the battery’s OCV.
- Calculate the SOC using the SOC-OCV relationship.
Note that the SOC-OCV curve changes with the battery’s usage and lifespan, requiring periodic calibration to maintain accuracy. Internal resistance also affects this method, and errors are more significant at high discharge states.
3. Combining Current Integration and OCV Methods
To improve accuracy, the current integration and OCV methods are often combined. The steps for this approach are:
- Use the current integration method to track the charging and discharging, obtaining SOC1.
- Measure the OCV and use the SOC-OCV relationship to calculate SOC2.
- Combine SOC1 and SOC2 to get the final SOC.
The formula is:
SOC=k1⋅SOC1+k2⋅SOC2
where k1 and k2 are weight coefficients summing to 1. The choice of coefficients depends on battery usage, testing time, and accuracy. Typically, k1 is larger for longer charge/discharge tests, and k2 is larger for more precise OCV measurements.
Calibration and correction are needed to ensure accuracy when combining methods, as internal resistance and temperature also impact results.
Conclusion
The current integration method and OCV method are the primary techniques for SOC calculation, each with its own pros and cons. Combining both methods can enhance accuracy and reliability. However, calibration and correction are essential for precise SOC determination.
Post time: Jul-06-2024
