The state of charge (SOC) calculation of a battery is one of the most critical components in a battery management system (BMS). Accurate SOC monitoring not only informs users about the available energy but also serves as a foundation for charge/discharge control and balancing strategies within the BMS. Therefore, achieving high accuracy in SOC measurement is essential for reliable battery operation and safety.
In this paper, we present a lithium battery SOC estimation system that utilizes the LTC6804-2 chip for voltage acquisition, a Hall sensor for current measurement, and an I2C temperature sensor to monitor the battery case temperature. The LPC2478 microcontroller acts as the central processing unit, enabling the integration of these components into a functional SOC application system.
**1. SOC Measurement System Principle**
**1.1, Ampere-Hour Integration Method**
The ampere-hour (Ah) integration method calculates SOC by integrating the current over time. The formula is as follows:
$$
SOC = SOC_0 + \frac{1}{C_N} \int_{0}^{t} I(t) dt
$$
Where $ SOC_0 $ is the initial SOC value, $ C_N $ is the rated capacity of the battery, $ t $ is the charge/discharge time, and $ I $ is the charge/discharge current. This method is straightforward and widely used. However, it requires precise current measurement, and any small errors can accumulate over time, leading to significant inaccuracies. Additionally, time drift can further affect the reliability of the results.
**1.2, Open Circuit Voltage Method**
The open circuit voltage (OCV) method relies on the strong correlation between the OCV and the SOC of a lithium battery within a specific range. By fully resting the battery and measuring its terminal voltage, the SOC can be estimated based on a pre-established OCV-SOC curve. This method offers high precision but requires long rest periods, making it unsuitable for real-time or online applications. Due to its limitations, it is typically used in conjunction with other methods to enhance overall accuracy.
To improve the reliability of SOC estimation, the system combines both the ampere-hour integration method and the open-circuit voltage method. During active charging/discharging, the ampere-hour method is used, while during idle periods, the open-circuit voltage method is applied along with temperature compensation to refine the SOC value. This hybrid approach leverages the strengths of both techniques, resulting in more accurate and stable SOC estimation.
**2. System Hardware Design**
The system is built around the LPC2478 microcontroller, which serves as the main control unit. It integrates 12 lithium battery cells, a voltage measurement circuit, a temperature sensor, a temperature acquisition circuit, a Hall current sensor, a current measurement circuit, and a communication interface. The hardware block diagram of the system is illustrated in Figure 1.
Figure 1: System hardware block diagram
This design ensures accurate data collection from multiple battery parameters, enabling efficient and reliable SOC estimation for lithium battery systems.
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