The calculation of the state of charge (SOC) of the battery is the most fundamental part of the battery management system (BMS). The accurate monitoring of the state of charge of the battery not only provides the user with the battery energy supply status, but also the basis of charge and discharge management and equalization control management in the battery management system. Therefore, it is of great significance to accurately measure the value of the battery SOC.
In this paper, LTC6804-2 chip is used for lithium battery voltage acquisition control chip. Hall sensor is used to collect charge and discharge current. I2C bus temperature sensor chip is used to collect battery case temperature. Using LPC2478 as the main control chip, a design based on LTC6804-2 is designed. Lithium battery SOC application system.
1, SOC measurement system principle 1.1, An time integration methodThe ampere-hour integral method obtains the SOC by integrating the current flowing through the battery pack per unit time, and the calculation formula is as follows:
SOC0 represents the initial value of the lithium battery SOC, CN represents the rated capacity of the lithium battery, t represents the charge and discharge time of the lithium battery, and I represents the charge and discharge current of the lithium battery. The Ansue integration method is simple and is a commonly used method. Since the method needs to integrate the current, the accuracy of the current collection is high, and the error will accumulate due to the integral, and the error in the use of the time will become larger and larger.
1.2, open circuit voltage methodThe basic principle of the open circuit voltage method is that when the SOC of the lithium battery is within a certain range, the open circuit voltage has a strong correlation with the SOC of the battery, and the correlation curve can be obtained by an experimental method. The method restores the battery terminal voltage to the open circuit voltage by fully resting the lithium battery, and performs SOC calculation according to the magnitude of the open circuit voltage. The method is simple and easy, and the precision is high, but the battery needs to be left for a long time, which is not suitable for online measurement, and the real-time performance is poor.
The system uses the ampere-hour integral method and the open-circuit voltage method to estimate the SOC, that is, the SOC estimation is performed by the ampere-hour method when the battery pack is charged and discharged, and the open circuit voltage method and the battery case temperature coefficient are used when the battery pack is in the idle state. Ampere integral method correction, fully utilize the characteristics of the two methods to improve the accuracy of SOC estimation.
2, system hardware designThe system uses LPC2478 as the main control chip. The whole system consists of 12 lithium battery packs, voltage measurement circuit, temperature sensor, temperature acquisition circuit, Hall current sensor, current measurement circuit and communication circuit. The system hardware block diagram is shown in Figure 1.
Figure 1 system hardware block diagram
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