Research on lithium battery SOC application based on LTC6804-2

Battery state of charge (SOC) measurement and calculation is the most basic and important part of the battery management system (BMS). Accurate monitoring of battery state of charge can not only provide users with battery energy supply status, but is also the basis for charge and discharge management and balance control management in the battery management system. Therefore, accurately measuring the value of battery SOC is of great significance. The balance of lithium batteries is essentially the balance of state of charge. Its value is the ratio of the battery's current capacity to its rated capacity. There are currently many methods for measuring SOC, including the ampere-hour integration method, the open circuit voltage method, the internal resistance method and the Kalman filter method. The internal resistance method is unstable in measurement and has a large amount of data disturbance; the Kalman filter algorithm has high real-time requirements and has many initial parameters and a large amount of calculation. Based on full consideration of the pros and cons and practicality of various algorithms. Combined with the specific conditions of the system, the ampere-hour integration method is used to estimate the SOC of the battery pack, and then the SOC value is corrected through the open circuit voltage method and the temperature coefficient.

This article uses the LTC6804-2 chip as the lithium battery voltage acquisition and control chip, uses the Hall sensor to collect the charge and discharge current, uses the 12C bus temperature sensor chip to collect the battery case temperature, and uses LPC2478 as the main control chip. A lithium battery SOC application system based on LTC6804-2 is designed.

What does the SOC of lithium battery mean?

The abbreviation of State of Charge is generally the ratio of charging capacity to rated capacity, expressed as a percentage.

A battery generally has a rated capacity. After charging at a certain rate for a certain time, you can get the charging capacity. The ratio of this capacity to your rated capacity is the SOC.

There is no strict definition of SOC. It is generally believed that SOC=100%-DOD

DOD is the discharged power of the battery. 100% is the fully charged state of the battery, which is the ampere-hour value of the battery.

This definition comes from the battery test manual published by USABC in 1996.

The biggest problem in this definition is that 100% is a time-varying value, which changes as the battery ages. DOD is also an uncertain value. DOD is different under different working conditions. In the same working condition, the DOD value is also different depending on the measurement method. USABC The battery test manual gives a DOD measurement process.

SoC Measurement System Principle

ampere-hour integration method

The ampere-hour integration method obtains SOC by integrating the current flowing through the battery pack per unit time. Calculated as follows:

SOC represents the initial value of the lithium battery SOC, c represents the rated capacity of the lithium battery, t represents the charge and discharge time of the lithium battery, and represents the charge and discharge current of the lithium battery. The ampere-hour integration method is simple and is a commonly used method today. Since this method requires current integration, the accuracy of current acquisition is high, and the error will accumulate due to integration, and the error will become larger and larger over time.

open circuit voltage method

The 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 its correlation curve can be obtained through experiments. This method allows the lithium battery to rest sufficiently to restore the battery terminal voltage to the open circuit voltage, and calculate the SOC based on the open circuit voltage. This method is simple and easy to implement, and has high accuracy. However, the battery needs to stand for a long time, which is not suitable for online measurement and has poor real-time performance.

This system uses the ampere-hour integration method plus the open circuit voltage method to estimate SOC. That is, the ampere-hour method is used to estimate the SOC when the battery pack is charging and discharging. When the battery pack is in a resting state, the open circuit voltage method and the battery case temperature coefficient are used to correct the ampere-hour integral method, making full use of the characteristics of the two methods. Improve the accuracy of SOC estimation.

System hardware design

The system uses LPC2478 as the main control chip. The entire system consists of a 12-cell lithium battery pack, voltage measurement circuit, temperature sensor, temperature acquisition circuit, Hall current sensor, current measurement circuit, and communication circuit.

The system hardware structure block diagram is shown in Figure 1.

Voltage measurement hardware circuit

The LTC6804-2 is a third-generation multi-cell battery pack monitor that can measure the voltage of up to 12 series-connected cells with a total measurement error of less than 1.2mV. The voltage of all 12 cells can be measured within 290Ixs, and a lower data acquisition rate can be selected for high noise rejection. The LTC6804-2 is a 16-bit delta-accumulate ADC with frequency programmable third-order noise filter. The LTC6804-2 battery voltage measurement circuit is shown in Figure 2. ISOMD connected to low level indicates that LTC6804 is configured as an SPI slave device. The SPI interface and LPC2478 are isolated by silicon chips to reduce interference from the battery side to the digital master circuit. The voltage of each cell of the lithium battery is 3.3V.

Temperature acquisition hardware circuit

The temperature acquisition chip uses the TMP100 temperature sensor chip from Texas Instruments. The temperature ADC acquisition hardware circuit design is shown in Figure 3.

The different combinations of TMP1O0-D1-ADD0 and TMP100-D1-ADD1 represent the communication address of the slave device of the chip. The slave address definition of the device is shown in Table 1.

Each TMP100 chip can support 8 different 12C slave addresses. The system design has 12 lithium batteries, which requires two TMP100 chips and two IC interfaces of the main control chip LPC2478.

Current measurement hardware circuit

The current measurement data is obtained through the Hall current sensor and the ADC voltage acquisition chip. This system design uses the HCS-LSP-20A Hall current sensor of Nanjing Zhonghuo Sensing Technology Co., Ltd. This sensor is a Hall current sensor based on the closed-loop magnetic balance principle and can measure DC, AC, pulse and various irregular currents. The output of the sensor can truly reflect the real wave of the energized conductor.

The output voltage range of the current sensor is 0.5~4.5V. The output voltage is collected through the ADC acquisition chip. The ADC chip uses Maxim-IC Company's MAX1062. The chip supports 5V input. 14bit acquisition accuracy, 200kS/s sampling rate, SPI communication interface, one SPI interface of the LPC2478 chip is connected to the ADC acquisition chip.

LPC2478 system main control hardware circuit

Taking into account that it can realize various functions of voltage measurement, current measurement, and temperature measurement, while meeting the low power consumption characteristics of the system, the ARM chip, which is currently the most widely used, was selected after comprehensive consideration. Select NXP's LPC2478 chip.

One SPI interface of LPC2478 implements communication with the LTC6804-2 and MAX1062 chips. Select the chip that needs to communicate through the GPIO port: the external physical layer chip of the Ethernet uses the KSZ8041NLI chip. The Ethernet MAC interface of the LPC2478 is connected to the KSZ8041NLI chip. The KSZ8041NLI is connected to the KSZ8041NLI chip through the RJ one. 45 interface to communicate with the outside world; LPC2478 chip has two serial ports and one is connected to the isolated RS. 422/485 chip ADM2582E, connected to isolated RS all the way. 232 chip MAX3250EAI, the two IC ports of LPC2478 are connected to the TMP100 temperature acquisition chip, and the LPC2478 debugging and programming interface is the JTAG interface.

System software design

LTC6804-2 configuration and communication

After the LPC2478 is powered on or reset, it first initializes the LTC6804-2 through the SPI port, mainly setting the SPI communication rate, the ADC working mode of the LTC6804, and selecting the channel to be measured. The specific functions used are as follows:

void LTC6804-initialize(); //Initialization function of hc6804-2

//Set ADC working mode, battery measurement channel, discharge allow bit setting

void set— adc (uint8-tMD, uint8-tDCP, uint8-tCH, uint8-tCHG);

void LTC6804_adcv(); //Start LTC6804-2 battery measurement

//Read the measured voltage of 12 batteries

uint8_tLTC6804_rdcv (uint8_treg, uint8_ttotal_.ic, uintl6_.tcell_

codes[][12]);

void LTC6804_wrcfg(uint8_tnIC, uint8_tconfig[][6]); //hc6804

— 2 write configuration register

int8_tLTC6804_~dcfg(uint8_tnIC, uint8_tr_conflg[][8]); //

LTC6804-2 read configuration register

Temperature acquisition chip configuration and temperature data reading

There are 4 data registers and 1 pointer register inside the TMP100. The accessed data register is indexed through the pointer register. The reading and writing of the data register are determined by the last two bits of the pointer register. The definition of the pointer register is shown in Table 2, and the definition of the data register is shown in Table 2. 3. See Table 4 for configuration register definition.

SD (shutdown): When this bit is 1, TMP100 enters standby power-saving mode and the chip stops working; when this bit is 0, it enters normal working mode and the temperature value can be read normally: R1/R0 (CONVERTER RESOLUTION): This bit indicates For conversion resolution, the system sets R1/R0 to 11, that is, the resolution is 12 bits, 0.0625oC, and the conversion time to complete a temperature data collection is 320ms. The temperature register is a 16-bit register, using 2's complement calculation.