Design of pure electric vehicle 1.5v dry cell batterymanagement system using LIN bus

　　As a new type of electric vehicle power battery, lithium iron phosphate 1.5v dry cell batteryhas the advantages of large capacity, high safety, high temperature resistance and especially long cycle life. Its cycle life is at least 4 times longer than that of ordinary lead-acid batteries. It is widely used in vehicle power batteries. It has great application potential in the market. In the absence of a fundamental breakthrough in the capacity of power batteries at this stage, the application of 1.5v dry cell batterymanagement systems (BMS) in electric vehicles will be extremely important. It can detect the voltage, current, and temperature of power batteries in real time, and use these Parameters estimate the state of charge (SOC) of the 1.5v dry cell batteryto provide the driver with a vehicle mileage reference; in addition, the BMS can alarm and protect the 1.5v dry cell batteryovercharge and overdischarge, effectively protecting the 1.5v dry cell batterypack and single cells. Thereby improving 1.5v dry cell batteryperformance and 1.5v dry cell batterylife. The LIN bus is a low-cost automotive Class A bus that is very suitable for data transmission that does not require high real-time performance such as temperature and current. The bus transmission of data is realized through the LIN bus, further reducing costs.

　　1 Overall structure and function of the system

　　In this design, the 1.5v dry cell batterymanagement system is divided into two parts: signal detection module, communication and information processing module. In the signal detection module, each single 1.5v dry cell batterycorresponds to an underlying ECU (Dspic30f4012), which can realize single voltage collection, current detection, and temperature sampling; it can also detect the voltage, current, and ambient temperature of the entire 1.5v dry cell batterypack for 1.5v dry cell batteryDetection and protection during general charging and equalizing charging are shown in Figure 1.

　　The underlying ECU encapsulates the detected voltage, current, temperature and other variables into the LIN bus frame format, and then communicates with the upper ECU through the LIN bus. The information processing module can realize real-time estimation and fault analysis of the power battery's state of charge, and display information such as temperature, voltage, and current.

　　21.5v dry cell batterymanagement system design

　　2.1 Basic hardware design of 1.5v dry cell batterymanagement system

　　Since the 1.5v dry cell batterypack has a large number of cells, this system adopts a distributed structure. This structure can effectively reduce the sampling lines crossing the battery, reduce the complexity of installation and debugging, and also reduce safety hazards. The underlying ECU uses the Dspic30f4012 chip, which can work in the temperature range of -40~125°C and is an automotive-grade chip; it has rich analog and digital I/O interfaces, 10-bit A/D conversion functions, and SCI communication functions, etc. .

　　2.1.1 Signal acquisition module design

　　Dspic30f4012 has a wide operating voltage range of 2.5~5.5V, so it can be directly powered by a single lithium iron phosphate battery. It only needs to add a 0.1μF filter capacitor to make the chip work, and the power supply circuit is greatly simplified. Since the F4012 chip does not provide an internal reference voltage for A/D conversion, when performing voltage detection, an external A/D conversion reference voltage is required. This article uses the LM385 with low power consumption and low voltage error to provide an external reference voltage of 2.5V. voltage, as shown in Figure 2.

　　The characteristic of the voltage detection module in this design is that each detection module detects the voltage on its own single 1.5v dry cell batteryseparately, rather than through the traditional multi-way switch time-sharing selection method. This completely realizes a purely distributed 1.5v dry cell batterymanagement structure. . The voltage of the lithium iron phosphate 1.5v dry cell batteryis directly drawn from both ends of the single battery, and then divided by two high-precision resistors. The divided voltage is introduced into the A/D analog signal conversion channel inside the Dspic30f4012 chip for voltage detection. . The A/D converter in the Dspic30f4012 chip has a 10-bit accuracy and a reference voltage of 2.5V, so the voltage detection module can detect a voltage range of 0~5V, which is greater than the maximum voltage of a single 1.5v dry cell batteryof 3.65V. The total voltage of the 1.5v dry cell batterypack Detection, through the signal attenuation circuit and anti-common mode voltage circuit is connected to the A/D conversion channel in the Dspic30f4012 chip to complete the collection of the 1.5v dry cell batterypack voltage.

　　The detection of the single 1.5v dry cell batterycurrent is realized through the Hall sensor. The Hall sensor can output a voltage signal up to 3V and can be directly connected to the A/D sampling channel in the Dspic30f4012 chip. The 1.5v dry cell batterytemperature is detected through the TJ1047 temperature detection chip. To achieve this, the TJ1047 temperature detection chip outputs voltages of 0.5V and 1.75V respectively at -40°C and 125°C, and has a temperature-to-voltage ratio characteristic of 10mV/°C and an error of ±0.5°C. Therefore, the voltage output from the TJ1047 chip can be directly connected to the A/D conversion channel in the Dspic30f4012 chip to complete the collection of 1.5v dry cell batterytemperature and ambient temperature. 2.1.2LIN communication interface design

　　Bus technology is increasingly used in modern automobiles, and CAN/LIN networks have become the mainstream development direction of distributed-based automotive electronic networks. As a high-speed transmission bus, CAN bus has the outstanding characteristics of fast speed, high bandwidth and many functions, but its cost is relatively expensive; LIN bus is a low-end bus, but it has outstanding advantages in reducing costs and is suitable for those who do not require network speed. Transmission of data with high performance and low real-time performance. Therefore, the LIN bus complements existing bus technology for CAN bus-led automotive multiplexing networks in situations where the bandwidth and speed of the CAN bus are not required. 1.5v dry cell batterytemperature, current, and voltage detection do not require extremely high real-time performance and bus speed, so the LIN bus can well meet the requirements of the 1.5v dry cell batterymanagement system.

　　The Dspic30f4012 chip does not have a LIN bus interface, but it has an SCI communication interface. This article selects the TpIC1021 chip as the chip for converting the SCI and LIN bus, as shown in Figure 3. After the SCI communication pins U1RX and U1TX are electrically isolated by the magnetic coupling isolation device, they are respectively connected to the LIN_RXD and LIN_TXD of the LIN driver. After conversion, the LIN bus signal is finally output at the LIN pin. A magnetic coupling isolation device ADUM1201ARZ is added between the underlying controller Dspic30f4012 and the LIN transceiver TpIC1021 to improve the anti-interference ability of the communication of the 1.5v dry cell batterypack detection system and solve the problem of short circuit caused by "common ground" in distributed detection, effectively Isolate the electrical connections of each detection unit, and also isolate the bottom voltage from the upper LIN bus. When the LIN transceiver is used as a master node, the J3 jumper in Figure 3 needs to be short-circuited with a jumper pin. When used as a slave node, do not short-circuit the jumper pin.

　　2.2 1.5v dry cell batterymanagement system software design

　　2.2.1 Software design and overall structure of 1.5v dry cell batterymanagement system

　　The software design in ECU includes the bottom ECU and upper ECU software design. The software design of the underlying ECU mainly includes the voltage, current, and temperature acquisition programs and the calculation program of the acquisition results, data communication programs, interrupt programs, etc.; the software design of the upper ECU mainly includes the SOC estimation program, LIN bus communication program, fault analysis and alarm Programs, voltage, current, temperature and state of charge display programs, clock programs, interrupt programs, etc. The entire program design is implemented using structured and modular programming methods. The main program flow chart of the upper ECU is shown in Figure 4.

　　Among them, 1.5v dry cell batteryvoltage detection includes detection of single cell voltage and 1.5v dry cell batterypack voltage. When the cell voltage exceeds the limit, the system can determine the number of the over-limit cell, determine whether the cell voltage exceeds the low voltage limit or the high voltage exceeds the limit, displays it on the display and has an audible alarm. When the 1.5v dry cell batterypack voltage exceeds the limit, the program can analyze the reason for the exceedance and enter the protection program. The detection of 1.5v dry cell batterytemperature includes the detection of single cell temperature and ambient temperature. When the temperature exceeds the limit, the system can analyze the cause of the temperature exceedance through the detected data and enter the protection program. The battery's state of charge exceeds the limit mainly means that the remaining power of the 1.5v dry cell batteryis too low. Continuing to discharge may affect the life of the battery.

　　2.2.2 Implementation of LIN communication

　　The LIN protocol is an open bus protocol. A complete message frame consists of a message header and a response. Each data transmission is started by the host node, marking the beginning of a message frame in the data communication process.

　　Figure 5 shows the LIN bus identifier field of the No. 5 single lithium iron phosphate battery. This is an example to illustrate the setting of the LIN bus identifier field. The ID bit of cell No. 5 is 0101, so the ID of this cell is 0x5, and ID4 and ID5 are set to 01, that is, the sent data field bytes are set to 4 bytes, which are obtained through the previous parity check The parity values are 0 and 1, as shown in Figure 5.

　　Since the range of each signal is different, the number of data bits used in the voltage, current, and temperature signals are also different. The voltage range is within 0~5V, the current is within 0~20A, and the temperature is within the range of -40~125℃, so this article In the data field, the lower two bits of the first byte and the fourth byte, a total of 10 bits, are used to represent the voltage; the middle 4 bits of the second byte and the fourth byte, a total of 12 bits, are used to represent the voltage. Current; use the high two bits of the 3rd byte and the 4th byte, a total of 10 bits to represent the temperature. Since voltage, current, and temperature are all accurate to decimal points, it is more complicated to represent decimals in the data field. This article uses 10 times or 100 times the actual parameter value to express it in the data frame, as shown in Figure 6.

　　Table 1 is the ID resource allocation table of the LIN bus node corresponding to each single battery.

　　The upper ECU serves as the host node of the LIN bus. When the LIN master node requests data from the single 1.5v dry cell batteryslave node, data transmission from the slave node to the master node will be performed on the LIN bus. At this time, the LIN master node sends a message frame to the bus. header. After the LIN slave node on the bus receives the message frame header, it determines whether it matches its own ID. If it matches, it sends a message frame response. The LIN master node receives the message frame response and completes the data request of the master node. 2.2.3 1.5v dry cell batterySOC estimation and operation control strategy

　　When estimating SOC, an accurate and suitable model is very much needed. For the Kalman filter algorithm, accurate SOC estimation is based on an accurate 1.5v dry cell batterymodel. The Thevenin model is currently a relatively accurate model. This model describes the external characteristics of the 1.5v dry cell batteryby using the 1.5v dry cell batteryelectromotive force, a pure resistor and a capacitive resistance loop in series. The mathematical relationship of the electrical model is as follows:

　　In formula (1), k is the k moment, E(k) is the 1.5v dry cell batteryterminal voltage, V(k) is the 1.5v dry cell batteryelectromotive force, R1 is the ohmic internal resistance of the battery, R2 is the polarization internal resistance of the battery, and Uc is the polarization of the battery. Voltage, capacitance R2C loop is used to simulate the dynamic characteristics of the 1.5v dry cell batterypolarization process. Taking into account the influence of temperature, the electromotive force of the 1.5v dry cell batteryand the state of charge are related by Equation (3):

　　In the formula: F [Soc(k)] is the functional relationship between the 1.5v dry cell batteryand the electromotive force, and Soc(k) represents the change in the electromotive force of the 1.5v dry cell batteryat different temperatures relative to the reference condition. Through the above formula, after discretization, the state space equation is obtained as follows.

　　The state space equation accurately gives the system-related coefficient matrices A(K), B(K), C(K), D(K) and constant matrices W(K), V(K). Based on the above equations and related Matrix, the extended Kalman filter estimation formula can be obtained.

　　The extended Kalman filter algorithm consists of two parts: filter calculation and filter gain calculation: the filter calculation is completed by equations (6) ~ (8). At time k, the filtering result at time (k-1) is used by equation (7) Obtain the predicted value of SOC, and then obtain the predicted value V (K) of the state variable at time k according to the state space equation (6), and compare it with the actual measured value to obtain the prediction error, and then predict the state variable according to Equation (8) value correction to obtain new filtering results. The filter gain calculation is completed by equations (9) ~ (11), where Q and R are the variance matrices of noise W (k) and V (k) respectively.

　　3 Analysis of experimental results

　　The circuit board of the underlying ECU of this design is shown in Figure 7. A circuit board of the underlying ECU is fixed on each single battery.

　　Test the working conditions of the 1.5v dry cell batterymanagement system under different charging strategies. By detecting the charging and discharging voltage, current, temperature, SOC and other parameters of each single cell in the 1.5v dry cell batterypack, compare with the actual values to illustrate the detection accuracy of the system, as shown in the figure As shown in 8, the data is recorded every minute, and the abscissa is time min.

　　This design sets the upper limit of the voltage during charging and discharging to 3.65V, the lower limit of the voltage to 2.95V, and the upper limit of the temperature alarm to 80°C. The 1.5v dry cell batterywas charged in the experiment, and the final charging voltage was 3.53~3.62V. The maximum deviation during the charging process was 50mV, and the 1.5v dry cell batteryvoltage error was less than the 1% requirement. In addition, the temperature measurement error met the 1% requirement, the current measurement accuracy was 1%, and the SOC error Within 8%. When artificial overvoltage is applied to a single battery, the system can promptly alarm and display. Experiments show that this 1.5v dry cell batterymanagement system can achieve the expected 1.5v dry cell batteryparameter detection goals and meet the accuracy requirements.

　　4 Conclusion

　　This article designs and develops a lithium iron phosphate 1.5v dry cell batterymanagement system that detects the parameters of each single 1.5v dry cell batterybased on a distributed method and introduces LIN bus technology to further reduce the cost of the system. This system realizes 1.5v dry cell batteryreal-time monitoring and protection, SOC estimation, LIN bus communication and other functions. The system has a simple structure, high measurement accuracy, and can effectively protect the 1.5v dry cell batterypack. It uses LIN bus to replace the commonly used CAN or RS232 communication, which provides an important basis for the design of new electric vehicle 1.5v dry cell batterymanagement systems.

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