Intelligent lead-acid battery management system

Introduction

    The lead-acid battery industry is closely related to the development of power, transportation, information and other industries. It is in a controlling position in vehicles such as automobiles and forklifts and large-scale uninterrupted power supply systems. It is indispensable in social production and business activities and human life. The scale of my country's battery industry is quite large, and its applications are also very wide. In view of the problems caused by improper use of lead-acid batteries (such as vulcanization, reduced capacity, shortened service life, etc.), it is very necessary to realize the intelligent management of batteries. There are few embedded system products used in this field. This design uses the 8-bit microcontroller MB95F136 to realize the intelligent management of the lead-acid battery, including battery charge and discharge monitoring and control, battery capacity detection, display and alarm, etc., so as to effectively realize the intelligent management of the lead-acid battery system. The service life of the storage battery is improved, and the maintenance cost is reduced.

1. System overview

    This design makes full use of the characteristics of MB95F136 to realize real-time online monitoring of battery voltage, current and temperature. The charging and discharging process of the intelligent control system can display the power of the battery, control and alarm the use condition that is incorrect or greatly damage the battery life, and can remind the user to charge the battery in time or switch the backup power when the battery needs to be charged. , To prevent overcharging and overdischarging. In order to realize the intelligent management of the lead-acid battery, the system obtains accurate calculation basis by automatically correcting the dynamic parameters of the battery in real time, thereby calculating the accurate power and battery status information, and obtaining the charging parameters of the battery.

    The battery management system designed in this article mainly has the following functions:

    ①Monitor the temperature of the battery in real time, calculate the charge and discharge parameters of the battery through temperature and other parameters, to avoid shortening the battery life due to improper use or high battery temperature.

    ②Real-time monitoring of the battery terminal voltage and current, if the battery capacity is found to be less than the warning threshold, it will prompt to charge or automatically switch the backup battery.

    ③The remaining capacity of the battery can be calculated by analyzing the parameters, and it can be displayed in real time through the digital tube.

    ④The system can automatically modify the internal parameters of the battery to adapt to some changes brought about by the battery, and it can also obtain a better charging effect by controlling the charging and discharging circuit.

2. System hardware design

2.1 System control core

    This system adopts F2MC-8FX series MCU MB95F136 as the control core of the system. In the system, MB95F136 not only needs to monitor the current, voltage, temperature and other parameters of the battery and the operating status of the system in real time, but also must process the collected data and output control signals to the charging control module to realize the intelligent management of the battery system; At the same time, it is also responsible for the realization of button control and system status output display. Fujitsu’s MB95F136 uses O. With 35μm low-leakage technology, mask products can operate in low-power operating mode (clock mode) of 1.8 V and 1μA. The pipeline bus architecture can provide double execution speed, and the minimum instruction cycle is 62.5 ns. It has the characteristics of fast processing and low power consumption, and is equipped with a wealth of timers; it integrates an 8-channel 8/10-bit optional A/D converter, which can be easily applied to the voltage and current in the system. collection. Dual-operation flash memory is also one of the features of F2MC-8FX series 8-bit microcontrollers. When a program is running in one storage area, it can be rewritten in another storage area, thereby reducing the number of external memory components and shrinking the circuit. The surface area of the board. In addition, LVD (low voltage detection) and CSV (clock monitor) functions can improve the stability and reliability of the system.

2. 2 power circuit design

    In this system, in order to enhance the flexibility of system application, the system power source is taken from the managed battery. For this reason, a DC-DC module must be used for isolation. Since the selected DC-DC module requires an input voltage ≥ 24 V, the battery managed by the system must be a battery pack of 2 or more nominal 12 V batteries, otherwise, an additional power supply circuit needs to be designed; in order to enhance the reliability of the system, the system can Set up a 3 V battery box for the backup battery. Once the power source from the battery fails, the system can still operate as usual. The schematic diagram of the system power circuit is shown in Figure 2.

2.3 Current and voltage acquisition circuit

    The monitoring objects are mainly the voltage and current of the battery pack. The voltage is obtained by the precision resistor of the voltage divider, and sent to the A/D port of the single-chip microcomputer after corresponding amplification. The charging and discharging current of the battery passes through O. The 01Ω sampling resistor samples, amplifies, and then sends it to the A/D port POl of the microcontroller. The key to testing the battery is the accuracy of the voltage sampling, so whether the sampling circuit is properly designed is very important to the entire system. Because the A/D converter embedded in MB95F136 can work under the 5 V reference voltage, the current and voltage acquisition circuit shown in Figure 3 is adopted. The biggest advantage of this circuit is that it can not only ensure that the sampled value can change in real time with the change of the battery terminal voltage, but also make the data more accurate and reliable. This circuit is a typical linear circuit. According to the characteristics of the operational amplifier, the output voltage after the sampling circuit can be calculated to be 0. 01 Q×I×23.

2.4 Parameter storage module

    Before the system is put into operation, parameters (such as product sequence, zero point adjustment, battery standard voltage, etc.) must be set, and the system will write these parameters into EE-PROM. In order to reduce the number of times of reading/writing EEPROM, the data is read from the EEPROM when the system is turned on and stored in the RAM of the microcontroller. The main function of EEPROM is the storage and quantitative backup of parameter data, which is mainly used to store some system operating parameters, such as reference data and correction coefficients for calculating battery power.

    This system uses EEPROMAT24C02 with a capacity of 2 Kb. The chip is a serial using I2C bus protocol. EEP-ROM can store important data in the system for a long time without power supply, and its working life can reach 1 million times. The I2C bus greatly facilitates the design of the system, without the need to design a bus interface, and helps to reduce the PCB area and complexity of the system.

2.5 Design of temperature acquisition module

    This design adopts DSl8820 single-bus digital intelligent temperature sensor produced by Dallas Company of the United States, which directly converts the physical quantity of temperature into digital signals, and transmits them to the controller for data processing in a bus mode. DS18B20 provides 9 to 12 bits of data and alarm temperature register for the measured temperature. The temperature measurement range is 55 to +125°C, and the measurement accuracy is ±0.5°C in the range of 10 to +85°C. This sensor can be applied to automatic measurement and control systems in various fields and environments. It has the advantages of miniaturization, low power consumption, high performance, strong anti-interference ability, and easy to equip with microprocessors. In addition, each DS18820 has a unique serial number, so multiple DS18820s can exist on the same single-wire bus, which brings great convenience to the application.

2.6 Controllable charge and discharge module

     This module is a hardware difficulty in actual design. It is connected to the external power grid to charge the on-board battery; it can charge the battery in stages with different currents according to instructions or flags issued by the control circuit; and has the function of automatic power-off, which can realize intelligent charging. This system is mainly for the management of electric vehicle battery packs, and the current used to charge the battery packs is relatively large. For this reason, an IGBT-based Intelligent Power Module (IPM) was selected for high-current charge and discharge management. IPM is an advanced hybrid integrated power device, which is composed of high-speed, low-power IGBT, drive circuit and protection circuit; there are fault detection circuits such as overvoltage, overcurrent, short circuit and overheating, and it has automatic protection function.

Q1 and Q2 are integrated in one IPM. When Q2 is opened, the battery pack is charged, when Q1 is opened, the battery pack is discharged through R1; when the battery pack supplies power to the load, both Q1 and Q2 are closed. In order to improve the working state of the power switching device, soft switching technology is adopted in the main circuit. In the case of high-current charging, due to the long-term charging of the battery pack, the charge accumulates on the battery electrodes to generate a reverse voltage, which is actually manifested as an increase in the internal resistance of the battery, and not only the effective chemical substances in the battery cannot be fully incorporated. The chemical reaction reduces the capacity utilization of the battery pack, and also causes serious heating of the battery pack, which affects the charging speed and quality, and then affects the performance and life of the battery pack. The effective way to eliminate it is to use the negative pulse method: instantaneous discharge at both ends of the battery to remove the charge accumulated on the electrode, thereby changing the inherent charge acceptance characteristics of the battery in the form of an exponential curve, and improving the battery's power receiving capacity. To this end, a charging strategy of "charge-stop-discharge-charge-stop-discharge" cycle charging is adopted. Its pulse charging characteristic is shown in Fig. 6, the time parameter is decided by the battery parameter.

2.7. Power and status output indication and alarm module

    In order to reduce the complexity and cost of the system, this design uses three 8-segment digital tubes to display the system status. Simple parameter setting can be carried out, real-time display of status, temperature and other data to achieve better human-computer interaction. This design adopts the solution of debounce processing on the input on the software, and continuously judges the state of the button until the button is released, and then executes the corresponding processing program. The data display adopts 3-digit 7-segment digital tube dynamic display mode, and uses 74HC595 to latch the dynamic display data. This design cleverly shared the key input and the dynamic display digital selection port, thereby reducing the application of the single-chip port, achieving the purpose of system optimization and reducing product costs. The alarm uses a buzzer.

3. System software design

    The software design process of this system is shown as in Fig. 7. After the system is started, the system initialization program is executed immediately, and the parameters obtained from the last operation are read from the EEPROM. Then begin to read the data in the temperature sensor to obtain the current system temperature, and then call the A/D sampling subroutine to obtain 10-bit precision voltage and current signal data. After processing, the final battery operating state can be obtained, and the respective processing procedures are carried out according to different states, and the state data is output to the digital tube display. When the system is running, it will automatically modify the parameters based on the existing data and the monitored data to accurately reflect the internal parameters of the battery and realize the intelligent management of the system.

Conclusion

     This system uses MB95F136 as the controller, taking full advantage of its multiple peripheral interfaces, strong functions, integrated high-precision A/D converter, convenient operation, low actual cost, and easy system modularization and miniaturization. The system can monitor the status of the battery and display the battery power in real time and accurately, and can automatically switch the power supply system to implement self-protection when the battery is insufficient. The update of the parameter data is based on the results of multiple experiments, comparison and calculation of the measured parameters. Through the experiment, the calculated value of the remaining power is closer to the actual value than when the parameter is not updated. Practice has proved that the intelligent lead-acid battery management system has a high degree of intelligence and accurate measurement. It can detect and control improper use of the battery in time, provide self-protection, and accurately determine the operating status of the system, which not only greatly improves the power supply The stability of the system, but also helps to improve the service life and efficiency of the battery.