Intelligent lead-acid battery management system

introduction

The lead-acid battery industry is closely related to the development of industries such as electricity, transportation, and information. It plays a controlling role in transportation tools such as automobiles and forklifts and large uninterruptible power supply systems, and is indispensable to social production and operation activities and human life. my country's battery industry is quite large and widely used. In view of the problems caused by improper use of lead-acid batteries (such as sulfation, reduced capacity, shortened service life, etc.), it is very necessary to realize the intelligent management of batteries. However, there are few embedded system products currently used in this field in China. This design uses the 8-bit microcontroller MB95F136 to realize the intelligent management of lead-acid batteries, 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, improve the service life of the battery, and reduce maintenance costs.

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 intelligent control system's charging and discharging process can display the battery power, control and alarm the incorrect use or use that has a great harm to the battery life, and remind the user to charge or switch to the backup power supply in time when the battery needs to be charged to prevent overcharging and over-discharging. In order to realize the intelligent management of lead-acid batteries, the system automatically corrects the dynamic parameters of the battery in real time to obtain accurate calculation basis, thereby calculating the accurate power and battery status information, and obtaining the battery charging parameters.

The battery management system designed in this paper has the following main functions:
① Real-time monitoring of battery temperature, and calculation of battery charge and discharge parameters through temperature and other parameters to avoid shortening the battery life due to improper use or excessive battery temperature.
② Real-time monitoring of battery terminal voltage and current. If the battery capacity is less than the warning threshold, it will remind charging or automatically switch to the backup battery.
③ The remaining capacity of the battery can be calculated by analyzing the parameters and displayed in real time through the digital tube.
④ The system can automatically correct the internal parameters of the battery to adapt to some changes in the battery caused by use, and can also achieve better charging effect by controlling the charge and discharge circuit.

The system structure is shown in Figure 1.

2 System Hardware Design

2.1 System Control Core

This system is designed with the F2MC-8FX series single-chip microcomputer MB95F136 as the control core of the system. In the system, MB95F136 not only monitors the battery current, voltage, temperature and other parameters and the system operation status in real time, but also processes the collected data and outputs 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 key control and system status output display. Fujitsu's MB95F136 uses 0.35μm low leakage process technology. The mask product can operate in a low power consumption working 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. While it has fast processing and low power consumption characteristics, it is equipped with a rich timer; it integrates an 8-channel 8/10-bit optional A/D converter, which can be easily used in the system to collect voltage and current. Dual-operation flash memory is also one of the features of the 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 parts to reduce the surface area of ​​the circuit board. In addition, LVD (low voltage detection) and CSV (clock monitor) functions can improve system stability and reliability.

2.2 Power supply circuit design

In this system, in order to enhance the flexibility of system application, the system power supply 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 more than 2 batteries with a nominal voltage of 12 V, otherwise a separate 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 backup batteries. Once the power supply from the battery fails, the system can still operate normally. The schematic diagram of the system power supply circuit is shown in Figure 2.

2.3 Current and voltage acquisition circuit

The objects to be monitored are mainly the voltage and current of the battery pack. The voltage is obtained by the voltage-dividing precision resistor and sent to the A/D port of the single-chip microcomputer after corresponding amplification. The charging and discharging current of the battery is sampled and amplified by the 0.01Ω sampling resistor and then sent to the A/D port PO1 of the single-chip microcomputer. The key to detecting the battery lies in the accuracy of the voltage sampling, so whether the sampling circuit is designed properly is crucial to the entire system. Since the A/D converter embedded in MB95F136 can work under the reference voltage of 5 V, 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 sampling 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 adjustment, battery standard voltage, etc.) must be set, and the system will write these parameters into the EEPROM. In order to reduce the number of times the EEPROM is read/written, the data is read from the EEPROM when the system is turned on and saved in the RAM of the microcontroller. The main function of the EEPROM is to save and quantitatively back up parameter data. It is mainly used to store some system operating parameters, such as reference data for calculating battery power, correction coefficients, etc.

This system uses the 2 Kb EEPROMAT24C02. This chip is a serial EEPROM using the I2C bus protocol. It can store important data in the system for a long time and reliably 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 Temperature acquisition module design

This design uses the DS18820 single bus digital intelligent temperature sensor produced by Dallas, USA, which directly converts the temperature physical quantity into a digital signal and transmits it to the controller via the bus for data processing. DS18B20 provides 9-12 bits of data and alarm temperature registers for the measured temperature. The temperature measurement range is -55~+125℃, and the measurement accuracy is ±0.5℃ in the range of -10~+85℃. This sensor can be used in various fields and various environments for automated measurement and control systems. It has the advantages of miniaturization, low power consumption, high performance, strong anti-interference ability, and easy matching with microprocessors. In addition, each DS18820 has a unique serial number, so multiple DS18820 can exist on the same single-wire bus, which brings great convenience to the application.

The temperature measurement circuit design is shown in Figure 4. The system uses a heat-conductive adhesive to adhere the device to the battery surface, and the difference between the core temperature and the surface temperature is approximately within 0.2°C. When the ambient air temperature is different from the battery temperature being measured, the back of the device and the leads should be isolated from the air. The ground pin is the main heat path to the core, and it must be ensured that the ground pin also has good thermal contact with the battery being measured.

2.6 Controllable charging and discharging module

This module is the hardware difficulty in the 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 the instructions or flags issued by the control circuit; and it 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) is selected for high-current charge and discharge management. IPM is an advanced hybrid integrated power device, consisting of high-speed, low-power IGBT, drive circuit and protection circuit; it has fault detection circuits such as overvoltage, overcurrent, short circuit and overheating, and has automatic protection function. The main circuit of battery charging and discharging is shown in Figure 5.

In Figure 5, Q1 and Q2 are integrated in an IPM. When Q2 is turned on, the battery pack is charged, and when Q1 is turned on, the battery pack is discharged through R1; when the battery pack supplies power to the load, Q1 and Q2 are both closed. In order to improve the working state of the power switch device, soft switching technology is used 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 electrode and generates a reverse voltage, which actually manifests as an increase in the internal resistance of the battery. Not only can the effective chemical substances in the battery not fully participate in the chemical reaction, reducing the utilization rate of the battery pack capacity, but also causing serious heating of the battery pack, thereby affecting the charging speed and quality, and then affecting the performance and life of the battery pack. An effective way to eliminate it is to use a negative pulse method: instantaneous discharge at both ends of the battery to remove the charge accumulated on the electrode, thereby changing the inherent exponential curve charge acceptance characteristics of the battery and improving the battery's power acceptance capacity. For this reason, a charging strategy of "charge-stop-discharge-charge-stop-discharge" cycle charging is adopted. Its pulse charging characteristics are shown in Figure 6, and the time parameters are determined by the parameters of the battery.

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 settings can be performed, and real-time display of status, temperature and other data can be achieved to achieve better human-computer interaction. This design adopts a solution to de-bounce the input in software, and continuously judges the key status until the key is released, and then executes the corresponding processing program. The data display adopts a 3-digit 7-segment digital tube dynamic display mode, and uses 74HC595 to latch the dynamic display data. This design cleverly shares the key input and dynamic display digital selection ports, thereby reducing the application of the microcontroller 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 in Figure 7. After the system is started, the system initialization program is executed immediately to read the parameters obtained from the last operation from the EEPROM. Then the data in the temperature sensor is read to obtain the current system temperature, and then the A/D sampling subroutine is called to obtain the voltage and current signal data with 10-bit accuracy. After processing, the final battery operation status can be obtained, and the respective processing procedures are carried out according to different statuses, and the status data is output to the digital tube display. When the system is running, it will automatically correct the parameters according to 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, making full use of its advantages of multiple peripheral interfaces, powerful functions, integrated high-precision A/D converter, easy 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. When the power is insufficient, it can automatically switch the power system to implement self-protection. The update basis of parameter data is the result of multiple experiments, comparison and calculation of measured parameters. Through experiments, the calculated value of remaining power is closer to the actual value than when the parameters are not updated. Practice has proved that this intelligent lead-acid battery management system has a high degree of intelligence and accurate measurement. It can timely detect and control improper use of batteries, provide self-protection, and can accurately judge the operating status of the system. It not only greatly improves the stability of the power supply system, but also helps to improve the service life and efficiency of the battery. ■