Application of single chip microcomputer in fuel cell monitoring system

1 Introduction
Fuel cell power generation is the fourth type of power generation technology after hydropower, thermal power and nuclear power generation. It is an efficient power generation device that converts the chemical energy of fuel into electrical energy directly through electrochemical reactions without combustion. Theoretically, as long as the fuel is continuously supplied, the fuel cell can generate electricity continuously. Due to the advantages of high power generation efficiency and less environmental pollution, fuel cells have always been considered as one of the future power generation technologies and have been applied to new environmental protection projects such as electric vehicles.
Due to the characteristics of fuel cells, if one of the fuel cells in a group is damaged and not discovered in time, it will have a relatively large impact on the entire battery group. Therefore, the system should monitor the single cell voltage of the fuel cell in real time. The monitoring system described in this article uses a microcomputer, an industrial control board, a system monitoring board and combines the LabView development platform to well complete the functions of fuel cell single cell voltage acquisition, analog/digital conversion, faulty battery display and alarm, etc., providing a guarantee for the normal operation of the fuel cell system.

2. Structure of fuel cell and function of monitoring system
The fuel cell group is composed of several 5mm wide single cells connected in series. Hydrogen circulates through the inside of the battery. The voltage of each single cell is approximately 0V when no hydrogen is passed through, and it quickly rises to about 0.9V after a certain flow of hydrogen passes through. When the voltage of a single cell differs from that of other single cells by more than 0.2V, it can be considered that the cell is damaged. At this time, the monitoring system retrieves the location of the cell, displays it on the light-emitting tube, and issues a warning.

3. Software and hardware design of monitoring system
(1) Software design
The software of the monitoring system is divided into two parts: the microcontroller part is programmed in assembly language. The microcomputer receiving part adopts LabView development platform. The functions of each part are described below:

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After collecting all the single-cell battery voltages and converting them into digital signals, the single-chip microcomputer transmits the data to the microcomputer through the serial communication port on the one hand, and processes the data on the other hand to search for damaged single-cell batteries. If there are damaged batteries, the location of the damaged battery can be displayed and an alarm can be sounded.
The flowchart of the single-chip microcomputer assembly language programming part is shown in Figure 1:
Programming points:
a. The single-chip microcomputer and the microcomputer must be synchronized during serial communication, otherwise the microcomputer will receive wrong data. This system adopts mode 3, and the initial value of the timer is set to E6H.
b. When the external interference of the system is relatively large, several groups of data can be collected during A/D conversion. After removing the extreme data, the remaining data can be calculated by arithmetic or geometric average.
c. After collecting each voltage value, the pointer should point to the next one so that the program can run repeatedly. The LabView development platform
for the microcomputer
is powerful. After the voltage value is collected in the microcomputer, it is first restored to the original value, and then the voltage value is stored once an hour in a queue arrangement, and the data for one month is retained to observe the performance changes of the fuel cell. When a single cell fails to work properly, the software controls the industrial computer to close the hydrogen supply valve and perform manual maintenance.
(2) Hardware design
The hardware design schematic diagram of the system is shown in Figure 2:

a. Since the working environment of the system has a large interference (AC and DC circuits are in the same control cabinet), if a flat 40-core bus cable is used, although the wiring is simple and convenient, due to the large number of high-frequency signals in the system, the capacitance effect in the cable cannot be ignored. Therefore, combined with the actual situation, the transmission line of the single cell voltage uses a multi-core shielded cable.
b. The single-chip microcomputer MPU uses 89C51, which has a 4K RAM and does not need to be expanded separately. It is equipped with a 6MHz crystal oscillator to ensure that the A/D converter ADC0809 can operate normally.
c. The multi-channel integrated analog switch array AD7506 has a multi-select transmission function. Two AD7506 chips can be used together to collect the voltage difference of a single cell at a certain moment. The voltage of a fuel cell cell is relatively small, and the system must not only monitor the voltage of each single cell, but also know the time it takes for it to rise from 0V to the normal working voltage. In order to make the data more accurate, the voltage value needs to be amplified. To achieve this link, this system uses an integrated 4-op amplifier LM324 to amplify the collected voltage difference by 3 times in the form of negative feedback.
d. Serial communication uses MAX232C, which has the advantages of long transmission distance and strong anti-interference, and the peripheral circuit is simple, only 5 tantalum capacitors are needed.
e. The monitoring board is connected to an external ±15V power supply, and the +5V power supply is obtained from 7805. Because the dual seven-segment code display requires a large current, the 7805 should be connected to an external heat sink.
f. Since the system has a lot of external interference, in order to prevent the MPU from entering a dead state due to interference, this system not only adds anti-interference capacitors to the power supply part of each chip, but also adds anti-interference parts to the program to make the system more stable.

4 Conclusion
This monitoring system, which uses a monitoring board and a microcomputer, has a quick response, strong anti-interference ability, and stable performance, and has achieved good experimental results. The staff can accurately grasp the status of the fuel cell in real time without leaving the operating room.

References
[1] Li Hua, ed., MCS-51 Series Single Chip and Application Interface Technology, Beijing University of Aeronautics and Astronautics Press, 1993.8
[2] Lin Weiming, ed., Fuel Cell System, Chemical Industry Press, 1996.6