Abstract: To protect fuel cell stacks, a single-cell voltage detection system for fuel cell stacks was designed. This system employs a voltage measurement method based on opto-isolated relays, solving the problems of high accuracy requirements, numerous voltage channels, and high potential accumulation in single-cell voltage measurement of fuel cell stacks. Experimental results show that the time for a single voltage detection of 124 channels is 110ms, and the measurement error is less than 0.01V. This system can effectively protect the fuel cell stack. Keywords: Fuel cell stack, single-cell voltage detection system, opto-isolation method, proton exchange membrane fuel cell engine. The proton exchange membrane fuel cell engine is one of the important development directions of new automotive capabilities, consisting of five parts: fuel cell stack, hydrogen intake and exhaust system, air intake and exhaust system, thermal management system, and control system. Among them, the fuel cell stack is the core of the fuel cell engine. A fuel cell stack typically consists of tens to hundreds of single cells. Due to the influence of operating parameters, the voltage of a single cell in the fuel cell stack varies greatly. The normal operating voltage of a single cell in the stack is typically 0.7V, and about 0.9V under no-load conditions. Abnormal voltage indicates a system malfunction, requiring immediate action; otherwise, the fuel cell stack will be damaged. To protect the fuel cell stack, a single-cell voltage detection system needs to be developed to measure the voltage of each cell in real time and to collaborate with the fuel cell engine's main controller to handle voltage anomalies. 1. Fuel Cell Stack Single-Cell Voltage Detection System Design The on-board fuel cell stack single-cell voltage detection system consists of two parts: a voltage detection card and a communication network. In non-mobile applications, such as laboratory research and stationary power station applications, computer monitoring functionality should also be added. This design is applied to laboratory research, and the system structure is shown in Figure 1. A fuel cell stack with hundreds of individual cells may use multiple voltage detection cards. Considering the communication between the voltage detection card and the main controller, and also taking into account future on-board applications, the communication of the fuel cell stack single-cell voltage detection system adopts a CAN network. The computer monitoring part uses a third-party CAN card to simulate CAN communication, receiving the single-cell voltage data sent by the voltage detection card, and simultaneously displaying and saving this data. The voltage detection card is the core of the system, realizing continuous acquisition of the voltage of individual fuel cell stack cells and judging the voltage value. If the voltage value is abnormal, the corresponding abnormality indicator code and cell number are sent to the fuel cell stack engine main controller via the CAN network. The main controller then takes corresponding actions based on the received information. The difficulty in measuring the voltage of individual fuel cell stack cells lies in the high accuracy requirements (around ±10mV), the large number of voltage channels, and the high potential accumulation. Considering that the leakage current of the opto-isolation relay is extremely small, the measurement loss of the 1V level voltage is negligible and can meet the accuracy requirements; at the same time, by controlling the input control terminal of the opto-isolation relay, the voltage of any cell in the fuel cell stack can be effectively selected, solving the problems of the large number of voltage channels and potential accumulation; in addition, the opto-isolation relay has the advantages of being contactless, highly stable, and having a long lifespan. Therefore, the opto-isolation relay method is used in the design. In addition, the designed system must also meet the requirements of inspection speed and vibration resistance. 2 Design of fuel cell stack cell voltage detection card 2.1 Overall design of voltage detection card The fuel cell stack cell voltage detection card is mainly divided into two parts: a signal acquisition module and a digital core module. The signal acquisition module acquires the voltage of a specific cell from multiple individual cells in the fuel cell stack and sends it to the digital core module. The digital core module primarily handles analog-to-digital conversion, control signal acquisition, and communication with the main controller and microcomputer. The specific structure of the voltage detection card is shown in Figure 2. The input control terminals of adjacent opto-isolated relays are connected to the output terminals of a pair of decoders; the signal input terminals of the opto-isolated relays are sequentially connected to the voltage signal terminals at both ends of the individual cells in the fuel cell stack; the signal output terminals of odd-numbered opto-isolated relays are grouped together and designated as the COMA terminal; similarly, the signal output terminals of even-numbered opto-isolated relays are grouped together and designated as the COMB terminal. The COMA and COMB terminals are connected to the analog signal input terminals of the A/D conversion chip. The corresponding cell voltage value can be measured simply by ensuring that only one pair of adjacent opto-isolated relays are closed at any given time. This can be achieved by controlling the decoder using a microcontroller. For example, when measuring the voltage of the first chip, the microcontroller first disables the other decoder pairs, turns on the first decoder pair, and then controls the decoder pair to close the first and second opto-isolated relays, thus leading the voltage signal of the first chip to the COMA and COMB terminals. It is worth noting that when measuring the voltage of odd-numbered and even-numbered chips using this method, the voltage at the COMA terminal may be positive or negative relative to the COMB terminal. This problem can be solved by adding an external absolute value circuit or an external bipolar A/D converter chip. This design chose an external A/D converter. 2.2 Speed ​​and Accuracy Analysis of the Voltage Detection Card Based on the real-time requirements of the signal, the voltage monitoring system must be able to complete signal acquisition and data transmission within 1 second. From the acquisition algorithm, it can be seen that if the sum of the chip selection, A/D conversion, and microcontroller operation time is on the order of μs, then the time to complete the acquisition of 120 voltage signals is on the order of ms. Simultaneously, the CAN bus communication rate can reach 1Mbps. It can transmit 8000 frames per second, while transmitting 120 voltage signals only requires 40 frames (assuming one frame can transmit three signal data channels), indicating that the communication rate is sufficient. Factors affecting measurement accuracy mainly include the accuracy of the A/D converter and noise error. When the A/D conversion input is bipolar and the full-scale voltage is 12V, to achieve an accuracy of 0.01V, the required conversion accuracy is 0.01/12/2 = 1/2400 (divided by 2 because of the bipolar input), therefore an A/D conversion chip with at least 11 bits of accuracy is required. Noise error can be reduced by averaging multiple measurements. 2.3 Main Hardware Selection The selection of the microcontroller mainly considers its computing power and its integrated functional modules. To ensure that the microcontroller's operating speed matches the A/D acquisition rate and communication rate, the time for the microcontroller to execute a single instruction should be around 1μs. The integrated functional modules should mainly include a CAN communication module, an SPI communication module, and a 16-channel digital output module (taking the measurement of 124 voltage channels as an example). Considering future development, the C8051F040 microcontroller was ultimately selected. This microcontroller has the following features: · CAN bus 2.0B · Pipeline instruction structure · 25MHz clock frequency, speed up to 25MIPS · 4352 bytes of internal data RAM · 64KB of FLASH memory · 64 I/O lines, all 5V withstand voltage · Can simultaneously use hardware SMBus™ (I2C compatible), SPI™, and two UART serial ports. The selection of the A/D converter chip mainly considers accuracy and its interface with the microcontroller. A bipolar A/D converter chip with at least 12-bit accuracy and SPI communication should be selected. The selected A/D converter chip is the Max1132. The Max1132 has the following features: · Bipolar 200ksps and unipolar 100ksps sampling rate · 16-bit conversion accuracy · Input voltage range -12V to 12V · SPI bus interface. Its accuracy and conversion rate fully meet the requirements of voltage detection. The decoder selected is the 4-line/16-line 74h154 decoder. Since decoders are widely used, further details are omitted. The selection of the opto-isolation relay mainly considers leakage current, withstand voltage, and size. The AQW210S opto-isolation relay was ultimately chosen. The AQW210S has the following characteristics: • Dual-unit opto-isolation relay • Resistance of only tens of ohms when on • Withstand voltage of 350V, leakage current not exceeding 100pA • Ultra-small SOP package . Experimental Analysis This system is currently being used on a small proton exchange membrane fuel cell stack at the State Key Laboratory of Automotive Safety and Energy Conservation, Tsinghua University, and the speed and accuracy of the system have been tested. The system can scan the voltage of 124 single cells 9 times per second, with a measurement accuracy within 0.01V, which meets the needs of detecting and protecting the fuel cell stack. Experimental research found that the opening and closing time of the opto-isolation relay is the main factor affecting the number of scans, with each scan taking approximately 0.5ms. Measuring the voltage on 120 opto-isolation relays takes approximately 60ms. It accounts for half of the total inspection time. The system enables the measurement of the output voltage of a single cell at any given time, solving the problems of high accuracy requirements, multiple voltage channels, and high potential accumulation; and the experiment verified that the system's acquisition accuracy and speed can meet the actual requirements.