Design of vehicle-mounted supercapacitor test system based on single chip microcomputer

With the advancement of science and technology, electric vehicle technology has developed rapidly. Compared with internal combustion engine vehicles, electric vehicles have the advantages of zero emissions, high performance efficiency, low noise, low heat radiation, easy operation and maintenance. They will be the direction of future automobile development and a hot topic in current research.

There are three types of power batteries for electric vehicles: fuel cells, batteries and supercapacitors. Fuel cells, batteries and supercapacitors are complementary in energy density and power density [1]. It is difficult to use batteries, wound batteries or supercapacitors as a power source for electric vehicles. Hybrid batteries are a relatively ideal solution. The hybrid battery drive system uses a hybrid battery drive system, especially the fast charging and discharging of supercapacitors to achieve vehicle braking energy recovery, and the super-large energy density of fuel cells supports the long-term driving of vehicles, making the hybrid drive system composed of fuel cells/supercapacitors the best solution for electric vehicle driving [2].

For vehicle power supply, in order to achieve higher power and energy, supercapacitors often use multiple monomers in series. With the increase of capacitor cascade, the overall voltage of the battery also increases. For vehicle batteries, the working voltage of supercapacitors often reaches hundreds of volts, and the fluctuation caused by such high peak voltage will bring strong electromagnetic interference, which brings great difficulties to the detection of capacitor components. At the same time, since series supercapacitors often use large current charging and discharging (usually between 50A-150A), the voltage and current change very quickly. For example, when the supercapacitor used in medium-sized buses is discharged with a current of 150A, the terminal voltage will decrease from 300V to 70V within 1 minute, and when the 200V constant voltage is charged, the current will also increase from 50A to about 150A within a few minutes. The noise impact caused by such rapid charging and discharging speed and amplitude is also very huge.

In view of the special working conditions of supercapacitors, this paper gives a supercapacitor battery detection system. By collecting its voltage and current parameters through charge and discharge cycle tests on supercapacitor components and comparing them with standard parameters, it is verified that this detection system can quickly achieve high detection accuracy under strong voltage and current changes.

1 Principle of the detection system and implementation of each module

1.1 Detection object The

supercapacitor used for the test adopts two sets of series-connected asymmetric electrode double-layer supercapacitor components provided by Shanghai Aowei Technology Development Co., Ltd.

1.2 System Principle Introduction

The supercapacitor management system can realize real-time acquisition of the working current and voltage of the supercapacitor. The overall structural block diagram of the supercapacitor management system is shown in Figure 1. The system consists of three main modules: on-site voltage, current, acquisition and conditioning module (i.e., acquisition module), signal isolation and MCU signal processing module (i.e., central processing module), and power management module. In the acquisition module, Hall voltage and Hall current sensors are used to collect the voltage and current of the supercapacitor on-site respectively. The collected signals are amplified by the instrument and then converted into 4mA-20mA current signals and sent to the central processing module. In the central processing module, the 4mA-20mA current signal sent by the acquisition module is isolated, amplified, and AD converted after current and voltage conversion and sent to the MCU. The MCU processes the data and transmits it to the host computer through the CAN interface. When abnormal data is detected, the MCU outputs a fault signal so that the staff can take timely measures. The power management module provides stable isolated voltage for each functional module and adds an RS232 communication serial port for MCU program burning.

1.3 Implementation of each main module

This test system uses four circuit boards to implement three major functional modules - acquisition module, central processing module and power management module. That is, voltage acquisition and preliminary conditioning board, central processing board and power board. The following focuses on the implementation of voltage, current acquisition module and central processing module.

1.3.1 Implementation of acquisition module

The acquisition module includes two parts: bus current acquisition and bus voltage acquisition. Figure 2 is the current acquisition principle diagram. The Hall current sensor is used to isolate the system under test. It has higher accuracy, better safety performance and stronger anti-interference ability than the traditional current voltage divider circuit based on resistor sampling. This paper uses the Hall closed-loop current sensor CSNK591 based on the magnetic compensation principle of Honywell Company, with a measurement range of ±1200A, a linear accuracy of 0.1%, an overall accuracy of 0.5%, and a response speed of less than 1μs, which fully meets the requirements of the system. The collected signal is converted into a voltage signal by a precision resistor, and then amplified into a ±5V bipolar voltage signal by an instrument amplifier. The system uses the AD620BR instrument amplifier chip, which has a large common-mode rejection ratio when the gain is low (when G=10, the minimum common-mode rejection ratio is 100dB), and can strongly suppress the common-mode interference caused by factors such as temperature and electromagnetic noise. The amplified signal is raised to a 0-10V unipolar signal through the OP27GS chip, and sent to the transmitter XTR110KU through an emitter follower, and converted into a 4mA-20mA current signal to the central processing module. The reason for converting the collected signal into a 4mA-20mA current signal is to take into account the unification with the industrial interface standard, and the current sensor has strong anti-interference ability. The

bus voltage collection also uses the closed-loop Hall voltage sensor VSM025A based on the magnetic compensation principle, and the implementation principle is the same as the current collection. [page]

1.3.2 Implementation of Central Processing Module

The central processing module is the core part of the test system, including MCU and AD unit, analog signal secondary conditioning unit, fault output unit and CAN interface unit, as shown in Figure 3.

The 4mA-20mA current signal input by the acquisition module first passes through the analog signal secondary conditioning unit for signal transmission, isolation, filtering and amplification. There are many ways to isolate analog signals. Commonly used methods are isolation amplifiers, linear optocouplers and voltage-frequency conversion. Isolation amplifiers and linear optocouplers have high isolation voltage, strong anti-interference ability and high linearity, but the linear optocoupler isolation circuit is complex, and there are many parameters that need to be adjusted. In addition, when the output voltage is relatively small, the linearity is poor. Therefore, this paper uses BB's high-precision ISO124U isolation operational amplifier to complete the isolation of the input analog signal. The isolated signal is filtered by the 5th-order Butterworth low-pass filter MAX280 circuit to filter high-frequency interference, and then sent out through an emitter follower. The

acquisition signal after secondary conditioning is sent to the MCU through the 12-bit high-speed AD7891. The MCU processes the data and transmits the data to the host computer through the CAN interface. The single-chip microcomputer uses the STC series 8-bit high-speed single-chip microcomputer STC89C58RD+. The single-chip microcomputer has strong anti-interference, 4kV fast pulse interference (EFT) and high anti-static (ESD), and can pass 6000V static electricity, which well meets the working environment of supercapacitors with high voltage and high current. The single-chip microcomputer can realize 6 clock modes. In the case of 24M crystal oscillator in this system, the single-chip microcomputer operating frequency can reach 4MIPS, which is equivalent to 4 times the running speed of ordinary 51 series single-chip microcomputers. In addition

, the test system is equipped with 3-channel fault diagnosis output, which can display undervoltage, overvoltage, overcurrent and other states. The test system and the host computer adopt CAN communication mode with strong anti-interference ability and good stability to ensure the reliability of the data sent to the host computer by the test system.

The actual system has analog ±15V, digital ±5V, and analog ±12V power supply requirements. The power management module isolates analog and digital circuits while providing the required voltages for each part of the system, thereby avoiding the mutual influence of the two types of voltages. TVS protection is added to the power input of each part to prevent surge voltage from damaging the system. At the same time, corresponding filter circuits are set at many power inputs, such as adding a π-shaped filter circuit at the AD power input to better eliminate the interference of power supply signals on the supplied circuits.

In addition, shielded wires are used for external connections, which can strongly shield electromagnetic interference in line transmission. All current boards are packaged in profile aluminum boxes and connected to the outside world using standard aviation connectors, so that the external magnetic field is isolated while protecting the circuit board.

2 Comparison and analysis of actual test results of the test system

2.1 Test content

The experiment selected 70A and 150A modes to test the charge and discharge of two sets of supercapacitor components in series. First, the capacitor is charged with constant current. When the bus voltage reaches 300V, it is switched to constant voltage charging. When the bus current drops to 10A, 70A constant current discharge is performed. This cycle test is repeated for 5 cycles.

2.2 Experimental results and analysis

Figures 4, 5 and 6 show the comparison of test curves under two conditions. Figure 4 shows the current change curve under two standard test conditions of 70A and 150A, and Figures 5 and 6 show the voltage curve characteristics under two conditions. It can be seen that the two are well matched, and the voltage test accuracy is higher than the current test accuracy. This is because on the one hand, the voltage of the charging and discharging system itself has higher control accuracy than the current. On the other hand, the current sensor is placed in the capacitor box and only relies on single capacitors. The noise interference generated when the capacitor is charged and discharged is relatively serious. At the same time, the aperture of the Hall current sensor is large, and there is still a certain gap after passing through the current bus, which affects the test accuracy to a certain extent. By comparing the current curves of each group, it can be seen that as the current increases, the relative error of the test result decreases, but the absolute error remains consistent and does not exceed 3A. [page]

This paper presents a vehicle-mounted supercapacitor test system, which uses Hall closed-loop current and voltage sensors based on the magnetic compensation principle to collect bus signals, uses the STC51 high-speed microcontroller that is resistant to high-voltage pulse interference for signal processing, and uses instrument amplification, current transmission, analog signal isolation, and 5th-order low-pass filtering to minimize the noise in the signal transmission process. Through the charge and discharge test of the supercapacitor component, it is shown that this system has the advantages of strong anti-interference ability and high detection accuracy, and can well meet the test requirements of vehicle-mounted supercapacitors in high-voltage and high-current environments. (end)