**Abstract:**
With the rise of the Internet of Things (IoT), intelligent remote management of fast chargers has become a reality, and the design of monitoring terminals plays a crucial role in this process. This paper presents an overall design scheme for a fast charger monitoring terminal, combining a single-chip microcontroller STM32 with the real-time operating system μC/OS-II. The study focuses on the formulation of protocols for high-power charger CAN bus communication and GPRS data transmission, as well as an economic analysis of GPRS traffic costs. The results demonstrate that the monitoring terminal ensures network stability, enables real-time monitoring of the charger's operational status, and supports remote management.
**0. Introduction**
With strong government support for new energy technologies, electric vehicles have become a key focus in the development of the new energy vehicle industry. As electric vehicles become more widespread, charging stations and fast chargers are essential infrastructure. A large number of off-board intelligent fast chargers installed in residential areas and parking lots are expected to dominate the market due to their efficiency, safety, and smart capabilities. However, current fast charging systems often operate without human supervision, which demands higher reliability, automation, and remote maintenance features in the chargers.
This leads to the trend of distributed, modular, and intelligent fast charger development. High-performance, low-cost monitoring terminals are considered key technologies in this field. To optimize resource utilization and management across multiple chargers, it is inevitable that these terminals interact with the internet.
**1. Overview of the Monitoring Network**
As shown in the structure diagram of the charger monitoring network (Figure 1), the monitoring terminal acts as a critical gateway between the charger and the monitoring center. Its communication links include: the monitoring center to the terminal, and the terminal to the charger or other devices such as the Battery Management System (BMS) or the electric vehicle itself.
Through the monitoring terminal, a communication link is established between the monitoring center and the charger or electric vehicle. The terminal communicates with the charger, BMS, and vehicle via the CAN network, collecting relevant data and storing it. It then sends the collected information back to the charger, enabling intelligent battery charging based on the received data. Additionally, the terminal connects to the monitoring center through a GPRS connection, transmitting charger, battery, and vehicle data. This allows the monitoring center to perform remote control and real-time monitoring, record operational statuses, and track any failures. Users can also check the availability of chargers through the monitoring center, maximizing resource usage.
**2. Function Modules of the Monitoring Terminal**
**2.1 Overall Design of the Monitoring Terminal**
The monitoring terminal serves as a bridge between the monitoring center and the charger. Its overall structure (Figure 2) consists of six main modules: the STM32ZGT6 core module (based on the Cortex-M3 architecture), a data acquisition module (CAN network), a user charging interaction module, a data storage module, a real-time clock module, and a GPRS communication module. The STM32ZGT6 microprocessor provides rich hardware resources, including a CAN 2.0B controller and up to four serial ports, meeting the requirements for both CAN and GPRS interfaces.
The terminal works by reading user information through the billing module and sending corresponding charging commands to the charging module via the CAN network. At the same time, it reads and stores key data frames from the CAN network, such as the charger’s operational status. Periodically, the terminal sends user information and operational parameters to the monitoring center via GPRS. It can also print user balances or charging credentials upon request.
**2.2 CAN Bus Module**
To ensure reliable CAN bus communication, the system defines a common application-layer protocol. The message ID in the CAN 2.0B protocol is allocated and defined accordingly. The priority determination mechanism in the protocol ensures that messages with smaller IDs have higher priority. This non-destructive competition algorithm enhances efficiency. The protocol also includes type codes, source addresses, and segment codes to manage different types of messages and data segments.
For example, the BMS node may send multiple data frames to transmit information about battery voltage, current, SoC, temperature, and status. Each piece of data occupies 2 bytes, requiring multiple frames for full transmission.
**2.3 Data Transmission Module**
The terminal connects to the internet using a serial port-connected GPRS module (ZWG-23A). After connecting to the server via the GPRS network, the terminal sends data periodically according to the communication protocol. The protocol includes a message start identifier, version number, command word, message length, data content, and checksum. Different command words determine the data composition and byte count. The communication between the terminal and the monitoring center involves four stages: registration, response, readiness, and timing transmission.
**3. Software Design**
**3.1 Multi-task Management with μC/OS-II**
μC/OS-II is used as the real-time operating system for the monitoring terminal. It offers real-time performance, modularity, and reliability. The system supports up to 56 user tasks, along with services like semaphores, message queues, and memory management. Thirteen tasks were designed, including display, keyboard input, printing, data storage, IC card reading/writing, GPRS transmission, CAN data reception, and alarm handling. Task priorities are set based on urgency, importance, and real-time requirements, ensuring efficient task scheduling.
**3.2 Configuration of ZWG-23A Module**
The ZWG-23A GPRS module is connected via a serial port to enable mobile internet access. Since no configuration program was available for μC/OS-II, a custom configuration program was developed. The program flow is illustrated in Figure 4. Assuming daily registration and a 30-second heartbeat interval, the GPRS traffic generated per day is approximately 5 MB, totaling around 1.7 GB per year. A 2 GB monthly package is sufficient for one terminal’s annual usage.
**4. Conclusion**
This paper explores the structure of the electric vehicle fast charger monitoring network and analyzes the communication protocols for CAN and GPRS. The CAN protocol demonstrates versatility, while GPRS offers low traffic costs and potential for expansion into other automation fields. The monitoring terminal proves effective in ensuring stable network operation, real-time monitoring, and remote management of fast chargers.
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