The electric vehicle (EV) is driven by the motor, and the power of the motor comes from the rechargeable battery, and the requirements of the electric vehicle for the working characteristics of the battery far exceed the traditional battery system. Therefore, the electric vehicle battery system has high voltage and high current, so the requirements for the electric vehicle charger are relatively high.
The electric vehicle charger needs to be able to charge the battery in minutes instead of hours. Based on an electric vehicle battery with a battery capacity of 30kwh, if it is fully charged within 15 minutes, the charging power will reach 120kw. Assuming that the charging voltage of the electric vehicle is 200 ~ 400V, the corresponding charging current will reach 300A. Such a large charging current is difficult to achieve if only a single power module is used. Facing the increasing capacity of charger, parallel current sharing is a good solution. Because software current sharing has the advantages of low cost, strong capacity expansion ability, convenient capacity expansion, easy scheme change and upgrade, the method of software current sharing is adopted in the implementation process, but the key problem to be solved in the implementation process is the communication problem between modules.
Can bus has been used more and more because of its high communication reliability, low cost, simplicity and practicality. Therefore, the software current sharing scheme based on CAN bus is adopted for the communication between internal modules of charger; The electric vehicle charger needs to communicate with the battery management system (BMS). At the same time, because can bus also has the characteristics of high network security and has gradually developed into the mainstream bus of automotive electronic system as an international standard, can bus will be used as the communication mode between charger and battery management system; Moreover, the communication distance of CAN bus is far (10km) and the reliability is high, so the communication between monitoring center and charger also adopts can communication.
In this paper, the research on CAN bus will focus on how to apply can bus to electric vehicle charger, and complete the communication process between charger and battery management system, internal power module and monitoring center.
2. Communication topology of electric vehicle charger
In the process of working, the electric vehicle charger needs to communicate with the on-board battery management system (BMS), the centralized monitoring center of the charging station and the internal power module of the charger.
As shown in Figure 1, the communication system of the charger includes three can communication networks:
Communication network (Can1) between charger main controller and battery management system (BMS): realize data exchange between charger and on-board battery management system, and provide parameter information for power battery charging.
The communication network (can2) between the main controller of the charger and the charging monitoring system: realize the real-time data acquisition, monitoring and control functions of the monitoring center and the charger, master the working state of the charger in real time through the monitoring center, and indirectly obtain the information of the battery through the charger.
Network communication network (can3) between charger main controller and power module: realize data exchange between charger main controller and independent power module, and realize software current sharing scheme based on CAN bus, in which n power modules are used as working modules and N power modules are used as standby power modules.
The application of can network in electric vehicle charger is mainly to formulate an appropriate work flow chart based on CAN bus communication according to the working principle of charger and the working characteristics of CAN bus.
Fig. 2 work flow chart between charger and battery management system
3. Can communication between charger and BMS
The communication flow chart between the battery and the battery management system is shown in Figure 2. In Figure 2, the work flow of the charger and the work flow of the battery management system are linked together, and the data is transmitted through the CAN bus. The data transmission between charger and battery through CAN bus mainly includes the following parts:
(1) Before charging the battery, BMS first establishes contact with the charger, and then sends the type, capacity, maximum allowable charging voltage and current of the battery to the charger through the CAN bus.
(2) During the charging process, BMS sends the battery charging parameters voltage, current, SOC and other information to the charger regularly (500ms), so as to provide reference for the charger to change the charging strategy and adjust the charging parameters.
(3) After charging, BMS sends the information of charging completion to the charger through CAN bus, and cuts off the communication with the charger to complete charging.
The communication system between charger and charging monitoring system is an independent can network, including monitoring computer and several chargers. The communication network between charger and charging monitoring system shall be able to realize the following functions at the application layer:
(1) Data transmitted from monitoring computer to charger:
Charger parameter setting
(2) Data transmitted from charger to monitoring computer:
Voltage, current and temperature data transmission
Cumulative input and output power
Charger fault code
(3) The monitoring computer reads the data of the battery through the charger:
Identification, type and parameters of storage battery
Battery voltage data
Battery temperature data
Battery SOC data
Real time status of battery
The communication between the charger and the power module is mainly to realize the software parallel current sharing function of the charger. In the process of working, what needs to be transmitted is the parameter values of voltage and current in the charging process, as well as the working state of the charger. Its workflow includes the following parts:
(1) Before charging, initialize the power module, check the working state, determine whether it can work normally, set the flag bit of allow or prohibit charging, and send the information to the main controller;
(2) After receiving the battery information and determining the charging strategy, the charger main controller will select the appropriate charging module to participate in the charging, and send the charging parameters to the power module;
(3) After the main controller of the charger confirms that the required power module can work, issue the command to start charging and collect the charging parameters;
(4) The main controller changes the charging strategy in real time according to the collected data, adjusts the charging parameters, and monitors the working state of the module in real time;
(5) When the power module sends alarm information during charging, start the standby module immediately and cut off the charging state of the problem module;
(6) After synthesizing the charging state information, the main controller of the charger makes a judgment to stop charging, ends the charging process and restores the standby state.
Figure 4 is the working flow chart of parallel current sharing inside the charger, which specifies the detailed working flow of the charger main controller and power module, and includes the series function of CAN bus.
Fig. 3 charging flow chart of charger and monitoring center
Fig. 4 flow chart of internal parallel current sharing of charger
As a reliable network bus, can bus has been widely used in many industrial fields. Because can bus has many advantages, and with the continuous application and promotion of CAN bus technology, can bus will be more and more widely used in automobile charger. Combined with the charger working standard of the State Grid and according to the actual working situation of the charger, this paper applies the CAN bus to the electric vehicle charger, applies the CAN bus to the specific work flow of the charger, and makes corresponding modifications to the work flow of CAN communication according to the actual test process, which can meet the requirements of high reliability of the charger.