The communication function is an important part of the battery management system, which realizes the information exchange between the battery management system and the external equipment. The communication function mainly involves the internal communication between the main control module and the slave control module of the battery management system, the communication between the battery management system and the vehicle controller/motor controller, the communication between the battery management system and the charging equipment, and the typical BMS communication The connections are shown in Figure 1. In order to facilitate debugging, the general battery management system also has the function of communicating with the host computer; in order to monitor the status of the battery and strengthen its safety supervision, the battery management system may also communicate with the remote monitoring background through wireless communication technologies such as GPRS.
In the battery management system, the communication mode currently adopted is mainly the CAN bus communication mode. In 1986, Bosch first proposed the CAN protocol. CAN is the abbreviation of Controller Area Network and is an ISO international standardized serial communication protocol. It is a serial communication network that effectively supports distributed control or real-time control, and adopts the CSMA/CD protocol with priority to arbitrate the bus, so the CAN bus allows multiple sites to send at the same time. The CAN communication medium can be twisted pair, coaxial cable, or fiber optics. CAN can realize reliable transmission of long-distance real-time data in electromagnetic interference environment, with strong fault tolerance and anti-interference ability, high transmission security, and low hardware cost. The CAN bus is a multi-master bus with the following characteristics.
(1) One of the biggest features of the CAN protocol is that the traditional station address encoding method is abolished, and the method of encoding communication data is expanded, so that the number of nodes in the network is theoretically unlimited. In this way of encoding by data blocks, different nodes can also receive the same data at the same time.
The communication mode is flexible, which can realize point-to-point and broadcast data transmission.
(2) The CAN bus transmits data in units of messages. The short frame structure is used for data transmission, and the data length is up to 8 bytes. 8 bytes will not take up too long bus time, thus ensuring real-time communication. At the same time, the transmission time is short and the probability of being interfered is low. The communication rate of CAN can be as high as Mb/s level. The CAN protocol adopts the cyclic redundancy CRC check, and can provide the corresponding error handling function to ensure the reliability of data communication.
(3) Adopt non-destructive priority-based bus arbitration technology. It has the function of judging temporary faults and permanent faulty nodes and automatically leaving the faulty nodes. Nodes can be arbitrarily attached or removed without closing the bus, so that the communication of other nodes in the system is not affected, and the flexibility and expansibility of the system are enhanced.
(4) Adopting unified standards and specifications, each device has good interoperability and interchangeability. In 1991, the CAN2.0 technical specification was formulated and released, which includes two parts, A and B. Since then, the CAN international standard ISO11898 was officially promulgated in 1993. CAN technical specifications 2.0A and 2.0B and ISO11898 are the basic basis and basic specifications for designing automotive high-speed network systems. The SAE J1939 standard was released in 1994, and its physical layer and data link layer are the CAN bus communication protocol developed based on the CAN2.0B protocol.
The SAEJ1939 standard is a CAN bus communication protocol specially used for trucks, buses, construction machinery, agricultural machinery, etc. It describes a network application of heavy vehicle field bus, including CAN network physical layer definition, data link layer definition, application layer definition, network layer definition, fault diagnosis and network management, etc. In the SAEJ1939 standard, not only the transmission type, message structure and its segmentation, flow inspection, etc. are specified, but also the content of the message itself is precisely defined. Figure 2 is the layered structure model of SAEJ1939 corresponding to the OSI model. The CAN protocol only defines the MAC layer of the physical layer and the data link layer in the OSI model. As can be seen from Figure 2, SAE J1939 is based on CAN2.0B. In addition, it also defines the network layer and application. layer protocol.