Design of NiMH battery management system based on CAN bus
1 Introduction
Accurate measurement of the remaining capacity of the battery has always been a very critical issue in the development of electric vehicles. An effective battery management system is conducive to the improvement of battery life. Therefore, the accurate estimation of the battery SOC has become a central problem of the electric vehicle battery energy management system. If the SOC of the battery can be correctly estimated, the electric energy provided by the battery can be reasonably used to extend the service life of the battery pack.
The scheme adopts a bus-type network, and uses field bus to complete the data exchange between each node. In the distributed scheme, the multi-energy controller is the main control ECU, which communicates with multiple lower-level ECUs through the field bus. During the work process, the communication sub-module of each controller runs in the background in the form of a timer or an interrupt to complete the sending and receiving of data, saving the main process resource expenditure. As shown in Figure 1.
The battery SOC value is sent by the battery controller to the multi-energy controller through the CAN bus, and the working mode of the vehicle is determined by the multi-energy controller by collecting information from each ECU through a certain logic algorithm. Once these parameters are determined, then we can decide whether to start the engine or shut down the engine, and also determine the state in which the motor should work. For example, when the SOC value of the battery is between 50% and 70%, at this time the multi-energy controller calculates that the vehicle's working mode is in the starting mode, then it means that the current system has sufficient electric energy and does not need to start the engine Can work in a driven manner.
2 System hardware composition
As shown in Figure 2, the battery controller can communicate with other control systems in the external car via the CAN bus network. A battery management ECU (electronic control unit) and 4 battery pack information detection ECUs; the single cells we use are combined into 24 battery packs. We configure a measurement unit for every 6 battery packs, that is, a total of battery packs ECU1 to ECU4. 4 battery pack ECUs and battery pack ECUs form a CAN bus network, and a CAN controller and battery pack ECU form a CAN within the battery management system Network, another CAN controller and other control systems in the car make up the vehicle's fiber optic CAN bus network.
Figure 2 Block diagram of the battery management ECU
As shown in Figure 3, the embedded microcontroller used in the battery pack ECU is the P87C591 single-chip microcomputer, and its internal hardware integrates the CAN controller and the A / D analog-to-digital conversion module. Each battery pack ECU manages 6 battery packs. The completed function is to measure the voltage and temperature information of the 6 battery packs and send the collected information to the battery management ECU via the CAN bus. The voltage of the 6-channel battery pack is connected to the 6-channel A / D input port of P87C591 after the voltage conditioning circuit. The signal wires of 6 temperature sensors are connected to the same IO port of P87C591.
Figure 3 Circuit diagram of the battery pack ECU
3 Circuit design of CAN interface
In this design, P87C591 is used as the microcontroller. Among them, the interface circuit design of P87C591 and CAN driver chip is shown in Figure 4. It is mainly composed of P87C591, photoelectric isolation circuit, CAN driver and other three parts. Photoelectric isolation circuit: In order to further suppress interference, the photoelectric isolation circuit is often used in the CAN bus interface. The photoelectric isolator is generally located between the CAN controller and the transceiver.
Figure 4 CAN communication module hardware design circuit diagram
The overall system program includes the initialization program and the main loop program, and its flow chart is shown in Figure 5:
Figure 5 Main program diagram
The system first powers on, then initializes the CAN and the timer, the system waits for an interrupt, if there is an interrupt, judges the interrupt type, if it is an interrupt of the SJA1000 controller, it reads the data of the SJA1000 controller, and releases the buffer, the operation is completed Interrupt return, if it is a timer 50ms cycle interrupt, AD conversion is performed on the voltage and current data, the SOC value is calculated, and the relevant data is sent by CAN, and the interrupt is returned after the operation.
4 Conclusion
The data communication technology based on CAN bus has high reliability, real-time and flexibility. The CAN bus has broad application prospects and development space in the application of the hybrid electric vehicle nickel-metal hydride battery management system.
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