Design of NiMH battery management system based on CAN bus

1 Introduction

The 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 improving the life of the battery. Therefore, the accurate estimation of the battery SOC has become the central issue of the battery energy management system of electric vehicles. If the battery SOC can be correctly estimated, the power provided by the battery can be reasonably utilized and the service life of the battery pack can be extended.

The solution adopts bus networking and uses fieldbus to complete data exchange between nodes. In the distributed solution, the multi-energy controller is the main control ECU, which communicates with multiple subordinate ECUs through the fieldbus. During the working process, the communication submodule of each controller runs in the background in the form of timer or interrupt to complete the data transmission and reception, saving the main process resource expenditure. As shown in Figure 1.

The battery SOC value is sent to the multi-energy controller via the CAN bus by the battery controller, and the vehicle's operating mode is determined by the multi-energy controller through a certain logic algorithm by collecting information from each ECU. Once these parameters are determined, we can decide whether to start or shut down the engine, and we can also decide which state the motor should work in. For example, when the battery's SOC value is between 50% and 70%, the multi-energy controller calculates that the vehicle's operating mode is in the starting mode, which means that the current system has sufficient electrical energy and does not need to start the engine, and the motor can work in a driving mode.

2 System Hardware Composition

As shown in Figure 2, the battery controller can communicate with other control systems in the external car through the CAN bus network. One battery management ECU (electronic control unit) and four 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, there are battery pack ECU1 to ECU4. The four battery pack ECUs and the battery pack ECU form a CAN bus network, one CAN controller and the battery pack ECU form a CAN network inside the battery management system, and another CAN controller and other control systems in the car form a fiber optic CAN bus network for the entire vehicle.

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, which has an internal hardware integrated with a CAN controller and an A/D analog-to-digital conversion module. Each battery pack ECU manages 6 battery packs, and its function is to measure the voltage and temperature information of the 6 battery packs and send the collected information to the battery management ECU through the CAN bus. The voltages of the 6 battery packs are connected to the 6 A/D input ports of the P87C591 after passing through the voltage conditioning circuit. The signal lines of the 6 temperature sensors are connected to the same IO port of the P87C591.

Figure 3 Circuit diagram of battery pack ECU

3 CAN interface circuit design

In this design, P87C591 is used as the microcontroller. The interface circuit design between P87C591 and CAN driver chip is shown in Figure 4. It mainly consists of three parts: P87C591, photoelectric isolation circuit, and CAN driver. Photoelectric isolation circuit: In order to further suppress interference, photoelectric isolation circuit is often used in CAN bus interface. Photoelectric isolator is generally located between CAN controller and transceiver.

Figure 4 CAN communication module hardware design circuit diagram

The system overall 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 is first powered on, then CAN and timer are initialized, and the system waits for an interrupt. If there is an interrupt, the interrupt type is determined. If it is an interrupt from the SJA1000 controller, the data from the SJA1000 controller is read and the buffer is released. The interrupt is returned after the operation is completed. If it is a 50ms period interrupt from the timer, AD conversion is performed on the voltage and current data, the SOC value is calculated, and the relevant data is sent by CAN. The interrupt is returned after the operation is completed.

4 Conclusion

The data communication technology based on CAN bus has high reliability, real-time and flexibility. CAN bus has broad application prospects and development space in the application of nickel-hydrogen battery management system of hybrid electric vehicles.