- System framework and overview
The battery management system mainly implements the collection and reporting of voltage and temperature signals on a single battery module from the board, and performs balancing operations on the cells on the module when the balancing function is executed.
Multiple battery modules are used in the pack system design, and each battery module uses a slave board. The slave board is mainly used for the collection, reporting and equalization functions of battery cell voltage and temperature. The slave board hardware generally includes a dedicated battery acquisition chip, an isolation chip, a single-chip microcomputer, and a communication circuit. The block diagram of the slave board system is shown in Figure 1.
The MCU main control unit is placed on the high-voltage battery side, only the CAN module is placed on the low-voltage side of the slave board, and a power chip is placed on the high-voltage side and the low-voltage side, which can reduce the isolation of one power supply and reduce the EMC problem of the entire board. The functional requirements of the slave board system are as follows:
(1) Slave board channel: Each slave board has 4 voltage acquisition channels and is compatible with 5-channel mode, and 5 temperature acquisition channels. Only 4 channels can be selected as the acquisition interface.
(2) Acquisition accuracy: single voltage accuracy ± 5mV, measurement range 0~5V, temperature measurement range -40°C~85°C, required accuracy at -30°C~60°C ≤±1°C, 60°C~85°C required Accuracy≤±1.5℃.
(3) Acquisition time: the single voltage reporting period is 50ms, and the temperature reporting period is 50ms.
(4) Communication mode: The communication between the slave board and the main board adopts high-speed fault-tolerant CAN, with a rate of 500kbit/s.
(5) Acquisition method: special acquisition chips are required to simplify system design and ensure scalability.
(6) Power supply mode: the power supply current of the high-voltage side does not exceed 50mA, and the power supply current of the low-voltage side does not exceed 30mA.
- Processor and chip and power supply
The slave board processor is used to monitor, process and report the battery voltage and temperature signals, and the functions can be realized by using the single chip microcomputer. The processor uses the freescale chip MC9S08DZ60. The processor must include at least ROM, RAM, and flash storage space, of which the EEPROM requirement is not less than 1K, the RAM requirement is not less than 2K, and the flash requirement is not less than 32K. Hardware watchdog function: The processor power-on completion time is required to be within 1s, and it can wake up and sleep through hard wires; the processor has the CAN interface function to communicate with the motherboard, and the SPI or I2C interface function to communicate with the internal chip.
2.2. Acquisition chip and isolation chip
The acquisition chip is used to manage the acquisition function of battery voltage and temperature. The function can be realized by using a special acquisition IC. The LTC6804HG-1 acquisition chip of Linear Technology is selected, which can provide the following resources: the single voltage acquisition channel is 12channel, 5 GPI0 ports (can be multiplexed into 5 temperature acquisition channels); multiple acquisition chips are allowed to be used in parallel, and a daisy-chain structure can be provided. The acquisition resolution of the voltage channel is 16bit, the total voltage acquisition accuracy (including analog front-end and back-end processing) meets -2.8~2.8mV, the acquisition time of the acquisition chip is 130us, the withstand voltage requirement of the acquisition chip is not less than 60V, and the voltage acquisition channel The measurement range is 0~5V, and the acquisition chip with hardware diagnosis function channel can perform hardware diagnosis on the undervoltage and overvoltage of the battery cell voltage, and report the fault status. The acquisition chip has the SPI interface function to communicate with the processor, and the chip temperature range is -40℃~125℃. The isolation chip is required to meet the electrical isolation of the battery side and the low-voltage side, and the electrical isolation meets the requirements of insulation and safety regulations. The electrical design defines the RMS voltage of 400V, and the minimum clearance and creepage distance of 4.00mm, which are mainly considered in the PCB layout. The isolation chip uses ADI’s I Coupler digital isolation chip ADuM12011.
2.3. Power supply
The slave board is powered by the low-voltage system and the high-voltage system, and the power supply system is designed as follows: the low-voltage power supply comes from the low-voltage battery of the whole vehicle (about the knowledge of the battery, I accidentally found an article before, and found that the author knows the knowledge of the battery Very thorough, if you are also interested, you can visit Tycorun Battery to read)
), the normal working voltage is 12V, the voltage range is 6~16V, and the working current does not exceed 50mA. Use the power management chip for power supply control, provide internal 5V or 3.3V, and supply power for CAN and isolation chips. The high-voltage power supply comes from the module of the high-voltage battery, and the voltage range is 8~25V. The high-voltage module directly provides power for the acquisition chip, and converts it into 5V through DC-DC or LDO to power the MCU and other chips on the high-voltage side. Power management can support power-on and power-off management of hard-wired Enable. Enable high level wakes up the power management chip and performs power-on initialization. It is required to complete initialization and start measurement within 120ms, and send a normal CAN signal; after Enable low level, the power management has a self-locking function, which supports the processor after the power-off management is completed. , go to sleep again.
- Interface definition and CAN communication
3.1. Interface Definition
The slave board is respectively connected to the battery terminal and the main board. The design requirements for the interface are as follows: the voltage input interface channel of the battery terminal is 4 channels, the temperature input interface channel is 4 channels, the connection terminals are designed separately, and the on-board connection terminals are used. The power supply terminal and the main board communication terminal have Redundant design to ensure the cascading of multiple acquisition sub-boards. The cascading method is shown in Figure 2.
The slave board has communication functions such as CAN. The communication follows the following requirements: the external port of the slave board needs to provide at least one high-speed CAN communication with the main board, the communication rate is 500kbit/s, and the CAN2.0 communication protocol. The acquisition chip and processor need to have communication functions such as SPI or I2C for internal communication; CAN needs to reserve a terminal resistance, and the CAN network ID can be configured independently. The voltage and temperature signals are collected from the board and reported to the main board through the CAN signal. At the same time, the equalization command of the main board is sent to the slave board through the CAN signal to realize the equalization function.