Category Research on thermal management system of power battery

Power battery-battery energy management system hardware slave board design

Power battery-battery energy management system hardware slave board design

  1. 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.

Figure 1 Block diagram of the slave board system
Figure 1 Block diagram of the slave board system

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.

  1. Processor and chip and power supply

2.1. Processor
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.

  1. 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.

Figure 2 Slave board cascade mode
Figure 2 Slave board cascade mode

3.2.CAN communication
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.

Read more: What are the characteristics of lithium-ion power batteries

What is the hardware motherboard design of power battery energy management system?

Power battery-battery energy management system hardware motherboard design architecture

As the core controller of the power battery system, the hardware main board of the battery energy management system mainly realizes the BMS strategy operation and execution functions, including information collection of total voltage / total current, real-time SOC estimation, fault diagnosis and storage, real-time control of the main relay, etc.

  1. System framework and overview

The hardware circuit block diagram of the designed motherboard system is shown in Figure 1, including power supply circuit, digital / analog input, high / low-end drive output, PWM input / output, can communication, etc.

System hardware circuit framework
System hardware circuit framework

The designed microprocessor adopts Infineon’s tricore32bit series tc1728, the main frequency is 133MHz, and the storage space is 1.5MB flash; Abundant peripheral IO resources can meet the design requirements.

Mainboard system framework
Mainboard system framework

1.1. working voltage

The working voltage range of the main board is 6 ~ 16V, and the static current of the main board shall not exceed 1mA.

1.2. Self programming and diagnosis ability

The motherboard can support self programming in the offline stage through UDS protocol. The mainboard is required to have hardware diagnosis functions, such as detecting the open circuit and open circuit status of the output IO port (including short circuit to ground and short circuit to power), and turning off the output function in time according to the fault status. The requirements for shutdown time are as follows: high voltage relay drives IO, acquisition board / high voltage board enable signal, and the response time after fault detection is 10ms; The rest output IO, and the response time after fault detection is 100ms.

1.3. Electrical interface and can communication interface

1.3.1 power interface

The power interface block diagram is shown in Figure 3, and the pin definition is shown in Table 4.

The designed maximum withstand voltage of the power supply is 65V and the maximum current is 5A; The designed maximum current resistance of the power supply ground is 25A, and the offset voltage to the ground shall not exceed ± 0.5V.

power interface block diagram
Power interface pin definition 1
Power interface pin definition 1
Power interface pin definition 2
Power interface pin definition 2
  1. Digital input and analog input

The high-level interlocking detection is designed to be effective at high level. It is required to pull down 1.2k Ω resistance to the ground, and the filtering time constant shall not exceed 1ms. The emergency diagnosis line is designed to be effective at high level. It is required to pull down 1.2k Ω resistance to the ground, and the filtering time constant shall not exceed 1ms. The fast charging wake-up line is designed to be effective at high level. It is required to pull down 3.3k Ω resistance to the ground, and the filtering time constant shall not exceed 1ms. The slow charging wake-up line is designed to be effective at high level. It is required to pull down 3.3k Ω resistance to the ground, and the filtering time constant shall not exceed 1ms.

Fast charge signal detection input port, the internal resistance is 4.64k Ω, pulled up to 5V, the connected effective signal is lower than 1V, and the non connected invalid signal is 5V; The temperature sensor adopts NTC, the typical value is 10K Ω, and the designed pull-up resistance is 3K Ω to 5V.

2.1. Digital output

Main board digital output pin pin, main positive relay control, main negative relay control and fast charging relay control. The design is equipped with small relay drive. The relay model is acb33401, the interface is active at low level, and the driving current does not exceed 300mA. Precharge relay control, slow charge relay control, design direct drive relay evr10-12s, the interface is low-level effective, and the drive current does not exceed 300mA. Accessory relay control, designed to directly drive relay acb33401, the interface is effective at low level, and the driving current does not exceed 300mA.

2.3. Communication interface, storage space and mechanical interface

Pin pin of the main board communication interface. The main board uses two-way can, in which hcan is used as the whole vehicle can to communicate with the whole vehicle controller; As an internal can, Ican communicates with acquisition slave board and high voltage board; Both channels of can adopt high-speed can, and the communication rate is 500kbit / s.

The flash storage space of the main board is 1.5MB, which is divided into application software area, calibration area, test area, production data retention area and boot loader area according to function. The external EEPROM storage space is 8KB, which is used to store vehicle data. The connector adopts Tyco series automotive connector, which is divided into a / B two parts, with a total of 121pins, as shown in Figure 6.

Main board contact structure
Main board contact structure

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What are the principle and design functional requirements of power battery energy management system?

Power battery-principle and functional design requirements of power battery energy management system

  1. Principle of power battery energy management system

The function of power battery energy management system in electric vehicle is to control the energy flow of high-voltage electric energy between high-voltage electric equipment such as energy storage device, motor, inverter and air conditioning compressor, as well as the energy transfer between electronic power converter, control system and auxiliary devices, so as to make high-voltage electric energy use efficiently and safely [86].

The power battery energy management system takes the chip processor as the center and forms a system together with various sensors and actuators [87]. The energy management system obtains the voltage, current and temperature status information of the power battery through the sensor. These information, together with the monitored relay status, hvil status, insulation status and the status of each high-voltage component, are used as the basis for real-time judgment and calculation, and the corresponding processing actions are completed through the actuator. For example, the energy management system monitors the voltage, electric quantity and temperature information of each battery unit and performs calculation and processing, and the equalization unit completes the electric quantity equalization and temperature equalization of the battery pack [88, 89].

The following figure is a typical battery management system architecture, which includes daughter board module, measurement module, relay module, safety module, communication module, etc. with the motherboard as the core. The seven sub boards in the figure are used to detect the voltage, current and temperature of the battery module; The measurement module realizes the voltage measurement of the main board to the power line and low-voltage line; The relay module includes the action control of the main positive and negative relays, precharge relays, fast charge and slow charge relays; The safety module realizes the insulation detection of the total positive and negative ends of the power line and the high-voltage interlock safety circuit; The mainboard interacts with the sub board and insulation detection through can data communication, and the mainboard interacts with external communication and fast charging charger through can data communication.

Architecture principle of battery management system
Architecture principle of battery management system
  1. Functional requirements of power battery energy management system design

From the perspective of vehicle high-voltage control strategy, this content focuses on the management of energy management and safety protection of power battery system, that is, power battery energy management system. The functional requirements for the design of power battery energy management system include:

(1) Power on / off control. Strictly control the power on and power off sequence and process to meet the power on and power off requirements of the whole vehicle.

(2) Relay control and disconnect the relay in case of emergency. The high-voltage relay is controlled according to the instructions of the vehicle controller (VCU), and the control mode is low side drive. VCU shall be able to directly disconnect the high-voltage relay during emergency power down.

(3) Precharge control function. According to the power on and power off sequence requirements, the precharge time is ≤ 300ms, and the precharge time is the time from the beginning of closing the precharge relay to the time when the bus voltage outside the load reaches 90% of the total voltage of the power battery.

(4) Communication function. Meet can2 0b protocol, baud rate is 500kbit / s, BMS shall have 120 Ω terminal resistance, and the communication protocol between DC charging pile and battery energy management system meets the requirements of national standard GB / t27930.

(5) Fault diagnosis function. The fault diagnosis contents of battery energy management system shall include but not limited to: over temperature (including over temperature, under temperature and over temperature difference), over voltage (over voltage of single body or total voltage, under voltage of single body or total voltage and over voltage difference of single body), low insulation resistance, hvil status, relay status, communication status, over-current, etc.

(6) High voltage safety protection function. The battery energy management system shall have the following protection functions: overvoltage protection, low voltage protection, discharge overcurrent protection, charging overcurrent protection, high temperature protection, emergency power failure, high voltage interlock and collision protection.

(7) Insulation status detection, relay status detection and hvil status detection.

(8) Various signal acquisition, including current, total voltage, monomer voltage, temperature, etc.

(9) Charging control function. Realize the control of fast charge and slow charge according to the charging requirements.

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What is a related battery model study?

What is a related battery model study?

As shown in Figure 1, a battery pack consists of some interfaces (such as electrodes) and several battery modules consisting of several battery cells. In a battery module, all battery cells are all connected in parallel, thereby reducing the possibility of battery failure caused by the failure of a single battery cell. The battery modules of the same group are usually connected in series to provide high voltage and energy to the battery pack. The BMS is responsible for protecting hundreds of battery cells from damage and keeping the batteries working properly while driving a pure electric vehicle.

What is a related battery model study?
Figure 1 battery and battery pack model

1. Battery efficient management and scheduling
Rate capacity and recovery effects are the most important physical performance parameters for efficient battery management. The larger the battery discharge current, the smaller the effective capacity of the battery, this phenomenon is called the rate capacity effect; the output voltage of the battery does not decrease with the battery discharge, but rises, this phenomenon is called the recovery effect. We can increase battery capacity by minimizing the discharge rate of each cell and hibernating the battery cells.

The state of charge (SOC) of the battery is used to characterize the remaining power of the battery, and the value ranges from 0 to 1. When SOC=0, it does not discharge externally, and when SOC=1, it means it is fully charged. Because large battery pack performance depends on the electrical state of the aging cells within the pack, balanced state of charge is the most critical factor affecting the performance of large battery systems. In order to obtain better battery performance, a series of battery scheduling and discharge rate reduction measures based on SOC balance are taken to control the battery cells to release energy at a suitable discharge rate. For example, when the motor requires high power, the battery management system connects all battery cells to discharge to increase battery power; on the contrary, when the motor requires low power, the battery management system cuts off some cells with low remaining power to achieve output voltage recovery and Balance of state of charge.

2. Battery thermal characteristics
In addition to discharge behavior and SOC balance, battery thermal characteristics are also important for battery efficiency, operation, and safety. First, cell efficiency is temporarily improved at “immediate high temperature” due to increased chemical reaction rates and ion mobility, but cumulative exposure to high temperatures accelerates irreversible side reactions leading to a decrease in the permanent battery life. Therefore, most BMSs will require limiting each cell to a well-defined temperature range to achieve the desired performance. Every electric vehicle must therefore be equipped with a thermal management system that includes cooling and heating to keep the temperature of each battery cell within a reasonable operating range. When the temperature of the battery pack deviates from the operating temperature range, the thermal management system is activated to ensure the thermal stability of the battery.

In a high temperature environment, the radiator takes away the heat of the battery through the coolant and exchanges heat with the outside air to cool the battery; in a low temperature environment, the battery needs to be heated. For example, the GM Chevrolet Volt uses 144 fins to actively cool or heat 288 battery cells, and its radiator uses a coolant flow valve to control the flow of cooled or heated coolant. The Ford Focus also features an active liquid cooling and heating system for thermal management of its lithium-ion battery pack.

The simple method currently employed in battery cells can operate normally and efficiently during the shelf life of the car, but such a passive, extensive thermal control cannot take full advantage of the thermal management system. By understanding battery thermal characteristics, thermal management systems can be used to improve battery performance without sacrificing battery life: heating the battery cells at high power to improve instantaneous performance of the battery, and heating the battery cells when low power is required Cool down to delay battery life decay.

3. Relevant problem statement
The DC power generated by the battery pack of the electric vehicle is converted in the inverter to drive the electric motor of the electric vehicle. During the operation of the electric vehicle, the power inverter needs a suitable input voltage Vapp to drive the motor. After the battery pack is fully charged, the accumulated time to provide the required power Prep(t) within the output voltage range is defined as the running time top. Therefore, the BMS should ensure that its battery pack provides the required power to drive the motor, while keeping the output voltage not lower than the input voltage threshold during long-term operation, which is longer than the battery’s lifespan. Otherwise, EVs require a larger battery pack or frequent replacements.

By controlling the temperature of the battery cells to achieve a long enough operating time in the life, select the type of cooling liquid for each cooling fin to achieve the purpose of controlling the temperature of the battery each time it is cooled or heated, that is, the type of cooling liquid Used as a control knob for the BMS. Determine the type of coolant for each instant Cfin(t) to maximize the battery runtime top.

4. Overview of battery physical dynamic changes
The dynamic changes of the battery under stress conditions can affect the performance and safety of the battery system. For example, uncontrolled high temperatures can cause batteries to explode; extremely low temperatures can degrade battery performance and even make it impossible to drive electric vehicles. Therefore, the influence and interrelationship of control nodes and external conditions on battery dynamics should be analyzed to improve the safety and performance of battery systems. To this end, the factors affecting battery performance are first determined by bridging different abstract models of physical dynamic changes, and based on this unified abstract model, the dynamic changes of batteries under the influence of different temperatures are discussed.

What is a battery thermal management system?

What is a battery thermal management system?

As the main energy storage form of electric vehicles, the performance of power battery directly restricts the power, economy and safety of electric vehicles. Compared with other types of batteries, lithium-ion power batteries have great advantages in terms of energy density, power density and service life, making them the mainstream of current vehicle power batteries. But its performance, life and safety are closely related to temperature. As many studies have pointed out, temperature is one of the most important factors in battery design and operation. If the temperature is too high, the side reactions of the battery will be accelerated and the performance of the battery will be attenuated, and even lead to safety accidents. Therefore, it is very necessary to study the thermal management system (BTMS) of the battery.

The battery thermal management system obtains the temperature of battery cells at different locations through temperature measuring elements. Accordingly, the control circuit of the thermal management system needs to make the action decisions of the cooling actuators such as fans and water/oil pumps. At present, the temperature sensors of common power battery packs are mostly attached to the inner surface of the battery box or the outer surface of the battery cell. For example, in the third-generation Prius battery pack in 2010, part of the temperature sensor is arranged in the flow channel inside the battery pack; the other part is directly attached to the middle of the upper surface of the cells in some typical positions, and these cells are located in the front of the battery pack. top, middle and rear. The battery thermal management system usually performs hierarchical management according to the temperature region where the battery is located. Volt plug-in hybrid battery thermal management is divided into active (active), passive (passive) and non-cooling (bypass) three modes: when the power battery temperature exceeds a preset passive cooling target temperature, passive cooling mode starts ; and when the temperature continues to rise above the active cooling target temperature, the active cooling mode is activated. However, this is still an extensive control strategy, which leads to a larger safety margin of the battery and reduces the efficiency of the battery.

Next, an efficient and sophisticated battery thermal management system is investigated. The maximum operating time or accumulated time that the battery management system can provide the required power after being fully charged is used as the evaluation index of efficiency. as an indicator of reliability. The hardware and software relationship for integrating and coordinating battery temperature management is shown in Figure 1. For the hardware part, on the basis of understanding the influence of battery thermal properties and external temperature and pressure conditions on battery performance, by calculating these nonlinear physical properties and abstracting these features in cyberspace, an ideal solution to reduce the safety margin is developed accordingly. temperature management system, thereby improving the efficiency of the entire battery system of pure electric vehicles.

What is a battery thermal management system?
Figure 1 BTMS software and hardware relationship

Compared with the common temperature control system, the temperature is selected as the control node to realize dynamic control, and the influence of the thermal properties of the battery and the electrical state of the battery on the efficiency and reliability is analyzed. The cell-level thermal control is adopted as the control strategy of the battery thermal management system. Temporarily boost the performance of the cell layer when high power is required, while sleeping other layers to reduce stress, and the associated model is validated.

The main research contents include the following three points:
(1) Using cyberspace to summarize thermal properties to solve problems related to the efficiency and reliability of temperature management systems.
(2) Design the battery temperature management system, and systematically study the use of temperature as the control core of the temperature management system.
(3) The temperature management system used in the in-depth evaluation proves that it can effectively improve efficiency without sacrificing reliability.