Energy system structure and power flow analysis of pure electric vehicle

Energy system structure and power flow analysis of pure electric vehicle

The power battery energy management system is one of the key technologies of electric vehicles. At present, the research on power battery energy management system mainly focuses on the energy management strategies of hybrid electric vehicles and pure electric vehicles. Due to the complex powertrain system of hybrid electric vehicles, there are many control strategies and a large space for development. For example, the commonly used control strategies for series hybrid electric vehicles include thermostat strategy, power tracking strategy and basic rule strategy; the commonly used control strategies for parallel hybrid electric vehicles include static logic threshold strategy, instantaneous optimal energy management strategy, and fuzzy logic control. strategy and global optimal energy management strategy, etc.; the commonly used control strategies for hybrid hybrid vehicles include engine constant operating point strategy, engine optimal working curve strategy, etc.

Pure electric vehicles can be divided into multiple energy source systems and single energy source systems according to the number of energy sources. The multi-energy source is mainly a dual-energy source system composed of a battery and a supercapacitor. The main feature of the supercapacitor’s large charge and discharge rate is used to make up for the shortcomings of the power battery by cutting peaks and filling valleys. At the same time, due to the existence of supercapacitors, which increases the complexity of the powertrain system, the available control strategies are also much more than that of single energy sources, such as threshold control strategies and fuzzy logic control strategies. For pure electric vehicles with a single energy source, because the powertrain system is simpler than that of hybrid and multi-energy source systems, there is less room for control strategies.

Combining the energy management and control strategies of pure electric vehicles of Chinese and foreign OEMs, the main control strategies are as follows: one is to reduce the energy usage rate of the entire vehicle, and only retain the high-voltage load necessary for the vehicle to travel, so as to minimize the energy consumption of the entire vehicle. The power consumption of the high-voltage system is to allocate all the limited power to the drive system; the second is to improve the efficiency of battery use. Through the use of the thermal management system, the battery has been controlled in the high-efficiency range. Although the first method can save the power consumption during driving to the greatest extent, due to the lack of the battery thermal management system, the long-term high temperature operation will accelerate the aging of the battery, which will sacrifice the economic performance of the vehicle; the second method is the most commonly used threshold. Although the control strategy is simple and stable, it cannot optimally solve the power distribution problem due to the fixed control rules, thus affecting the dynamic performance of the vehicle.

  1. Energy system structure and power flow analysis of pure electric vehicle

The high-voltage system components of pure electric vehicles are mainly divided into air-conditioning system components, drive system components, low-voltage power supply system components and charging system components. Among them, the low-voltage power supply system components convert high-voltage electricity into low-voltage electricity to support the entire vehicle electronic components , the operation of low-voltage equipment such as power steering, water pumps and fans. Charging system components replenish energy from the grid through chargers or other charging equipment. The air conditioning system components are mainly used to improve driver comfort and thermal management of the power battery. The high-voltage components are the electric heater (PTC) for heating and the air compressor (ACP) for cooling, and the drive system components are used to drive the motor to the outside. Doing work, you can also brake to recover part of the energy.

From the above-mentioned components of the power battery system and the high-voltage system, a schematic diagram of the power flow of the vehicle energy management system as shown in FIG. 1 can be obtained. It describes the input-output relationship of power flow between the power battery system and the high-voltage system of the vehicle.

Figure 1 - Power flow of vehicle energy management system
Figure 1 – Power flow of vehicle energy management system

In the figure, Pbat represents the maximum output power of the power battery system. PA-c represents the maximum power allocated by the energy management system to the air conditioning system. Since the comfort system and the thermal management system share power devices, in order to describe their functions intuitively, the air conditioning system is equivalent to a thermal management system, which is called PT-M. Pmotor represents the maximum power that the energy management system can allocate to the drive system. PL-p represents the required power of the low-voltage power supply system. Pm represents the maximum rechargeable power of the charging system.
During the driving process of the vehicle: Pcha(t)=0; PL-p(t) is a fixed value, which is equal to the DC/DC rated power in value and must be allocated; PT-M(t) and Pmotor(t) The size can be determined according to the power required by the power bus under different working conditions, and it is an amount that can be adjusted and allocated. Let the power that can be allocated by the vehicle energy management system be Pe(t), then:

Obviously, the purpose of the vehicle energy management system is to reasonably allocate Pe(t) to PT-M(t) and Pmotor(t), so as to maximize the energy utilization efficiency of the vehicle.

Read more: What is a Lithium-Ion Power Battery Pack

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

Read more: What is the cycle life of a lithium-ion power battery?

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.

What is lithium-ion power battery charging optimization control?

What is lithium-ion power battery charging optimization control?

Lithium-ion power battery cells generally adopt a constant current-constant voltage charging method, that is, first use a fixed rate of current (0.3C or 1C, etc.) to charge, reach the set charging cut-off voltage (3.6V, 4.0V, etc.) For constant voltage charging, the charging is completed after the charging current is lower than a certain value (such as 0.03C). Relevant studies have shown that the charging current and charging cut-off voltage not only have a significant impact on the charging time and charging energy of lithium-ion power batteries, but also have an important impact on their service life. Lithium-ion power battery systems are generally used in groups of multiple cells in series. If the constant current-constant voltage charging method is still used, it may cause overcharging of some battery cells. On the basis of studying the influence of charging current, charging voltage, overcharge and other charging factors on the service life, with the purpose of prolonging the service life of the battery, an optimized charging method for lithium-ion power battery cells and battery packs is proposed.

1. Influence of charging factors on the life of lithium-ion power batteries

1) Influence of overcharging on the life of lithium-ion power batteries
When the lithium-ion power battery is overcharged, a lot of heat is generated inside the battery, and at the same time, a lot of bubbles are generated in the electrolyte, which causes the active material on the positive and negative plates of the lithium-ion power battery to peel off, which seriously affects the activity of the battery and increases the internal resistance. Capacity has also dropped. At the same time, overcharging may also cause the battery to expand and deform, and even cause serious consequences such as fire and explosion. Research by Wang Hongwei et al. shows that at an ambient temperature of 20°C to 40°C, overcharging will cause the lithium-ion power battery to expand and deform, and the higher the temperature, the faster the temperature rises when the lithium-ion power battery is overcharged, and the higher the maximum temperature. more likely to be dangerous. Therefore, during the use of the lithium-ion power battery, it is necessary to ensure the normal operation of the charger and the protection circuit to avoid overcharging.

2) Influence of charging current and charging voltage on battery life
The charging voltage and charging current directly affect the charging energy and charging speed of the lithium-ion battery. Taking a certain lithium-ion power battery as an example, as shown in Figure 1, as the charging current increases, the charging capacity in the constant current stage becomes smaller, and the constant current charging capacity at 100A charging current is reduced by 8.36% compared with 20A charging current. The total charging time of the battery decreases with the increase of charging current, and the total charging time of 100A constant current charging is reduced by 76.1% compared with the total charging time of 20A charging current. This shows that increasing the charging current has little effect on the total energy charged, but can significantly improve the charging speed. However, high-rate charge-discharge current will cause the battery system to deviate from the equilibrium state, and accelerate the aging of positive and negative materials, thereby shortening the battery life. Therefore, power battery manufacturers need to comprehensively consider charging time and battery life when designing charging strategies. Two charging modes can be set: under normal circumstances, low-current charging should be selected as far as possible when charging lithium-ion power batteries, so as to prolong the battery life; in urgent cases, high-current charging can be used to shorten the charging time, although this will damage battery life.

What is lithium-ion power battery charging optimization control?
Figure 1 The relationship between constant current charging capacity, total charging time and charging current of a lithium-ion power battery

In general, when the charging current is the same, the higher the charging cut-off voltage, the greater the total energy charged by the lithium-ion power battery. However, the higher charge cut-off voltage will cause partial decomposition of the battery cathode material, the performance of the electrolyte will also decline, and the separator will also be oxidized due to contact with the high-potential cathode material. Taking a lithium-ion power battery as an example, as shown in Figure 2, when the charging voltage is reduced from 4.2V to 4.1V, the capacity retention of the lithium-ion battery is better as the number of charging and discharging increases, that is, the battery Longer cycle life. Relevant studies have shown that reducing the charge cut-off voltage by 0.1~0.3V can prolong the battery cycle life by 2~5 times.

What is lithium-ion power battery charging optimization control?
Figure 2 The relationship between the usable capacity and the number of cycles at different charge cut-off voltages

2. Charging strategy based on lifetime optimization
By studying the influence of charging current, charging voltage and overcharge on the life of lithium-ion power battery, in order to prolong the service life of lithium-ion power battery cells and battery packs, this paper proposes the optimization of lithium-ion power battery cells and battery packs. charging strategy.

1) Charging strategy of lithium-ion power battery cells
In order to prolong the service life of the battery, the charger and the charging protection circuit should be safe and reliable. The thermistor can be used to detect the temperature of the battery, and stop charging when the battery temperature exceeds the high temperature threshold to prevent overcharging. After the battery is fully charged, disconnect the voltage in time, otherwise, metal lithium will be generated inside the battery, resulting in permanent capacity loss, and may cause a short circuit inside the battery.

According to different positive and negative materials and battery structure, the charging parameters of lithium-ion power batteries will be different. When determining the charging method of a single battery, factors such as the composition material and structure of the battery, charging time, charging capacity and battery life should be comprehensively considered, and parameters such as charging current, charging cut-off voltage and charging termination current should be optimized and designed. The charging methods of lithium-ion batteries can be divided into ordinary charging and fast charging: ordinary charging is suitable for general household charging or long-term parking charging, using small rate charging current and charging cut-off voltage, charging voltage can be 3.8~4.0V, And use a small charging termination current (such as C/10 or less) to strengthen the protection of the battery; fast charging is suitable for charging in emergency situations, which will greatly damage the battery life, use a large rate current for a short time (1h) Charge more than 90% of the battery inside. The number of fast charging times should be minimized during use.

Appropriately reducing the charge cut-off voltage can significantly improve the service life of the battery. In order to take into account the charging time, the cut-off voltage can be set according to the depth of discharge (DOD): when the depth of discharge is 100%, in order to shorten the charging time, the cut-off voltage can be increased, such as 3.8V; when the depth of discharge is 0, set the cut-off voltage The voltage is 3.5V. In this way, when the depth of discharge is between 0 and 100%, the charge cut-off voltage can be set between 3.5 and 3.8V according to a linear relationship. This approach helps extend battery life and can reduce the overall charging time for Li-ion power batteries.

2) Charging strategy of lithium-ion power battery pack
When charging a lithium-ion power battery pack, if a single constant current-constant voltage charging is used, it is likely to cause some cells to be overcharged. Therefore, the charging of the battery pack needs to be controlled according to the state of the lithium-ion power battery cells to prevent overcharging of the cells, protect the cycle life of the single cells, and help prolong the cycle life of the battery pack. Taking the charging process of a lithium-ion power battery system as an example, the standard charging steps are: charging with a constant current of 1C, when the voltage of a single battery reaches 3.5V or more, the charging current is reduced to C/2. When the voltage of the single battery reaches 3.55V or more, reduce the charging current to C/4, when the voltage of a single battery reaches 3.6V, reduce the charging current to C/8, when the highest voltage of the single battery reaches the cut-off voltage (3.65V) or the battery pack The charging process is completed after the total voltage reaches a certain cut-off voltage.

When the vehicle is parked for a long time, the power supply should be cut off, and the vehicle should be parked in a ventilated, rain-proof, moisture-proof, sun-proof, and fire-fighting place, and should be kept away from flammable and corrosive items. When the vehicle is parked for more than a month, the battery pack must be kept at a state of charge of about 50%, the connecting wire of the battery pack must be unplugged, and the power battery system must be charged and discharged with a small current every three months for maintenance.

What is the management of lithium-ion power battery consistency?

What is the management of lithium-ion power battery consistency?

The consistency of lithium-ion power batteries means that after the cells are used in groups, the voltage, internal resistance, capacity and other parameters of each cell are not exactly the same due to the influence of factors such as production and use environment. The performance parameters such as charge-discharge capacity and cycle life of a lithium-ion power battery are generally determined by the worst-performing monomer in the battery. Therefore, the consistency of the battery pack plays an extremely important role in its performance and cycle life. The impact of consistency on battery life will be discussed and ways to improve consistency will be suggested.

1. The impact of consistency on the life of the power battery pack
The consistency of lithium-ion power batteries mainly includes voltage consistency, capacity consistency and internal resistance consistency. As the power battery usage time increases, the degree of inconsistency will gradually increase. The most intuitive reflection is that the degree of inconsistency of the cell voltages in the battery pack increases. There are two main reasons for the poor consistency of lithium-ion power batteries: one is the production and manufacturing reasons. Due to slight differences in electrode plate thickness, chemical activity, microporosity, etc., there are some differences in parameters such as internal resistance and capacity of single cells. The second is the inconsistency in the use process. Lithium-ion power batteries have complex operating conditions, long-term work under harsh conditions such as high-rate charge and discharge current, vibration, etc., coupled with the structural layout of the battery system and the design of the heat dissipation system, which lead to the temperature, self-discharge degree, and electrolyte activity of each battery cell. There are differences, and the inconsistency of lithium-ion power battery packs gradually increases as the number of uses increases. It can be seen that the inconsistency of lithium-ion power batteries is inevitable. Figure 1 shows the causes of battery pack inconsistency and its transmission process.

What is the management of lithium-ion power battery consistency?
Figure 1 Causes and transmission process of battery pack inconsistency

As the inconsistency of the power battery pack increases, the performance and life of the battery pack are seriously affected. Scholars from various countries have already made some research results on the impact of inconsistency on the life of power batteries. Wang Zhenpo et al. [71] proposed a formula for calculating the remaining capacity of the battery pack after n times of use under the influence of inconsistency:
C(n)=fn(△C)(1-nP/N)C0

In the formula, C(n) represents the remaining capacity of the battery pack after n times of use; f(△C) represents the maximum value of the damage coefficient of the battery charge and discharge capacity during each charge and discharge process, which is a positive number less than 1; N represents the battery pack The service life of the battery pack; P indicates the specified capacity decay percentage at the end of the battery pack life; C0 indicates the initial capacity of the battery pack.

If f(△C) takes the maximum value of 0.999, the end of battery life is defined as the capacity decay of 20%. Assuming that there are three single cells with cycle life of 300 times, 600 times, and 1200 times, respectively, according to the formula, the cycle life when they are used in groups can be calculated as shown in Table 1.

sampleMonomer cycle life (times)Battery pack cycle life (times)
1#300132
2#600167
3#1200191
Table 1 Relationship between single cycle life and battery pack cycle life caused by inconsistency

According to the calculation results, the cycle life of the lithium-ion power battery pack is much lower than that of the corresponding monomer. Due to the inconsistency, the cycle life of the single cell is doubled, and the life of the battery pack can only be improved by dozens of times. If the battery pack is not repaired and maintained in time, the life of the battery pack can only reach a fraction of the life of the single unit. The lithium-ion power battery used in the demonstration operation of Beijing’s public transport has a single cell life of more than 1,000 times. The capacity of the power battery system applied to the vehicle will be seriously attenuated after 150 times of charging and discharging, and the capacity of some cells has been lower than 80% of rated capacity.

2. Measures to improve battery consistency
The cycle life of the battery pack is increased by increasing the cycle life of the lithium-ion power battery cells, which is ineffective and expensive. By optimizing the charging and discharging method of the battery pack, reducing the inconsistency caused by charging and discharging, and regularly repairing and maintaining the battery pack during use, the electrical performance of the lithium-ion power battery pack can be effectively guaranteed and the service life of the battery pack can be improved. Combined with the research on the life characteristics of lithium-ion power batteries for vehicles and the actual use of vehicles, the following measures can be taken to prevent the expansion of inconsistencies in the use of battery systems.

(1) To ensure the delivery quality of lithium-ion power battery cells, the initial voltage of each cell needs to be consistent, and the same batch of cells must be correlated with voltage, internal resistance and other data before leaving the factory to ensure the same batch of cells. performance as consistent as possible. The battery cells of the same batch, the same specification, and the same type must be selected when assembling the battery.

(2) Adopt practical battery balancing system and energy management system. At present, the most effective and practical equalization method is to equalize the voltage of each cell during the charging process of the battery pack, so that the cell voltage is as consistent as possible, and equalization management is realized from the source. Charging is terminated when the cell voltage reaches the charge cut-off voltage. Charge equalization is to use an active or passive equalization method to make the voltage of each cell consistent before charging is terminated, and the passive equalization method is currently used more. The principle of passive balancing is shown in Figure 2. Each cell is connected to a load resistor and controlled by a switch. According to the result of the cell voltage detection in the battery system, the balance management system closes the switch connected to the cell with the faster voltage rise during charging, thereby maintaining the consistency of the cell voltage and improving the electrical performance of the entire battery pack. Passive equalization is carried out by means of heat dissipation, the discharge current is generally controlled at about 0.1A, and the charging equalization takes several hours to complete. Active charge equalization requires an energy storage element (capacitor, magnetic field, etc.) to transfer energy between cells. The equalizing current is large and the power consumption is small, and no special cooling measures are required, which is beneficial to improve the consistency of the battery pack. However, this method has a complex structure and high cost.

What is the management of lithium-ion power battery consistency?
Figure 2 Schematic diagram of passive charge equalization

The purpose of the battery management system (BMS) is to avoid premature failure of battery cells due to excessive use, so that the main electrical performance of the battery pack can reach and maintain the performance level of poor cells. Its main task is to prevent overcharge and overdischarge. , which provides status information such as voltage, current, temperature, and remaining power. Using thermal resistance, semiconductor refrigeration device for temperature control, etc., and controlling the charging and discharging state of the power battery pack through BMS can effectively increase the cruising range of the vehicle, prolong the service life of the battery system, and at the same time ensure the safety and reliability of the battery pack during use. Sex is important.

(3) Strengthen the maintenance and maintenance of the battery pack during use. During the use of the battery pack, it is necessary to avoid the contamination of the battery poles by water and dust as much as possible, to ensure a good working environment for the battery pack, and to avoid excessive use as much as possible. The power battery pack should be maintained regularly, and the cells with poor performance should be replaced or adjusted in time through the analysis of parameters such as the voltage of each single cell of the battery pack. The power battery pack is charged with a small current at regular intervals to promote its balance and performance recovery.

How to optimize the air cooling and cooling system of the lithium-ion power battery system?

How to optimize the air cooling and cooling system of the lithium-ion power battery system?

1. Flow field design of battery pack thermal management system
The rate of heat dissipation per unit area of ​​the battery pack to the heat transfer medium is expressed as
·Q=h(Tbat-Tamb)

Among them, h represents the convective heat transfer coefficient on the surface of the battery pack, and the subscripts bat and amb represent the surface of the battery pack and the heat transfer medium, respectively.

First, the design of the flow field determines the order in which the heat transfer medium flows through different positions of the battery pack, which will affect the value of the Tbat-Tamb term, thereby affecting the local heat dissipation rate at different positions. Second, the design of the flow field determines the flow velocity of the heat transfer medium at different locations, and the flow velocity will affect the h term of the local convective heat transfer coefficient. Third, the design of the flow field determines the local shape of the flow channel, which will also affect the value of the local convective heat transfer coefficient h. Therefore, the rationality of the flow field design has a significant impact on the thermal management effect of the battery pack.

(1) Path design of flow field – serial flow channel and parallel flow channel. According to the passage of the heat transfer medium inside the battery pack, the flow field can be divided into serial flow channel type and parallel flow channel type, as shown in Figure 1. In the serial flow channel design, the heat transfer medium passes through each single cell or battery module in strict order, while in the parallel flow channel design, the heat transfer medium enters the battery pack box and passes through the parallel flow channels. Divide the current through different battery sub-modules in parallel. For serial runner designs, the battery modules behind the runners will not be able to dissipate heat effectively because the medium will gradually be heated in the serial runners. It has been pointed out that the parallel flow channel design results in better temperature uniformity at different locations of the battery pack compared to the serial flow channel.

How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 1. Fluid design of serial and parallel runners

(2) Velocity design of flow field—speed regulation and pressure regulation of parallel flow channels. For the parallel flow channel design, the flow rates of different flow channels must be as uniform as possible to reduce the non-uniformity of temperature at different positions inside the battery pack. Two methods to ensure uniform flow rate: speed regulation method and pressure regulation method, and the optimal combination of the two methods is given. The speed regulation method refers to reducing the width of each channel in turn in the direction of increasing the number of parallel channels to adjust the flow resistance of the heat transfer medium, so that the heat transfer medium can redistribute its flow according to the resistance of each channel, so as to achieve the purpose of adjusting the flow rate distribution. The pressure regulation method changes the pressure difference on both sides of different channels by changing the inclination angle of the inlet and outlet collector plates, thereby indirectly adjusting the flow rates of different channels.

The thermal management system of the lithium-ion power battery system is mainly divided into air cooling and liquid cooling according to the different cooling media. Among them, liquid cooling has better cooling effect, but it needs to arrange special pipes, has many parts, complicated control and high cost. The gas cooling system has low heat transfer rate and low volumetric efficiency, but is widely adopted due to its simple design, simple control and low cost. Due to the small convective heat transfer coefficient of gas, it is more difficult to use gas to heat or cool battery systems than liquids. Therefore, the design of gas cooling systems should be optimized to the greatest extent possible for battery packs. Taking the development of a power battery cooling system for an electric vehicle as an example, the optimization scheme of the air-cooled cooling system is proposed, and the final optimization scheme is determined by the simulation results.

2. Problems and solutions of cooling system
The air-cooled cooling system of an electric vehicle power battery is shown in Figure 2. There are two main problems in this cooling system: one is that the temperature difference between the battery modules is too large; the other is that the pressure loss is too large, and the structure of the air channel needs to be optimized. The main reasons for these two problems are: the arrangement of the battery modules is asymmetric, and the air flow between the battery modules is inconsistent; the battery module adopts a double-layer structure, which generates heat accumulation; there is a sudden contraction or expansion in the air channel. The cross section changes suddenly, the structure does not have enough corner radius at the corner, and the air cannot transition smoothly. In order to solve these two problems, the air-cooled heat dissipation system of the battery system is optimized: the battery module is arranged in a single-layer structure, the structure of the air channel is arranged symmetrically, and the cross-section is changed by using a small shrinkage angle and multiple cross-sections. , and design a large corner radius at the corner. There are two types of improved schemes: scheme one, the air inlet of the air channel is set at the top left side, the air outlet is set at the bottom end of the right side, and the air inlet and outlet are located on both sides; scheme two, the air inlet port of the air channel is located at the top left side, The air outlet is at the bottom left side, and the air inlet and outlet are at the left end. In the two schemes, the battery modules are arranged in a single-layer symmetrical arrangement, and a large rounded transition is designed at the turn of the air inlet, and a small-angle contraction is used to reduce the pressure loss. The improved scheme is shown in Figure 3 and Figure 4.

How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 2 The structure diagram of the heat dissipation system of the original lithium-ion power battery system
How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 3 Cooling system optimization scheme 1
How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 4 Cooling system optimization scheme 2

3. Evaluation indicators of air-cooled cooling system
In the design process of the air cooling system of the battery pack, it is necessary to evaluate it with relevant indicators to determine whether the optimization scheme is feasible. The main indicators are the maximum temperature difference of the system, the maximum temperature of the system, and the pressure difference between the inlet and outlet. The maximum temperature difference of the system refers to the difference between the highest temperature and the lowest temperature of all the cells in the lithium-ion power battery system, which reflects the uniformity of the cooling system and ensures that the cooling effect of each cell is consistent. The maximum temperature of the system refers to the maximum temperature of all cells in the lithium-ion power battery system, which can represent the cooling effect of the cooling system to a certain extent. The inlet and outlet pressure difference refers to the pressure difference between the air inlet and the air outlet in the air cooling system, which is closely related to the structure of the air flow channel of the cooling system.

4. Simulation analysis
The flow field and thermal field simulation are carried out for the two optimization schemes respectively, and the temperature cloud map, pressure cloud map and velocity cloud map are formed, as shown in Figure 5, Figure 6, and Figure 7. By comparing and analyzing the simulation results of flow, pressure, temperature, etc., the advantages and disadvantages of the two schemes are judged according to the evaluation indicators of the cooling system, and the one with better cooling effect is selected. The key parameters of the two optimization schemes are compared in Table 1. From the data in Table 1, it can be seen that the maximum temperature difference of the system, the maximum temperature of the system, and the flow uniformity of the scheme 2 are better than those of the scheme 1, but the pressure difference between the inlet and the outlet of the scheme 2 is slightly different. Therefore, the second solution with the same side design of the air inlet and outlet has a better heat dissipation effect, and further optimization design can be made on the basis of the second solution to obtain a better heat dissipation effect.

SchemeMaximum temperature difference (K)Maximum temperature (K)Inlet and outlet pressure difference (Pa)Flow unevenness (%)
Scheme 114.2326.311.880.08604
Scheme 213325.811.890.06196
Table 1 Comparison of the main evaluation indicators of the two optimization schemes
How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 5. Scheme 1 temperature cloud map
How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 6 Scheme 1 pressure cloud map
How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 7 Scheme 1 Velocity Cloud Map
What effect does temperature have on the life of lithium-ion power batteries?

What effect does temperature have on the life of lithium-ion power batteries?

Lithium-ion power battery is an electrochemical battery based on Li+ concentration difference. The level of ambient temperature during operation directly affects the activity of positive and negative electrode materials and electrolyte, and has an important impact on its life. The electrical performance and service life of the same lithium-ion power battery at different operating temperatures are very different. Generally, the lithium-ion power battery can exert its maximum efficiency at a room temperature of about 25 °C. When the ambient temperature is too low, the activity of the electrolyte is affected, and the internal resistance increases significantly, resulting in difficult battery charging, reduced power, reduced usable capacity, and impaired battery life. When used at high temperature, the heat dissipation of the battery system will be affected. When the internal temperature of the battery exceeds the limit temperature, the internal chemical balance will be destroyed, resulting in corrosion and aging of battery materials, seriously aggravating the battery life decay process, and causing the battery to fail prematurely.

Figure 1 shows the relationship between the capacity retention rate and the number of cycles of a lithium-ion power battery cell at different temperatures. In the cyclic charge-discharge test, the discharge system is: 1C constant current discharge to voltage 2.5V; charging system: 1C constant current charge to 3.7V, transfer to constant voltage 3.7V to charge until the current drops to 1/30C, stop charging, and complete the charging process. After charging, let it stand for 1h, and then re-charge and discharge test. The remaining capacity of the monomer was measured every 20 charge-discharge cycles completed.

What effect does temperature have on the life of lithium-ion power batteries?
Figure 1 The relationship between the capacity retention rate of a lithium-ion power battery and the number of cycles at different temperatures

It can be seen from Figure 1 that when the high temperature is 40~60°C, the battery decays faster as the temperature increases, especially when the battery is at an ambient temperature of 60°C, the battery discharge capacity decays to 80.63 after 20 charge-discharge cycles. %. When the low temperature is -10~10℃, with the decrease of the ambient temperature, the battery attenuation speed is accelerated, and the battery capacity attenuation speed is significantly accelerated in the -10℃ environment. Moreover, it can be seen that the high temperature environment has a greater impact on the life attenuation than the low temperature environment, which is more detrimental to the life of the battery. The temperature of the battery increases rapidly in the high temperature environment. When the charge and discharge test is carried out in the environment of 40°C, the temperature of the battery increases by 20°C after 7 cycles of charge and discharge.

In order to protect the lithium-ion battery and improve the service life of the battery, the ambient temperature of the lithium-ion power battery should be controlled within the range of 0~40 °C, and it is forbidden to work in a high temperature environment above 50 °C. In order to further ensure that the battery capacity decay rate is within a certain range, the working temperature of the battery should preferably be controlled at 0~25℃.

When the lithium-ion power battery cells are connected in series and parallel to form a power battery pack, the temperature field of the power battery pack is not a simple superposition of the temperature field of the single cell, and the temperature distribution of the battery pack is not as uniform as that of the battery cells. The stability is also not as good as the monomer. The non-uniformity of the temperature distribution of the battery pack leads to inconsistent cell activity at different positions inside the battery pack, thereby aggravating the expansion of the inconsistency. Therefore, the power battery system needs to design a special thermal management system to ensure that the battery works in an appropriate temperature range and the uniformity of the battery temperature distribution.