Lithium-ion battery packs for electric vehicles have large battery capacity, many series and parallel connections, complex systems, and high-performance requirements such as safety, durability, and power. In addition, the safe working area of the lithium-ion battery pack is limited by temperature and voltage. If it exceeds the allowable range, the performance of the battery pack will accelerate decay, and even safety problems may occur.
Therefore, the battery management system is indispensable and extremely important. On the one hand, the battery management system needs to detect, collect and initially calculate the real-time battery status parameters, and control the on and off of the power supply loop based on the comparison between the detected value and the allowable value; on the other hand, the battery management system needs to report the collected key data to the whole The vehicle controller, and receives the instructions of the controller, and coordinates with other systems on the electric vehicle.
The main task of the battery management system is to ensure the design performance of the battery pack system, which can be broken down into the following three aspects:
1. Safety: protect battery cells or battery packs from damage and prevent safety accidents;
2. Durability: make the battery pack work within a reliable and allowable range, and extend the service life of the battery pack;
3. Power: Maintain the battery pack to work in a state that meets the requirements of electric vehicles.
BMS is composed of various sensors, actuators, controllers and signal lines. In order to meet relevant standards or regulations, BMS should have the following functions.
1. Battery parameter detection.
Including the total voltage of the battery pack, the total current of the battery pack, the voltage of the single battery cell (to prevent overcharging, overdischarging and even reverse polarity), temperature detection (preferably each series connection, key cable connector, etc. have a temperature sensor) , Smoke detection (monitoring electrolyte leakage, etc.), insulation detection (monitoring leakage), collision detection, etc.
2. Battery status assessment.
Including state of charge (SOC) or depth of discharge (DOD), state of health (SOH), state of function (SOF), state of energy (SOE), fault and safety state (SOS), etc.
3. Online fault diagnosis.
Including fault detection, fault type judgment, fault location, fault information output, etc. Fault detection refers to the use of diagnostic algorithms to diagnose fault types through the collected sensor signals, and early warning.
Battery failure refers to sensor failures, actuator failures (such as contactors, fans, pumps, heaters, etc.) of various subsystems such as battery packs, high-voltage electrical circuits, and thermal management, as well as network failures, and various controller software and hardware failures Wait.
The failure of the battery pack itself refers to overvoltage (overcharge), undervoltage (overdischarge), overcurrent, ultra-high temperature, internal short-circuit failure, loose joints, electrolyte leakage, and reduced insulation.
4. Battery safety control and alarm.
Including thermal system control, high-voltage electric safety control. After the BMS diagnoses the fault, it informs the vehicle controller through the network and requires the vehicle controller to perform effective processing (the BMS can also cut off the main circuit power supply when a certain threshold is exceeded) to prevent high temperature, low temperature, overcharge, overdischarge, and over discharge. Current, leakage, etc. damage the battery and human body.
5. Charge control.
There is a charge management module in the BMS, which can control the charger to safely charge the battery according to the characteristics of the battery, the temperature level and the power level of the charger.
6. Balance control of battery cells.
The existence of inconsistency makes the capacity of the battery pack smaller than the capacity of the smallest cell in the pack. Battery balancing is based on the information of single cells, using active or passive, dissipative or non-dissipative balancing methods to make the capacity of the battery pack as close as possible to the capacity of the smallest cell.
7. Thermal management.
According to the temperature distribution information in the battery pack and the charging and discharging requirements, determine the intensity of active heating/dissipation, so that the battery can work at the most suitable temperature as much as possible to give full play to the performance of the battery.
8. Network communication.
The BMS needs to communicate with network nodes such as the vehicle controller; at the same time, the BMS is inconvenient to disassemble on the vehicle. It needs to be online calibration, monitoring, automatic code generation and online program download without disassembling the shell (program update without disassembling the product), etc., the general vehicle network adopts CAN bus technology.
9. Information storage.
Used to store key data, such as SOC, SOH, SOF, SOE, accumulated charge and discharge Ah number, fault code and consistency, etc. The real BMS in the vehicle may only have some of the hardware and software mentioned above. Each battery unit should have at least one battery voltage sensor and one temperature sensor.
For a battery pack system with dozens of batteries, there may be only one BMS controller, or even integrate the BMS function into the main controller of the vehicle. For a battery pack system with hundreds of battery cells, there may be a master controller and multiple slave controllers that manage only one battery module.
For each battery module with dozens of battery cells, there may be some module circuit contactors and balancing modules, and the slave controller manages the battery modules like measuring voltage and current, controls the contactors, balances the battery cells and communicates with the master controller Communication. Based on the reported data, the main controller will perform battery state estimation, fault diagnosis, thermal management, etc.
10. Electromagnetic Compatibility.
Due to the harsh operating environment of electric vehicles, BMS is required to have good anti-electromagnetic interference capability, and at the same time, BMS is required to have low external radiation.
In summary, the BMS system of the EV lithium-ion battery pack is very important, and it is related to the service life and safety of the battery pack.
Therefore, after the production of the lithium-ion battery pack for EVs is completed, it must be tested with a professional battery pack and BMS test system to ensure that the lithium-ion battery pack has qualified performance, safety, reliability and long life.
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