Definition of the battery management system
The main task of the battery management system is to ensure the design performance of the battery system, which can be broken down into the following three aspects:
1) Safety, BMS protects the battery cell or battery pack from damage and prevent safety accidents;
2) Durability, BMS make the battery work in a reliable and safe area, and extend the service life of the battery;
3) Power to maintain the battery working in a state that meets the requirements of the vehicle. The safe working area of the lithium battery is shown in Figure 1.
The BMS is composed of various sensors, actuators, controllers, and signal lines. To meet the relevant standards or specifications, the BMS should have the following functions.
1) Detection of all battery parameters: total voltage, current and each cell voltage (to avoid over-charging, over-discharging or even reverse polarity phenomenon), temperature detection (preferably, each string of batteries, key cable connectors, etc. have temperature sensors), smoke detection (monitoring electrolysis Fluid leakage, etc.), insulation detection (monitoring leakage), collision detection, etc.
2) Battery status estimation: which includes SOC (state of charge) or DOD (depth of discharge), SOH (state of health), SOF (functional state), SOE (energy state), SOS (fault and safety state).
3) Online fault diagnosis. It includes fault detection, fault type judgment, fault location, fault information output, etc. Fault detection refers to the use of collected sensor signals to diagnose the type of fault using a diagnostic algorithm and early warning.
Battery failure refers to sensor failures, actuator failures (such as contactors, fans, pumps, heaters, etc.) as well as network failures and various controller hardware and software failures. The failure of the battery pack itself refers to overvoltage (overcharge), under-voltage (over-discharge), 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 electrical 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 it exceeds a certain threshold) to prevent high temperature, low temperature, overcharge, over-discharge, and excessive Current, leakage and other damage to the battery and personal.
5) Charging control. BMS has a charging management module, 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) Battery balancing. The existence of inconsistencies 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 battery pack capacity as close as possible to the smallest single cell.
7) Thermal management. According to the temperature distribution information in the battery pack and the charge and discharge requirements, it is determined to the intensity of active heating/heat dissipation, so that the battery can work at the most suitable temperature as much as possible, and the performance of the battery can be fully utilized.
8) Network communication. It needs to communicate with network nodes, such as vehicle controllers. At the same time, BMS is inconvenient to disassemble on the vehicle, and online calibration, monitoring, automatic code generation, and online program download are required without removing the shell (program update without disassembly of the product) ) Wait, the general vehicle network uses 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 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 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 contactor and balancing modules, and the slave controller manages the battery modules like measuring voltage and current, controls the contactors, balances the cell, and communicates with the master controller Communication. Based on the reported data, the main controller will perform battery status 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 capabilities, and at the same time, BMS is required to have low external radiation. The basic framework of BMS software and hardware for electric vehicles is shown in Figure 2.
We will share the most BMS issues in another post. Welcome to make a comment or shore. Thanks in advance.