2、Definition of 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 three areas as follows.
1) safety, to protect the battery unit or battery pack from damage and prevent safety accidents;
2) Durability, to make the battery work in a reliable and safe area, to extend the 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 lithium-ion battery is shown in Figure 1.
Figure 1 shows the safe operation window of lithium-ion battery
BMS is composed of various types of sensors, actuators, controllers and signal lines, etc. In order to meet the relevant standards or specifications, BMS should have the following functions.
1. Battery parameter detection. Including total voltage, total current, single cell voltage detection (to prevent overcharge, overdischarge or even reverse polarity phenomenon), temperature detection (preferably with temperature sensors for each string of cells, key cable connectors, etc.), smoke detection (monitoring electrolyte leakage, etc.), insulation detection (monitoring leakage), collision detection, etc.
2, battery state estimation. 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 diagnosis of fault type and early warning by using diagnostic algorithm through the collected sensor signals. Battery failure refers to sensor failure, actuator failure (such as contactor, fan, pump, heater, etc.), as well as network failure and various controller software and hardware failures in various subsystems such as battery pack, high-voltage electrical circuit, thermal management, etc. Battery pack failure itself refers to over-voltage (overcharge), under-voltage (over-discharge), over-current, ultra-high temperature, internal short-circuit fault, loose joints, electrolyte leakage, insulation reduction, etc.
4、Battery safety control and alarm. Including thermal system control, high-voltage electrical safety control. after the BMS diagnoses the fault, it notifies the vehicle controller through the network and asks the vehicle controller to deal with it effectively (the BMS can also cut off the main circuit power when it exceeds a certain threshold), so as to prevent the damage to the battery and people from high temperature, low temperature, overcharge, overdischarge, overcurrent, leakage, etc.
5, charging control, BMS has a charge management module, it can be based on the characteristics of the battery, the temperature and the power level of the charger, control the charger to the battery for safe charging.
6, battery equalization. The existence of inconsistency makes the capacity of the battery pack is less than the capacity of the smallest individual in the group. Battery equalization is based on the single battery information, using active or passive, dissipation or non-dissipation and other equalization methods to make the battery capacity as close as possible to the capacity of the smallest single unit.
7、Thermal management. According to the information of temperature distribution in the battery pack and the charging and discharging demand, decide the intensity of active heating/heat dissipation to make the battery work at the most suitable temperature as possible and give full play to the performance of the battery.
8, network communication. BMS needs to communicate with the vehicle controller and other network nodes; at the same time, the BMS is not convenient to disassemble the vehicle, the need for online calibration, monitoring, automatic code generation and online program download (program updates without disassembling the product), etc., the general vehicle network are using CAN bus technology.
9、Information storage. Used to store key data, such as SOC, SOH, SOF, SOE, cumulative charge/discharge Ah number, fault codes and consistency, etc. The real BMS in the vehicle may only have some of the hardware and software mentioned above. Each battery cell should have at least one battery voltage sensor and one temperature sensor. For battery systems with dozens of cells, there may be only one BMS controller, or even the BMS functions may be integrated into the vehicle's main controller. For battery systems with hundreds of cells, there may be a master controller and multiple slave controllers that manage only one battery module. For each battery module with dozens of cells, there may be a number of modular circuit contactors and balancing modules, and the slave controllers manage the battery modules as if they were measuring voltage and current, controlling the contactors, balancing the cells and communicating with the master controller. Based on the reported data, the master controller will perform battery status estimation, fault diagnosis, thermal management, etc.
10、Electromagnetic compatibility. Due to the harsh environment of electric vehicle, BMS is required to have good anti-electromagnetic interference capability and small external radiation of BMS. The basic framework of EV BMS software and hardware is shown in Figure 2.
