One of the key components of an electric vehicle is a power battery. The power battery provides energy for the electric vehicle to ensure the cruising range of the electric vehicle.
The performance of the power battery, in addition to relying on its own materials, process and other hardware qualities, also depends on the performance of the battery management system, which is commonly referred to as BMS (Battery Management System).
BMS is like the brain of a battery. It receives the information of the battery and external interfaces, analyzes and processes the information, and issues execution instructions to complete the functions of battery charging, discharging, protection, equalization, fault detection and fault warning to ensure the battery. Normal, efficient, reasonable and safe operation.
The main components of BMS can be divided into three parts: closed-loop feedback: information acquisition, information analysis processing, and output decision execution instructions.
BMS also needs to communicate and interact with the charger and vehicle control system in the work to ensure the safe and efficient operation of the charging process and the discharging process.
BMS needs to monitor the state of the battery in real time, which requires various sensors to collect physical parameters such as voltage, current, and temperature of the battery.
Information analysis processingAfter the BMS collects relevant information, it needs to analyze and process the information to determine the action that needs to be taken.
For example, estimating the battery's power (SoC) based on the information of the voltage and current;
The operation of the heating or cooling system is determined based on the temperature, the command is output to the thermal management system, and the operational status of the thermal management system is monitored.
External interaction interfaceBMS has two main interaction objects: charging equipment and vehicle control systems.
When charging, the BMS interacts with the charging device to determine the output voltage and output current of the charging device to ensure safe and efficient charging of the battery.
When discharging, it is the interaction between the vehicle and the vehicle control system to determine the battery output strategy.
For example, when the power is low, reduce the output power to extend the cruising range;
After the power continues to drop to a certain threshold, discharge is prohibited to prevent the battery from being over-discharged and damaging the battery.
SoC, DoDSoC (State of Charge) is the remaining battery power ratio, which is equal to the remaining battery power/the total battery power. SoC=0% means that the battery is completely out of power. When SoC=100%, the battery is fully charged.
DoD (Depth of Discharge) indicates the depth of discharge, which is exactly the opposite of SoC. When DoD=100%, the battery is dead. When DoD=0%, the battery is fully charged.
The SoC terminology is generally used in the use of the battery, and the DoD terminology is used when describing the cycle life of the battery.
Unlike the fuel tank, the remaining oil in the fuel tank is relatively easy to measure. The remaining capacity of the battery cannot be directly measured, but requires indirect related information such as voltage and current, and is estimated by the relevant algorithm of the BMS software.
Therefore, the accuracy of the SoC is a problem that plagues the battery BMS industry. For the existing electric vehicles on the market, the SoC estimation error often reaches more than 10%, and the accuracy is within 10%.
SoC estimates, commonly used are chemical methods, voltage look-up table, current integration method, hybrid calculation method and Kalman filter method.
Because of the inaccuracy of the SoC, it will lead to the calculation error of the remaining cruising range of the electric vehicle, which will cause the anchoring phenomenon.
As a consumer, we hope that BMS industry personnel will work hard to improve the accuracy of SoC estimation and use electric vehicles with greater confidence.
Thermal managementThe performance of the battery is highly dependent on the temperature of the environment in which it is used.
In a safe situation, the right temperature maximizes the potential of the battery;
For safety reasons, it is necessary to avoid battery operation under extreme conditions, or in extreme cases, to ensure the safety of the system as much as possible, to prevent or reduce the damage caused by fire or explosion.
Then a thermal management system is needed to manage and control the temperature of the battery. Battery control has two goals, to warm it up when the battery is cold, and to cool it when the battery is hot.
From the scene, there are mainly two scenes of charging and discharging.
If you charge in a cold area when the temperature is extremely low, there is no accessory facility, and the car will not be charged.
At this time, it is necessary to add a preheating module to the battery, and then the battery is heated and then charged. If charging in high temperature conditions in the tropics, also pay attention to the battery temperature and start the cooling system.
In the case of discharge, the main consideration is the heat dissipation of the battery. At present, there are three solutions for the cooling of the battery, natural cooling, air-cooling equipment and liquid cooling equipment.
In terms of effect, liquid cooling is the best, and the familiar Tesla series electric vehicles use a liquid cooling solution.
Electricity balanceA typical automotive battery system consists of a large number of cells connected in series and in parallel. For example, the Tesla series is made up of thousands of 18650 cylindrical batteries.
We know that the performance of the overall battery system is optimal when the charge of each cell is good.
After the battery is shipped, due to internal resistance, self-discharge rate and other factors, after using for a period of time, the battery power of each battery system may be inconsistent.
Then at this time, it is necessary to balance the battery with a large amount of electricity and the battery with a small amount of electricity.
There are two types of power balance: active balance and passive equalization. Active balance is controlled by BMS. The power of a large battery is transferred to a small battery. Active balancing is complex and costly.
Passive equalization means that the battery with a large amount of electricity is partially discharged, generating heat and dissipating it. Passive equilibrium is simpler and less costly.
At present, the balance of electricity in the market is basically passive equilibrium.
summaryAs the brain of the battery of the key components of electric vehicles, BMS needs to make the battery eat well (charge), but also to do a good job (discharge), good health (balance), no scorpion (safety protection).
So BMS is a very important part of electric vehicles.
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