As we all know, the power output of pure electric vehicles depends on the battery, and the battery management system BMS (Battery Management System) is the core of which is responsible for controlling the charging and discharging of the battery and implementing functions such as battery state estimation. If an electric vehicle is compared to the human body, then the battery system is its heart, and the BMS battery management system is the brain that controls the operation of its body.
Why have a BMS?
Since it is called a battery management system, the main job of the BMS is to handle tasks related to the on-board battery. Although the current battery manufacturing process has narrowed the differences between individual cells, there are still differences in internal resistance, capacity, and voltage between single-cell lithium batteries. It is prone to uneven heat dissipation or excessive charging and discharging. Over time, these batteries in poor working conditions are likely to be damaged in advance, and the overall life of the battery pack will be greatly shortened.
Not only that, the battery is in a serious overcharged state and there is a danger of explosion, which not only damages the battery pack, but also threatens the safety of the user's life. Therefore, a set of targeted battery management system (Battery Management System, BMS) must be equipped for the power battery pack on the electric vehicle, so as to effectively monitor, protect, balance the energy and alarm the fault of the battery pack, so as to improve the overall power The working efficiency and service life of the battery pack.
What is the main role of BMS?
An electric vehicle has hundreds of cells, how does BMS manage it? If we have seen it, we will see that there are hundreds of cells inside the anatomy of the battery pack. How to manage these dense cell systems? The main work of the BMS system is divided into two major tasks - battery detection and battery safety.
Among them, the implementation of battery detection is relatively simple, mainly by collecting parameter information of the battery during use, such as temperature, voltage and current of each battery cell, and voltage and current of the battery pack. These data play a crucial role in the subsequent management of the battery pack. It can be said that without the support of these battery status data, the battery system management will be impossible.
If we regard the battery testing process as a "physical examination" of the battery, then this "physical examination" is online, continuous and uninterrupted. During the process, when abnormal data is found, the corresponding battery status can be inquired in time, and the battery in question can be selected, so as to maintain the reliability and efficiency of the entire battery group. When the "physical examination" of the battery is over, it will enter the stage of analysis, diagnosis, and calculation, and then generate a "physical examination report". This process can be understood as the status evaluation of the battery.
What is SOC?
If you have driven an electric car, you will definitely see the SOC logo on the dashboard. What does this mean? SOC stands for State of Charge, which is the abbreviation of the state of charge of the battery pack. We are more used to calling it the remaining battery power. SOC is the basis for judging a series of faults such as battery overcharge and overdischarge. Accurate estimation of SOC can prevent battery overcharge and overdischarge, prolong battery life, and improve battery utilization.
In fact, in addition to SOC estimation, there are also SOH (State of Health) and SOP (State of Power). Users can see these data through the on-board instrument display to confirm the working and functional status of the battery. Accordingly, on the basis of protecting the battery, the potential is maximized and the driving experience is greatly improved.
For battery life, the accuracy of SOC is very important
The SOC algorithm has always been one of the key technologies in the development and application of the battery management system (BMS). The accuracy of its calculation directly affects the difference between the displayed battery life and the actual battery life. In case the vehicle breaks down.
Regarding the estimation of the battery state, a series of complex calculations are required. That is to accurately estimate the remaining power of the battery, ensure that the SOC is maintained within a reasonable range, and prevent damage to the battery due to overcharge or overdischarge, so as to predict the remaining energy of the hybrid vehicle energy storage battery or the state of charge of the energy storage battery at any time. The estimation accuracy of SOC is high, and for the same amount of batteries, it can have a higher cruising range. Therefore, high-precision SOC estimation can effectively reduce the required battery cost.
Today, domestic battery manufacturers have mastered precise core algorithms, and through the estimation method based on battery parameters, the influence of accumulated errors can be effectively eliminated, and the estimation is more accurate. The accuracy of NCM estimation (NCM is the abbreviation of lithium battery cathode material, namely nickel cobalt manganese ternary material) is 3%, and the accuracy of LFP (another lithium ion battery cathode material, called lithium iron phosphorus) is about 5%. In terms of data, the technical standards of domestic battery companies have reached the international leading level.
The patron saint of battery and personnel safety
Another core function of BMS is to provide security for battery packs and personnel.
As we all know, overcharging and overdischarging of the battery will cause local overheating, which will affect the life of the battery. In severe cases, it will threaten the safety of the battery pack and cause personal safety hazards. At this time, the "charge and discharge management" module of BMS starts the protection function. On the one hand, it communicates with the whole vehicle and the charger, and on the other hand, it provides the battery status in real time, which is convenient for issuing commands and control in time, and effectively preventing high charge and low discharge. occur.
Balance is also a very important part of protecting the battery module, and it is a necessary means to protect and improve the battery life. In addition, the protection of the battery also includes the protection of overvoltage, undervoltage, overtemperature, and overcurrent. Simply put, when the actual parameter is higher or lower than a certain agreed value, the system will automatically make a judgment and take measures such as disconnection and pre-charging to protect the battery safety.
In terms of personal safety, the BMS is protected by means of high voltage control. The high voltage of the battery can reach 300-500V, far exceeding the safety voltage of 36V for the human body. There are great risks and hidden dangers. High voltage control must be done. The most common ones are relays, high voltage interlocks, and insulation protection. Comprehensive high-voltage protection control can effectively protect the personal safety of drivers, passengers and maintenance personnel.
How to understand the safety level of a battery?
Many domestic battery companies use the internationally accepted ISO26262 evaluation standard. The ISO 26262 standard divides the security requirements from A to D according to the degree of security risk.
(Automotive Safety Integrity Level, ASIL), of which D-level is the highest level and requires the most stringent safety requirements. The higher the level, the higher the security requirements of the system, and the higher the cost to achieve security, which means that the hardware diagnostic coverage is higher, the development process is stricter, the corresponding development cost increases, the development cycle is prolonged, and the technical requirements are higher. strict.
ASILD level, the failure rate is 10^-8/h, which means that a vehicle is assumed to run for 4 hours a day, and it takes 70,000 years to run for 1 functional failure caused by the BMS. And such a low probability of failure rate is comparable to the requirements of aircraft operation. Generally speaking, the requirements for components in the automotive industry are B or C grades.
