Demystifying the battery's brain BMS battery management system

　　As we all know, the power output of pure electric vehicles depends on the battery, and the battery management system BMS is the core of it, responsible for controlling the charging and discharging of the battery and realizing functions such as battery state estimation.

　　As we all know, the power output of pure electric vehicles relies on batteries, and the battery management system BMS is the core of it, responsible for controlling the charging and discharging of the battery and realizing functions such as battery state estimation. If one compares an electric car to a human body, then the battery system is his heart, and the BMS battery management system is the brain that governs the functioning of its body.

　　Why is there 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 difference between the individual cells, there are still differences in internal resistance, capacity, and voltage between single-cell lithium batteries. Therefore, in practical applications, the individual cells in the battery pack 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 is greatly shortened.

　　Not only that, there is also a danger of explosion when the battery is in a severely overcharged state, which causes damage to the battery pack and also threatens the safety of the user. Therefore, it is necessary to equip the power battery pack on electric vehicles with a set of targeted battery management system (Battery Management System, BMS), so as to effectively monitor, protect, energy balance and fault alarm of the battery pack, thereby improving the entire power battery pack Work efficiency and service life.

　　What is the main function of BMS?

　　An electric car has hundreds of batteries, how is BMS managed? If we have seen the analysis diagram of the battery pack, we will see that there are hundreds of cells inside. 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.

　　Battery detection is relatively simple. It mainly collects parameter information of the battery during use through sensors, such as temperature, voltage and current of each battery cell, and voltage and current of the battery pack. These data will play a vital role in the subsequent battery pack management. It can be said that without these battery status data as support, 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. In the process, when data is found to be abnormal, the corresponding battery status can be inquired in time, and the problematic battery can be selected, so as to maintain the reliability and efficiency of the entire battery pack. When the "physical examination" of the battery is over, it will enter the stage of analysis, diagnosis, and calculation, and then a "physical examination report" will be generated. This process can be understood as the battery state assessment.

　　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 StateofCharge, which is the abbreviation of the state of charge of the battery pack. We are more accustomed 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, extend battery life, and improve battery utilization.

　　In fact, in addition to SOC estimation, there are SOH (State of Health) and SOP (State of Power). Users can view 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 will be maximized and the driving experience will be greatly improved.

　　For battery life, SOC accuracy is very important

　　SOC algorithm has always been one of the key technologies for the development and application of battery management system (BMS). The accuracy of its calculation directly affects the difference between battery life and actual battery life. If the calculation is not accurate enough, it may even cause battery usage. As long as the vehicle broke down.

　　The estimation of the battery state requires a series of complicated calculations. That is to accurately estimate the remaining battery power, ensure that the SOC is maintained within a reasonable range, and prevent damage to the battery due to overcharging or overdischarging, so as to predict how much energy is left in the energy storage battery of the hybrid electric vehicle 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 battery, it can have a higher cruising range. Therefore, high-precision SOC estimation can effectively reduce the required battery cost.

　　Nowadays, domestic battery manufacturers have mastered the accurate core algorithm. Through the estimation method based on battery parameters, the effect of cumulative error is effectively eliminated, and the estimation is more accurate. NCM estimates (NCM is the abbreviation of lithium battery cathode material, that is, nickel-cobalt-manganese ternary material) accuracy is 3%, and LFP (another cathode material of lithium-ion battery, called lithium iron phosphorus) is about 5%. From the data point of view, 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 safety protection for battery packs and personnel.

　　It is well known that overcharging and overdischarging batteries will cause local overheating, which will affect the battery life, but in serious 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 the BMS starts the protection function. On the one hand, it communicates with the vehicle and the charger, and on the other hand, it provides the battery status in real time, which facilitates timely command control and effectively prevents high-charge and low-discharge. happen.

　　In the protection of the battery module, balance is also a very important part, and it is a necessary means to protect and enhance the battery life. In addition, the protection of the battery also includes the protection of over-voltage, under-voltage, over-temperature, over-current, etc. To put it simply, when the actual parameter is higher or lower than a certain agreed value, the system will automatically make a judgment and adopt methods such as disconnection and pre-charging to protect the safety of the battery.

　　In terms of personal safety, BMS is protected by means of high-voltage control. The high voltage of the battery can reach 300-500V, which is far exceeding the safety voltage of 36V for the human body. There are great risks and 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 batteries

　　Many domestic battery companies adopt the internationally accepted ISO26262 evaluation standard. The ISO26262 standard divides the safety requirement level (AutomotiveSafetyIntegrityLevel, ASIL) from A to D according to the degree of safety risk, of which D level is the highest level and requires the most demanding 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 higher the diagnostic coverage of the hardware, the stricter the development process, the corresponding increase in development costs, the longer the development cycle, and the greater the technical requirements. strict.

　　ASILD level, the failure rate is 10^-8/h, which means that a vehicle is supposed to run for 4 hours a day and needs to run for 70,000 years before a functional failure caused by BMS occurs. And such a low probability of failure rate is comparable to the requirements of aircraft operation. Generally speaking, the automotive industry's requirements for parts are B or C grades.

　　Many domestic battery companies have already gone abroad to provide products and technical support for more international brands. This shows that domestic battery and BMS technology does not lag behind Japan, Europe and the United States, and has even reached the world's advanced level in some areas.