"Battery balancing" is not a new term. It has been used on nickel-cadmium batteries for at least 30 years. It's just that the early technology was simple and inefficient. Its principle and method was: use power resistors to reduce the voltage of the highest battery cell in the series connection in the form of energy consumption to reduce the voltage difference; at the same time, it can prevent accidents caused by overcharging. Compared with today's technological development, there is no comparability in hardware and software management. But in terms of its mechanism, it is the same: to ensure the safety of the battery and reduce the differences between single cells. "Balance" technology is born from battery series connection The application of battery cells in the system is mainly to obtain high voltage in series to meet the load requirements. For example, the voltage of a rated 48V power supply is the voltage superposition effect of 12~13 single cells with a nominal voltage of 3.7V in series. Due to the differences in production processes, such as production date, environmental differences, electrode thickness, area, electrolyte filling amount, etc., the 12~13 individual cells cannot be exactly the same, and can only meet the deviation requirements within a certain range. Therefore, the difference of its intrinsic physical quantity is not difficult to understand. However, the current flowing through its series circuit is the same during charging and discharging. If r is used to represent the internal resistance of a battery cell, then the difference in internal resistance will cause the voltage of the single battery cell to be different. When charging, the battery with large internal resistance will be filled in advance or close to the upper limit voltage. At this time, in order to prevent overcharging damage or catastrophic results, it is necessary to pull down the highest single cell voltage to approach or maintain it within the upper limit value. This is a form of balancing, and it is also the prototype and basis of balancing technology. This technology was used in the earliest 2010 version of leaf. I often attribute it to "protective balancing". The reason is that its main function is to prevent overcharging.
"Balance" is the process of achieving or converging the intrinsic characteristics of the single cell In other words, balancing is to make each single cell parameter close to the average value of a group of batteries. Voltage is one of the intrinsic parameters of the battery. In many cases, the most direct form of balancing is voltage balancing. Before balancing, the individual cells in series show voltage inconsistency. Reducing the voltage difference to a specified value is another form of balancing and is also the function that needs to be achieved in the end.
Due to the difference between the cells in series, the upper limit of SOC can only be close to 100% Theoretically, if the cell voltage is calculated at the same value, its state of charge SOC should be 100%, U=V1×n. However, in real applications, SOC is impossible to be full. This requires the understanding of technicians. During the design process, when technical requirements are proposed, only values close to 100% can be used. For example, the upper limit SOC is 95%, which is reasonable. The determination of this data needs to be obtained through experiments based on the characteristics of the cell body and the integration level of the system. If it is some indicators facing customers, there is indeed a SOC 100% valuation, such as in the instrument display, but this is a practice for better understanding by customers. It does not completely represent the real data. How to increase the available capacity of the system through "balancing"? This question needs to be discussed from two aspects: On the one hand, from the perspective of charging, when there is no equalization circuit function, in the series connection, when the voltage of a single cell reaches the upper limit, the monitoring circuit reports and starts the control circuit to stop charging; when there is an equalization circuit function, the voltage difference and the upper limit voltage state will be monitored in real time, and the equalization function will be started in time, and the voltage will be pulled up and then down. In fact, it is equivalent to extending the system charging time. The capacity charged is naturally greater than that of the circuit without the equalization function. Its essence: the chargeable capacity is increased by controlling the voltage difference value and the upper limit value.
On the other hand, from the perspective of the usable range of SOC, when there is no equalization function, the elastic range at both ends of SOC is very "thick", and the virtual value is very large. In order to ensure safety, the battery is usually used in the SOC range of 20% to 80%, and only 60% is provided to the load. If the equalization function is added, the SOC range may be increased from 5% to 95%, and the percentage of usable values will be increased to 90%. In fact, it increases the usable part of the battery. Its essence is to change the DOD (depth of charge and discharge). Of course, the SOC window boundary value will vary depending on the battery cell and BMS. Don't stick to the case value. Therefore, the balancing function is more about changing which board is the shortest in the "barrel" principle. Changing the DOD depth makes the battery more efficient From the above analysis, the application of the balancing circuit can well expand the SOC application range. If a system of 10KWh can increase the utilization rate by 10%, the contribution of cost, weight and energy density are all considerable. Of course, the available part of the SOC range does not entirely depend on balancing. There are many other factors, such as: the SOC buffer zone caused by electrochemical factors, the SOC buffer zone caused by SOC algorithm estimation, etc., which are all reasons for the narrowing of the available range. Therefore, the battery system has the saying of "cutting off the head and tail". However, the contribution of the balancing circuit must exist in it. A comprehensive evaluation is required. We sometimes set lower limits based on experience, with each step being 5% or 10%. We do not calibrate the SOC boundary according to the inherent characteristics of the battery cell, ambient temperature, etc., and do not make careful calculations. This is a great waste of battery efficiency. Balance does not change the inherent nature of the battery cell The previous section focused on the essence of balance. Let's look back at the extent of the impact on the battery cell. The intrinsic nature of the battery cell is mainly reflected in the changes in internal resistance and voltage, which can be directly measured, and the potential corresponding to the calculated power and capacity. There have always been different opinions and disputes on the role of the balance function in improving the state of the battery system. However, one thing is certain: adding a balance function cannot change the inherent characteristics of individual batteries. However, whether it can slow down the attenuation of battery performance needs further research and use of data to prove it. Therefore, it is incorrect to blindly exaggerate the role of the balance function. Different applications and understandings of balancing functions in domestic and foreign products When analyzing foreign models such as Leaf, Volt, and BMW, we found that their balancing functions deviate from our understanding: on the one hand, from the perspective of balancing hardware, their balancing current is between 100 and 200 mA. Such a small current balance has little effect. Some people attribute it to the fact that high-quality batteries do not require large current balancing. On the other hand, they are very cautious in designing balancing hardware circuits. For example, they use chips that are exclusive to themselves and their products, and have comprehensive heat dissipation and spacing designs; sufficient redundant designs. In short, the core of the design is still completed from the perspective of system safety. High-quality batteries can indeed simplify the balancing circuit and effectively guarantee the safety and reliability of the system. At the same time, the essence of balancing is the internal circulation of energy within the loop. The occurrence of faults can lead to serious consequences. Therefore, do not take care of the quality of the battery and ignore safety. This will also mislead the development direction of balancing. Balancing technology is not a life-saving straw for low-quality batteries Some BMS manufacturers are desperately amplifying the balancing current to cater to the application of some low-quality batteries in China. In fact, this is not really the selling point and original intention of BMS manufacturers, but more of their helplessness, because they also have to survive in the domestic market. What needs to be reflected on is the rigor of the technical requirements put forward by the vehicle manufacturers and the quality of the battery cells provided by the battery manufacturers. There is still a big gap between domestic batteries and foreign batteries, which is a consensus. Therefore, it is urgent to improve the quality of domestic battery cells. Not only does it require good consistency in the front section, but the key is that the consistency in the back section also needs to be strong. Assisted by efficient balancing, this is a reasonable and correct development idea. Further research and exploration System balancing can first ensure that the system is safer; secondly, it can promote the increase of chargeable and dischargeable capacity. Its advantages can be better reflected in deep charging and deep discharging EV applications. With the deepening of new energy development, there are more technical issues that need to be studied and analyzed, such as: the optimal entry point of the balancing working point; how to accurately capture and judge the battery status; how to effectively reduce the individual battery attenuation rate, etc., which are all problems that need to be continuously solved.
提升卡
变色卡
千斤顶
京公网安备 11010802033920号
