System "balancing" technology is not a new concept. "Balancing" technology is born from the series connection of batteries.

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System "balancing" technology is not a new concept. "Balancing" technology is born from the series connection of batteries. [Copy link]

  "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 is: use power-type 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, which is to ensure the safety of the battery and reduce the difference between single cells.

Balancing technology was born out of 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 in series with a nominal single cell of 3.7V. Due to differences in production processes, such as production date, environmental differences, electrode thickness, area, electrolyte filling amount, etc., 12~13 individual cells cannot be exactly the same, and can only meet the deviation requirements within a certain range. Therefore, the differences in their intrinsic physical quantities are not difficult to understand. However, the current flowing through the series circuit is the same during charging and discharging. If r is used to represent the internal resistance of a cell, then the difference in internal resistance will cause the voltage of the single cell to be different. When charging, the cell with large internal resistance will be filled in advance or close to the upper limit voltage. At this time, in order to prevent overcharge 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 a single cell.

In other words, balance is to make each single cell parameter close to the average value of a group of cells. Voltage is one of the intrinsic parameters of the battery. In many cases, the most direct form of balance is voltage balance. Before balance, the individual cells connected in series show voltage inconsistency. Reducing the voltage difference to a specified value is another form of balance, and it is also the function that needs to be achieved in the end.

Due to the difference of series monomers, the upper limit of SOC can only be close to 100%

Theoretically, if the monomer 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 proposing technical requirements, it can only be a value close to 100%. 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 battery body and the level of system integration.

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 improve 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 balancing 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 the balancing circuit function is available, it will monitor the voltage difference and the upper limit voltage status in real time, and start the balancing function in time, pull it up and then it will be low. In fact, it is equivalent to extending the system charging time. The capacity charged is naturally greater than the circuit without balancing 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 balancing 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 balancing function is added, the SOC range may increase from 5% to 95%, increasing the usable value percentage 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.

So, the balancing function is more about changing which board is the shortest in the "barrel" principle.

Change the DOD depth to make the battery more efficient

From the above analysis, the application of the balancing circuit can well expand the SOC application range. For example, 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 depend entirely on the balance. There are many other factors, such as: the SOC range buffer caused by electrochemical factors, the SOC buffer 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. Comprehensive evaluation is needed. We sometimes set the lower limit based on experience, and one step is 5%, 10%. It is a great waste of battery efficiency if the SOC boundary is not calibrated according to the inherent characteristics of the battery cell and the ambient temperature, and it is not carefully calculated.

Balancing will not change the inherent nature of the battery cell

The previous section focused on the essence of balancing. Let's look back at the extent of its 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 potentially corresponds to the calculated power and capacity. There have always been different opinions and controversies on the role of the balancing function in improving the state of the battery system. However, one thing is certain: adding a balancing function cannot change the inherent characteristics of individual battery cells. However, whether it can slow down the attenuation of battery cell performance needs further research and use of data to prove it. Therefore, it is incorrect to blindly exaggerate the role of the balancing function.

Different applications and understandings of the balancing function of 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 100~200mA, and such a small current balance has little effect. Some people attribute it to the fact that the high quality of the battery cells does not require large current balancing. On the other hand, they are very cautious in designing the balancing hardware circuit, for example, using chips exclusive to themselves and their products, comprehensive heat dissipation and spacing design; and sufficient redundant design.

In short, I think the core of its design is still completed from the perspective of system safety. High-quality battery cells 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 failures can lead to serious consequences. Therefore, do not take care of the quality of the battery cells and ignore safety. This will also mislead the development direction of balancing.

Balancing technology is not a lifeline for low-quality batteries

[p=30, 2,I have seen some BMS manufacturers desperately amplify the balancing current to cater to the application of some low-quality batteries in China. In fact, this is not 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. This 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 a safer system; secondly, it can promote the increase of chargeable and dischargeable capacity. Its advantages can be better reflected in EV applications with deep charging and deep discharging. As the development of new energy deepens, there are more technical issues that need to be studied and analyzed, such as: the optimal entry point for the balanced working point; how to accurately capture and judge the battery status; how to effectively reduce the individual battery attenuation rate, etc., which are all issues that need to be continuously solved.

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