Current status, technical difficulties and solutions of cascade utilization of 2200mah 18650 batterys

　　1. The necessity of echelon utilization

　　The equipment that uses the largest amount of 2200mah 18650 batterys is electric vehicles. As the service life of the first batch of electric vehicles gradually approaches, a large number of vehicle lithium battery packs are facing decommissioning and scrapping. The quantity is measured in tens of thousands of tons. Such a huge number of 2200mah 18650 batterys, It is obviously inappropriate to deal with it directly according to the scrapping procedures and processes.

　　This is because not all batteries in retired or scrapped battery packs are in a scrapped state. Due to inconsistency, usually only the batteries of individual units are scrapped. Many batteries in the pack are still in a good life cycle and still have a high level of utilization. value, waste heat can continue to be utilized through appropriate echelon utilization plans.

　　In recent years, calls and research on the cascade utilization of battery packs have continued. This voice is positive and positive, and reflects the further enhancement of environmental awareness and resource reuse awareness. Some companies have already carried out attempts in this regard. .

　　2. Current status of echelon utilization

　　The ideal is full and the reality is cruel. It is a true portrayal of the echelon utilization of decommissioned battery packs. In order to extend the service life and residual value of retired batteries, the state and government encourage enterprises and social forces to carry out cascading utilization of a large number of retired batteries, reduce the number of scrapped batteries and reduce environmental pollution. However, the reality of the utilization of cascading batteries is very cruel, especially their Security Question.

　　Since the consistency problem of battery packs is a technical problem all over the world, there are no efficient, thorough and economical solutions and technologies, making the operational safety of the cascade-used battery packs far lower than that of the original battery packs, and the cascade-used battery packs are much safer than the original battery packs. Safety, cycle life and reuse value cannot be guaranteed. Therefore, the specific application level and market response to the echelon utilization of battery packs are far less than expected. Below are further explanations through several cases.

　　According to reports, 23 serious fires have occurred in South Korea’s energy storage industry since May 2018. On June 11, 2019, the South Korean government officially announced the results of the investigation. Among all 23 energy storage system fire accidents, 14 occurred after charging, 6 occurred during the charging and discharging process, and 3 occurred during installation and construction. . It can be seen from the number of fire accidents that the number and proportion of accidents caused by battery charging and discharging account for the vast majority, which shows how important battery safety management is in the safe operation of battery packs.

　　According to official statistics, at least 40 new energy vehicle fires occurred in my country in 2018. Since the beginning of this year, new energy vehicle fires have continued to occur frequently, with three consecutive fire accidents occurring within four days from April 21 to April 24. At the same time, new energy vehicle recalls also occur frequently, among which the number of recalls due to battery safety increased significantly compared with 2018.

　　Industry insiders believe that battery safety is the most critical factor in new energy vehicle fires. Battery safety management deficiencies, especially consistency management problems, may lead to overheating of the battery pack inside the vehicle during use, posing safety risks of thermal runaway fires. , are difficult problems and issues that need to be overcome and solved.

　　Although 2200mah 18650 batterys have advantages that other secondary batteries do not have, such as high energy density and long cycle life, their weaknesses are also very obvious, especially being very sensitive to charge and discharge current and voltage, and resistant to overcharge and overdischarge. The capacity is very poor, neither overcharging nor over-discharging is allowed, otherwise it will cause irreversible damage to the capacity, performance, and service life of the lithium battery. Therefore, single lithium battery (including multiple 2200mah 18650 batterys in parallel) modules are usually equipped with An independent lithium battery protection board prevents overcharge or over-discharge failure of the lithium battery during use, but this protection method is not suitable for series-connected lithium battery packs.

　　A large number of studies and tests have shown that to solve the problem of safe operation of lithium battery packs, the consistency of 2200mah 18650 batterys and the management of "thermal runaway" caused by them must be solved. The key technology to solve the consistency problem is battery balancing technology, so , the research and development of efficient battery balancing technology is a key topic that needs to be tackled currently and in the future. It is the core technology to ensure the safe operation of the battery pack.

　　3. Difficulties in battery balancing technology

　　In terms of cascade utilization of high-power and large-capacity batteries at home and abroad, they are mainly used in energy storage power stations for peak shaving, followed by communication base stations, gradually replacing traditional lead-acid batteries. In order to obtain the required voltage and power, all 2200mah 18650 batterys in the energy storage battery pack adopt a multi-parallel and multi-string method. The capacity is larger, the number of strings is more, and the safety management is more difficult. Therefore, the requirements for battery balancing are also higher, especially In terms of balancing current and balancing efficiency, it is necessary to solve the problem of fast balancing.

　　The use process of lithium battery pack includes charging period, recovery period after charging, resting period, discharge period, recovery period after discharging, resting period, recharging, and repeated cycles, as shown in Figure 1, which affects the service life of the battery. The two most important links are charging and discharging, which are also the stages most likely to cause battery "thermal runaway" accidents.

　　The most common problem that occurs during the charging period is overcharging. Usually not every unit battery is overcharged, but the battery with the smallest capacity in the group is easily overcharged, and it is overcharged for a long time. The voltage of most batteries It is usually in the normal voltage range, especially at the end of charging, the voltage difference is very obvious; the most likely problem during the discharge period is over-discharge.

　　Similarly, usually not every unit battery is over-discharged, but the battery with the smallest capacity in the group is easily over-discharged. The voltage of most batteries is usually within the normal voltage range, especially at the end of discharge, the voltage difference is very obvious. ; In other periods, charging and discharging behavior no longer occurs, and the voltage difference between small-capacity batteries and other batteries is usually not very obvious, and voltage identification can easily lead to misjudgment. Here we find that the battery with the smallest capacity is easily overcharged and overdischarged. Both states accelerate its decay. We need to focus on solving the problem of overcharging and overdischarging of small-capacity batteries.

　　The capacity fading of 2200mah 18650 batterys has a gradual and cumulative process. If the battery pack is activated, the battery balancing function intervenes and intervenes. Then, the attenuation difference speed of 2200mah 18650 batterys will be reduced due to external factors. Through the active control and intervention of external equalization hardware, the attenuation speed difference of all batteries can be controlled in the same range, that is, equal speed or equal rate attenuation, which can maximize the improvement. The cycle life of the entire battery pack needs to be achieved through battery balancing technology.

　　It is not difficult to find from the schematic diagram of the battery usage period that the charging period and the discharging period are only two links. Therefore, it is not the most ideal solution to balance the battery pack solely by relying on the charging and discharging periods. Even if it can be achieved, the performance requirements for the battery equalizer must be very high, which will bring high costs and make it difficult to popularize.

　　Judging from the actual use of the battery pack, according to the time ratio, the cumulative time of the charging period and the discharging period is far smaller than the sum of the recovery period and the resting period. Therefore, making more use of the recovery period and quiescent period of the battery pack for battery balancing can quickly shorten the difference between attenuated batteries and normal batteries. Therefore, from a practical point of view, practical battery balancing technology should preferably support static balancing. Only Supporting static balancing can reduce the balancing pressure during the charging and discharging periods, and better improve the charging and discharging balancing efficiency of the battery pack.

　　It should be noted that the starting condition for static equalization is that the voltage difference is greater than the set reference voltage difference of the equalization device until it is balanced. In order to prevent the equalizer from performing static equalization endlessly, the equalizer needs to support entering sleep or micro-power consumption after the equalization is completed. Detect status function to reduce unnecessary power loss.

　　At present, the trend of battery PACK modularization is becoming more and more obvious. Battery PACKs can be used to form large battery PACKs with greater capacity and power. Although the lithium battery protection board is installed in the PACK, the cycle life of the battery pack is short. The problem has hardly changed, and safety accidents still occur frequently. The root cause is that the consistency problem has not been solved. It can be seen that the demand for real-time high-speed equalization of battery packs in battery packs is very urgent.

　　When battery pack consistency problems occur, voltage difference characteristics are most obvious and are also commonly used and key quantitative indicators for battery pack consistency testing. From the perspectives of voltage detection, balance control and equipment cost control, by controlling the voltage The method of balancing is the most economical, most effective and easiest to implement solution, and is adopted by the majority of R&D personnel.

　　Based on this, there are three main types of battery balancing technology, namely resistive energy-consuming balancing, charging balancing and transfer battery balancing. Among them, resistive energy-consuming balancing is a typical passive balancing, and the other two types are active balancing. The order from low to high in terms of development cost and balancing efficiency is: resistive energy-consuming balancing < charging balancing < transfer battery balancing.

　　Charge balancing is actually a transitional battery balancing technology, which mainly solves the problems of small balancing current and serious heating of resistive energy-consuming balancing, while transfer battery balancing is the real and practical battery balancing technology, which is battery balancing. Regarding the future development direction of technology, the main performance differences of the three battery balancing technologies are detailed in Table 1.

　　Through comparison, it can be found that although the transferred battery balancing technology has performance advantages, it also has many weaknesses, such as high cost, complex technology, and difficulty in implementation. These are all problems that current research and development needs to face. It requires the unremitting efforts of the majority of scientific and technological workers. Efforts will be made to further carry out technical research to solve the problem.

　　4. Difficulties in the research and development of real-time battery balancing technology

　　Research and development based on real-time battery balancing technology is very difficult. The research and development of this kind of battery equalizer on the market is progressing very slowly. There are very few R&D institutions and companies that have mastered this technology. In particular, the efficient transfer type real-time battery balancing technology has not been available on the market. The sales and popularity of corresponding products can prove that there are only a few types of battery equalizers on sale, but due to cost reasons, sales are not ideal.

　　In the eyes of many people, transfer-type real-time battery balancing technology is almost an impossible technology. In fact, its research and development is more difficult than many people imagine. There are many technical difficulties that need to be overcome, and one technical difficulty is often overcome. New technical difficulties have emerged, and some technical difficulties are interfering and interfering with each other. Finding a technical solution that meets the requirements of multiple indicators requires a lot of calculations and experiments, and it is very difficult to determine the design solution. .

　　Even a normal parameter adjustment will lead to system disorder and abnormal work. Many R&D institutions, groups and companies in the society that were full of confidence at the beginning were mostly because of the difficulty of R&D, the long R&D cycle, and the high cost of equipment. After investing a lot of money in R&D to no avail, it chose to give up R&D.

　　Really practical battery balancing should have the characteristics of moderate cost, high power conversion efficiency, fast balancing speed, high voltage control accuracy, and real-time balancing. These unique index requirements actually form a contradiction between cost control and performance requirements, and must Solved through economical and reasonable software and hardware design.

　　5. Examples and Analysis

　　In order to achieve the above balancing goals, the author spent many years developing efficient real-time battery balancing technology, which successfully solved the conflict between cost and performance. He has carried out and completed balancing application experiments on various lithium battery packs with serious imbalances in consistency, achieving the design index. Through continuous charge and discharge equalization experiments on up to 13 strings of 18650 model echelon lithium battery packs, all performance indicators have met the expected requirements.

　　During the experiment, the minimum capacity battery did not suffer from overcharge or overdischarge problems, and all battery capacities were maximized. In terms of the performance of the entire group, both the charging capacity and the discharge capacity far exceeded the capacity of the worst battery in the group. During the process, the voltages of all batteries are within safe values. Especially at the end of charging and discharging, the voltage difference is always very small. The temperature rise of all batteries is within a reasonable range. The temperature rise of the worst battery is the lowest. This kind of The performance of temperature rise is very beneficial for controlling "thermal runaway".

　　This article uses the standard discharge and balanced discharge of 13 series of 18650 model cascade lithium battery packs as an example to compare and explain. The actual capacities and internal resistances of the 13 2200mah 18650 batterys are very different. The discharge standard is 1A constant current discharge. When the total voltage Discharge to 39.0V or stop discharging when the discharge voltage of any battery drops to 3.0V. To ensure a fair comparison of discharges, each battery is charged to the same voltage through equalization charging mode before each discharge.

　　In the conventional discharge mode, the effective safe discharge time is 35 minutes, and the 10# battery reaches the discharge termination voltage of 3.0V. As shown in Figure 2, the voltage of other batteries is generally higher, and the voltage consistency is very poor. The maximum voltage difference It reaches 0.581V, which is very serious. The total discharge voltage is still as high as 44.876V, which is much higher than the specified 39.0V. The average voltage is as high as 3.452V, which is much higher than the average discharge cut-off voltage. Judging from the remaining average voltage, there is still more electric energy. It is not utilized, a lot of capacity is idle, the capacity utilization rate is low, and there is serious waste;

　　When a high-efficiency battery equalizer is used throughout the process, under the same discharge standard, the effective safe discharge time is extended to 58 minutes, which is 1.66 times the standard discharge capacity and time. At this time, the voltage of the 10# battery is still as high as 3.0 V or above, as shown in Figure 3, the discharge voltage of other batteries is also close to 3.0V, and the voltage consistency is very good. The maximum voltage difference is only 70mv, which is basically normal. The total discharge voltage is 39.377V, close to 39.0V, average The voltage is only 3.029V, which is very close to the average discharge cut-off voltage of 3.0V, and the effective power is basically released. The reason why the voltage of the 10# battery is not the lowest is related to the different discharge curves of the experimental ladder batteries. The most important thing is the intervention of a high-efficiency battery equalizer, which fully adjusts and redistributes the power of other batteries, and the battery's power is fully use.

　　For the convenience of comparison, a comparison table of voltages at the end of the two discharge modes was made, as shown in Table 2, and a comparison chart of the voltage consistency at the end of the two discharge modes, as shown in Figure 4. The high-efficiency battery equalizer used in this article supports BMS joint control, which can enable and disable the equalization function on demand. It can control a unit independently or the entire equalizer group.

　　The comparison of experimental results under the two discharge modes fully demonstrates that the role and effect of balanced discharge are very obvious. Balanced discharge not only realizes the synchronous decrease of battery voltages of different capacities, prevents over-discharge of small-capacity batteries, prevents thermal runaway failures, and improves the cycle life of the battery pack, but more importantly, achieves safe and full utilization of different battery capacities. , improve the average capacity utilization of the battery pack and stabilize the battery life.

　　The transfer-type real-time battery equalizer described in this article efficiently supports static equalization and charge equalization. Due to space limitations, the corresponding comparative experimental screenshots and related data analysis are omitted.

　　6. Conclusion

　　The problems encountered in the echelon utilization of 2200mah 18650 batterys are not limited to decommissioning where consistency is difficult to control during grouping.

　　The echelon utilization of battery packs also includes standard battery packs with good consistency during assembly. For the cascade utilization of 2200mah 18650 batterys, the most important thing to pay attention to and solve is the operational safety of the battery pack. Without the premise of safety, cascade utilization is impossible.

　　The biggest threat to safety is the consistency problem and the "thermal runaway" caused by it. Effective battery balancing technology is the only option at present, and the transfer real-time battery balancing technology has the best performance, but it still cannot eliminate battery attenuation. It just optimizes and adjusts the attenuation speeds of different batteries so that they have approximately the same attenuation rate and synchronous attenuation, achieving long-life and safe operation of the battery pack through efficient balance management.

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