Under the influence of the consistency problem, the actual discharge capacity of the battery pack depends on the battery with the smallest capacity in the battery pack. The greater the number of battery packs connected in series, the greater the impact on the discharge capacity of the battery pack, and the lower the utilization rate of the battery pack. It not only affects the charge and discharge capacity and battery life, but also easily causes thermal runaway and other faults, especially for high-power power and energy storage battery packs. The intervention of a real-time, high-efficiency battery equalizer not only intelligently adjusts the charge and discharge current and charge and discharge rate of batteries of different capacities, but also significantly improves the capacity utilization of the battery, and the effect of controlling the attenuation of the temperature rise of the battery is also very obvious.

　　Under the influence of the consistency problem, the actual discharge capacity of the battery pack depends on the battery with the smallest capacity in the battery pack. The greater the number of battery packs connected in series, the greater the impact on the discharge capacity of the battery pack, and the lower the utilization rate of the battery pack. It not only affects the charge and discharge capacity and battery life, but also easily causes thermal runaway and other faults, especially for high-power power and energy storage battery packs. The intervention of a real-time, high-efficiency battery equalizer not only intelligently adjusts the charge and discharge current and charge and discharge rate of batteries of different capacities, but also significantly improves the capacity utilization of the battery, and the effect of controlling the attenuation of the temperature rise of the battery is also very obvious. This article fully proves the importance of a high-efficiency battery equalizer in stabilizing the battery life and capacity by comparing and analyzing the experimental data of conventional discharge and equalization discharge of a set of 13 strings of 48-volt scrapped lithium batteries with seriously deteriorated consistency.

　　Keywords: consistency, balanced discharge, equal rate, thermal runaway

　　1 Causes of battery pack consistency problems

　　Ideally, the battery pack should have the following characteristics: when charging or discharging, the voltages of all batteries rise or fall simultaneously, and the capacity, voltage, self-discharge rate and internal resistance between batteries are very close, that is, the performance of all batteries is basically the same and consistent. The performance is very good, all batteries can be fully charged or discharged almost at the same time, and there will be no problem of overcharge or overdischarge of the battery.

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　　However, in reality, most battery packs perform very poorly and have prominent consistency issues. The cycle life of the battery pack is usually only 1/3 to 1/5 of the design life of a single battery, which greatly affects the service life and endurance of the device. In serious cases, thermal runaway failure may occur, causing damage to equipment or personnel.

　　Through a large amount of experimental research and operation data analysis, it can be found that there are two main reasons for consistency problems in conventional battery packs: One reason is caused by differences in battery production processes, referred to as internal causes. Once the battery packaging is completed, the differences between batteries There are differences in capacity, self-discharge rate and internal resistance parameters, but the degree of difference is different.

　　The second reason can be called external factors, which are mainly caused by differences and fluctuations in charging and discharging voltage parameters, current parameters, and ambient temperature. These external factors will gradually accumulate and amplify the differences caused by internal factors, and the amplification of this difference will It exhibits exponential amplification characteristics, which is why consistency problems in the battery pack will quickly worsen once they occur.

　　2 Common Ways to Solve Consistency Causes

　　In terms of solving the consistency problem of the battery pack, there are two main technical solutions based on the main reasons for the battery consistency problem. One solution is to make a fuss about the battery production process and improve the battery by improving the production process level. Consistency at the factory, this solution has a certain effect and can slow down and delay the occurrence of consistency problems to the greatest extent, but it cannot be eradicated;

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　　Another solution is to use a battery equalizer to intervene. Battery balancing includes passive balancing and active balancing. Passive balancing is also called energy consumption balancing. The balancing current is small and the balancing efficiency is zero. It is only suitable for applications with good consistency and uniform heat dissipation. , and the battery pack capacity is small; the typical representative of active balancing is transfer battery balancing, the balancing efficiency and balancing current are much higher than passive balancing.

　　It is foreseeable that even if active balancing is the mainstream of future development, the design architecture and implementation methods are diverse. This article will not discuss it, but one thing is certain. All design goals are to support larger balancing currents and balanced Develop in the direction of high efficiency and balanced speed.

　　3Real-time, high-speed battery balancing technology and shunt characteristics

　　In battery balancing technology, the most difficult thing to solve is the matching and balancing of balancing current and balancing efficiency. It must be able to provide a larger balancing current and have a higher balancing efficiency. The reason for this requirement is mainly because of the existence of The balancing equipment generates heat under high current balancing conditions and affects the temperature rise of the battery pack.

　　In order to solve this contradictory problem, the author has developed a unique bidirectional synchronous rectification technology [1] after years of research, which not only supports large current balancing, but also has high balancing efficiency. Under full load operation, the temperature of the equipment is reduced. The temperature rise is also low and will hardly increase the temperature rise of the battery pack.

　　Reasonable distribution and optimization of power is achieved through high-speed voltage equalization [2]. In terms of discharge equalization, this technology automatically analyzes and determines the capacity of the battery by detecting the relative voltage difference between adjacent batteries in real time, and automatically adjusts the battery capacity. Batteries with high voltage (high voltage during discharge usually have large capacity) increase the discharge current, and the increased discharge current is transported to both ends of the low-voltage battery through efficient conversion by the equalizer. For low-voltage batteries (low voltage during discharge usually has small capacity) batteries Reduce the discharge current to make up for the lack of discharge capacity of small-capacity batteries, so that batteries with different capacities can be discharged at approximately the same rate.

　　In terms of charge balancing, the charging current is automatically reduced for batteries with high voltage (high voltage during charging means small capacity). The reduced charging current is efficiently converted by the equalizer and transported to both ends of the low-voltage battery. Low voltage during the period means large capacity) The battery increases the charging current, so that batteries with different capacities can be charged at approximately the same rate; this technology can also support high-speed static equalization at the same time, increasing the effective equalization time. The unique pulse technology is very useful for stably attenuating battery capacity. Obviously, the battery equalizer used in the example of this article adopts the newly developed bidirectional synchronous rectification technology [3].

　　4 Thirteen-string lithium battery pack discharge experiment

　　The experimental battery pack is shown in Figure 1. It is assembled from scrapped lithium battery packs after disassembly, selection and echelon utilization. They are all 18650 model lithium batteries. The longest storage time is more than 8 years. The initial voltage of the disassembled battery is only The voltage ranges from a few tenths of a volt to a few tenths of a volt, and most of them are in a scrapped state. The original design capacity of a single cell is between 2200mAh and 2500mAh. Most of the batteries have heavy leakage and generate serious heat during charging.

　　When the capacity is tested for 2 hours after being fully charged, the actual remaining capacity of the 1A discharge test is only between 550mAh and 2350mAh. See the remaining capacity column in Table 1. It can be seen from the detected capacity that the capacity difference is very large, with the maximum difference reaching 1.8Ah, there are 13 experimental batteries in total. The remaining power histogram of all batteries is shown in Figure 2.

　　The battery with the words "meter power supply" on the far right side of the experimental bench is only responsible for powering the high-precision voltmeters under the 7# and 13# batteries, and does not participate in the charge and discharge experiment. The other voltmeters are powered in cascade mode. The bottom of each battery corresponds to a high-precision voltmeter, which displays the current voltage of the upper battery in real time. The experimental platform (the platform has been modified to facilitate battery replacement) supports up to 2 and 14 strings of 18650 battery experiments. The example in this article only connected 13 strings. Battery, nominal voltage 48 volts.

　　4.1 Conventional discharge experiment

　　First use a battery equalizer to charge the 13-string battery pack (the voltages in the attached table still have a certain voltage difference, mainly due to leakage of most batteries and large leakage current, the same below). When the charging current of the charger no longer When it drops, it is considered to be fully charged, and then the battery pack is discharged at a constant current of 1A through the electronic load. When the discharge voltage of any battery drops to 3.00V, the discharge stops.

　　During this period, the current voltage of each battery is recorded every 10 minutes until a certain battery is discharged and stops discharging, and the actual total discharge time is recorded. The measured data during the discharge period are shown in Table 1 (organized according to the real-time recording video, and the shading color represents The highest and lowest voltage in the group, the same below), 6# battery capacity is the smallest, the corresponding battery voltage at the end of discharge is shown in Figure 3, the remaining power histogram of each battery at the end of conventional discharge is shown in Figure 4, conventional The remaining voltage curve of each battery at the end of discharge is shown in Figure 5.

　　Table 113 Conventional discharge data table of lithium battery pack

　　It can be seen from the discharge measurement data that when the discharge reaches 33 minutes, the 6# battery reaches the discharge cut-off voltage and stops discharging. At this time, the 2# battery is about to be discharged, but the other 11 batteries still have more power. Released, the voltage at the end of discharge speaks for itself, especially when batteries 1# and 7# still have a lot of power left that has not been released and cannot be used. This situation is especially like installing a BMS battery management system with single battery discharge protection. System power battery pack.

　　Although the 6# battery is discharge protected and will not over-discharge, the capacity of the vast majority of batteries in the battery pack has not been fully utilized and released, resulting in a serious waste of capacity; in addition, the maximum voltage difference data measured every ten minutes can also be It was found that as the discharge progressed, the maximum voltage difference gradually increased.

　　This means that the consistency performance of the battery is getting worse and worse, and the consistency problem is getting more and more serious. In addition, the measurement data shows that the 2# battery is about to be discharged, indicating that the 2# battery in the battery pack is also attenuated very seriously. There is another phenomenon during the discharge period. In the early stage of discharge, the voltage of the 6# battery is not the lowest, but reaches The voltage is at its lowest in the middle and late stages and continues until the end of discharge.

　　When the discharge was stopped, the voltages of all batteries began to rebound normally, but the voltage rebound of 2# and 6# batteries was much faster than that of other batteries, and soon rose to about 3.9V, further confirming the attenuation of batteries 2# and 6#. very serious.

　　4.2 Equilibrium discharge experiment

　　Various applications and practices have shown that the actual significance of balanced discharge is greater than balanced charging. Balanced discharge can reflect the actual available capacity of the battery pack. Both theoretically and practically, the balanced discharge capacity of the battery pack is greater than the conventional discharge capacity, especially For battery packs with consistency problems, the more serious the consistency problem, the greater the difference in actual discharge capacity.

　　The purpose of balanced discharge is to allow most of the battery capacity that is higher than the average capacity to be exerted and released, and to increase the overall discharge time. During this period, it must also be ensured that all batteries can be safely discharged, and no battery will be over-discharged. . Before equalizing discharge, use the same method as before to fully charge the battery pack. The discharge method, ambient temperature, and data recording method remain unchanged.

　　Similarly, when the voltage of any battery is discharged to 3.00V, the discharge is stopped. The only difference is that the battery equalizer in this article is connected throughout the equalization discharge period. The measured data related to the equalization discharge of each battery are shown in Table 2. At the end of the equalization discharge, each The remaining voltage of the battery is shown in Figure 6, and the corresponding remaining voltage curve of each battery at the end of equalization discharge is shown in Figure 7.

　　Table 213 String lithium battery pack equalization discharge data table

　　4.3 Equilibrium discharge cycle expansion experiment

　　Based on the very satisfactory experimental data obtained from the first equalization discharge, in order to verify whether the equalization discharge has universal characteristics, the author continued to equalize the experimental battery pack more than a hundred times while maintaining the equalizer prototype. Charge and discharge cycle experiments. The cycle experiment results show that after the intervention of a high-efficiency equalizer, the safe discharge time and discharge capacity of the battery pack are very close, significantly better than ordinary charge and discharge. The temperature rise of the severely attenuated 2# and 6# batteries is similar to that of the battery pack. Other batteries are basically the same or even slightly lower. The reduction of temperature rise is of positive significance in preventing thermal runaway.

　　5. Comparative analysis of discharge experiments

　　The initial conditions of the two discharge methods are basically the same, but the discharge results are very different. The only difference is that the connection of the equalizer is maintained during the equalization discharge. The following is a comparison of the voltage performance and actual discharge time of the attenuated battery in the two discharge methods. Analyze by comparison.

　　5.1 Analysis of voltage performance of attenuated batteries

　　In the conventional discharge experiment, although the voltage of the 6# battery was at a high level in the early stage of discharge, after 10 minutes of discharge, the voltage began to be at the lowest state until the end of the discharge. The voltage drop rate of the 2# battery followed closely, and also showed Severely attenuated state. The voltage of the 1# and 7# batteries with the least attenuation is always at the highest state.

　　In the equalization discharge experiment, before the end of the discharge, the voltage has been at a relatively lowest level, but it is the 11# battery, not the 6# battery, and also not the 2# battery. The main reason is that the intervention of the equalizer automatically changes the voltage of each battery. The actual discharge current of the 2# and 6# batteries is significantly reduced. In the conventional discharge experiment, the voltage of the 1# and 7# batteries is always at the highest state until the end of the discharge, showing the capacity of these two batteries. maximum.

　　Since they are close to the 2# and 6# batteries, during the equalization discharge period, they automatically provide a large amount of power to the severely attenuated 2# and 6# batteries. While increasing the discharge current, they reduce the actual power of the 2# and 6# batteries. The output current and voltage drop speed, and the voltage of the entire battery pack appears to drop approximately synchronously.

　　By comparing Figure 5 and Figure 7, it can be clearly found that at the end of equalization discharge, the voltage consistency of the battery is significantly better than that of conventional discharge. This improvement in consistency lasts throughout the entire discharge period of the battery pack, which is very beneficial. Safe and efficient operation of battery packs.

　　5.2 Analysis of maximum voltage difference changes

　　In conventional discharge, before discharge, the maximum voltage difference between batteries is only 0.026V, which has good consistency, and the overall voltage is slightly higher than the initial voltage of the balanced discharge experiment. As the discharge proceeds, the maximum voltage difference gradually expands until the discharge At 30 minutes, the maximum voltage difference has expanded to 0.770V. At this time, the remaining capacity of the battery pack is only about 10%, and the consistency performance is very poor.

　　In equalization discharge, the maximum initial voltage difference before discharge is 0.034V, and the overall voltage is slightly lower than the initial voltage of conventional discharge. In this unfavorable situation, the intervention of the equalizer completely changes the subsequent discharge state. Before the end, the maximum voltage difference is always in a small state, which is far lower than the maximum voltage difference of conventional discharge, and is in a gradually shrinking state. It is completely opposite to conventional discharge. It only starts to rise slowly at the end of discharge, but it rises The margin is small.

　　The main reason is that at the end of discharge, the dischargeable capacity of the battery drops sharply. Even if the battery is not degraded, the remaining available power is very small, and the equalizer cannot quickly provide more power to the degraded battery. It can be seen from past experimental data that for lithium-ion batteries, the discharge time is around 1C.At high rate, when the battery voltage drops below 3.20V, the remaining capacity is usually less than 5%.

　　If the releasable power is retained at 5%, the maximum voltage difference is less than 0.1V, and the equalizer can still perform high-speed equalization function normally. If the protective power is set at 10%, the maximum voltage difference is about 0.08V, and the capacity is the lowest The 6# battery still has remaining power, and the consistency is very good.

　　5.3 Comparative analysis of battery pack discharge time and discharge capacity

　　Without a battery equalizer, the total effective discharge time is only 33 minutes, and the discharge capacity is only 550mAh. The actual discharge capacity is equal to the capacity of battery 6#. Except for battery 2#, which has the second smallest capacity, the other 11 batteries still have a large amount of effective power that cannot be used. To be released and exert its effect, the capacity is wasted seriously and the utilization rate is very low. Compared with balanced discharge, the capacity utilization rate is only 47.8%.

　　After using the battery equalizer, when the initial voltage is relatively low, the effective discharge time reaches 69 minutes, and the effective discharge capacity is as high as 1150mAh. Both the discharge time and the discharge capacity have been greatly improved. This is because with the intervention of the equalizer, most of the excess power of the larger-capacity battery is released through the efficient conversion of the battery equalizer, which significantly improves the capacity utilization of the larger-capacity battery, thereby greatly increasing the actual discharge time. extend.

　　5.46# battery discharge rate analysis

　　In the standard discharge mode, the discharge rate of the 6# battery is about 1/0.55=1.82C; after using the battery equalizer, all batteries enter the balanced discharge state, and the average discharge rate of the 6# battery is only 33/(69*0.55)= 0.87C, that is, after using the battery equalizer, the actual discharge rate of the 6# battery is less than half of the conventional discharge rate. The discharge rate is reduced, and the discharge current naturally decreases. The benefit brought by this is that the actual discharge time increases, which is 6 #The reason why the battery has a very long discharge time despite serious degradation.

　　5.5 Analysis of battery discharge temperature rise

　　Measured by an infrared thermometer, in the standard discharge mode, after 30 minutes of discharge, the temperature rise of the severely attenuated 6# battery and 2# battery is more obvious, exceeding the temperature rise of other batteries. This is because the internal resistance of the attenuated battery is obvious. When the temperature rises, the calorific value caused by the internal resistance increases significantly, and the temperature rises the fastest. Theory and practice have proved that the temperature rise increases and the attenuation speed is accelerated. After using the battery equalizer, during the entire discharge period, the temperature rise of the severely attenuated 2# and 6# batteries is almost the same as that of other batteries, which is very beneficial to reducing the attenuation speed. During the equalization discharge period, the temperature rise of the equalizer prototype has no difference. obvious change.

　　6Conclusion

　　Through comparative experiments and data analysis of conventional discharge and equalization discharge of 13 strings of 48-volt scrapped lithium battery packs, this article can draw a conclusion that high-efficiency battery equalizers can fully utilize and regulate battery power, and have a significant effect on stabilizing and extending the effective discharge time of the battery pack. , the lithium battery pack that has been completely scrapped and lost its use value can, with the intervention of the equalizer, fully exert its energy storage and power functions, extending the actual service life of the battery pack.

　　The high-speed shunt function of the high-efficiency battery equalizer significantly reduces the discharge current of the attenuated battery, and the temperature rise caused by the increase in internal resistance is significantly reduced, thereby reducing the risk of thermal runaway. If it is used for high-power, large-capacity energy storage, Power battery packs include echelon-used battery packs. A large number of scrapped battery packs can be disassembled and screened into groups that can be reused, which is of great significance.

　　references:

　　[1] Zhou Baolin, Zhou Quan: A transfer-type real-time battery equalizer with synchronous rectification function

　　[2] Zhou Baolin, Zhou Quan: Research on the impact of transfer battery balancing technology on battery voltage and charge

　　[3] Zhou Baolin, Zhou Quan: Research and application of bidirectional synchronous rectification technology in transferred real-time battery equalizer

　　Introduction to the first author:

　　Zhou Baolin (1968-): Male, Daqing, Heilongjiang, Master of Engineering, senior engineer, main research direction: battery balancing technology.

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