102450 lipo battery

Comparative experiment and analysis of conventional discharge and balanced discharge of 13-series 48V scrapped 102450 lipo battery pack

Under the influence of consistency problems, the actual discharge capacity of the battery pack depends on the battery with the smallest capacity in the battery pack. The more battery packs are 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, which 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 real-time and high-efficiency battery equalizers not only intelligently adjusts the charge and discharge current and charge and discharge rate of batteries with different capacities, but also significantly improves the capacity utilization rate of the battery, and the temperature rise effect of controlling the attenuation battery is also very obvious. This paper fully proves the importance of high-efficiency battery equalizers in stabilizing the battery life and capacity of battery packs through the comparison and analysis of a group of 13-series 48V scrapped lithium batteries with severely deteriorated consistency in conventional discharge and balanced discharge experimental data.

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

1 Reasons for consistency problems in battery packs

Ideally, battery packs should have the following characteristics: when charging or discharging, the voltage of all batteries rises or falls synchronously, 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, the consistency is very good, all batteries can be fully charged or discharged at almost the same time, and there will be no problem of overcharging or over-discharging of batteries.

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

Through a large number of experimental studies and operation data analysis, it can be found that there are two important reasons for the consistency problems of conventional battery packs: one reason is the difference in battery production process, referred to as internal factors. Once the battery packaging is completed, the differences in capacity, self-discharge rate and internal resistance parameters between batteries exist, but the degree of difference is different.

The second reason can be called external factors, which are mainly caused by the fluctuation of charging and discharging voltage parameters, current parameters, and ambient temperature differences. These external factors will gradually accumulate and amplify the differences formed by internal factors, and the amplification of such differences is exponentially amplified. This is why the battery pack will rapidly increase once the consistency problem occurs.

2 Common methods to solve consistency problems

In solving the consistency problem of battery packs, according to the main reasons for the consistency problem of batteries, there are two important technical solutions. One method is to make a fuss about the battery production process and improve the consistency of the battery at the time of leaving the factory by improving the production process level. This method has a certain effect and can slow down and delay the occurrence of consistency problems to the greatest extent, but it cannot be eradicated;

Another method is to use a battery balancer for intervention. 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 applicable to the situation with good consistency, uniform heat dissipation, and small battery pack capacity; the typical representative of active balancing is transfer battery balancing, and 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, which will not be discussed in this article, but one thing is certain, all design goals are to develop in the direction of supporting large balancing current, high balancing efficiency and fast balancing speed.

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

In battery balancing technology, the most difficult problem to solve is the matching and balance of balancing current and balancing efficiency. It is necessary to be able to supply large balancing current and have high balancing efficiency. The reason for this requirement is that the balancing equipment will heat up under large current balancing and affect the temperature rise of the battery pack.

In order to solve this contradiction, the author has developed a unique bidirectional synchronous rectification technology [1] after years of research and development. It not only supports large current balancing, but also has high balancing efficiency. Under full load working conditions, the temperature rise of the equipment is also low, and there is almost no additional temperature rise on the battery pack.

The reasonable distribution and optimization of power can be achieved through high-speed voltage balancing [2]. In terms of discharge balancing, this technology automatically analyzes and determines the capacity of the battery by detecting the relative voltage difference and direction between adjacent batteries in real time, and automatically increases the discharge current of the battery with high voltage (high voltage during discharge usually means large capacity). The additional discharge current is efficiently converted by the equalizer and transmitted to both ends of the low-voltage battery. The discharge current of the low-voltage battery (low voltage during discharge usually means small capacity) is reduced to make up for the insufficient discharge capacity of the small-capacity battery, so that batteries of different capacities can be discharged at approximately the same rate.

In terms of charge balancing, the charging current of the battery with high voltage (high voltage during charging means small capacity) is automatically reduced. The reduced charging current is efficiently converted by the equalizer and transmitted to both ends of the low-voltage battery. The charging current of the low-voltage battery (low voltage during charging means large capacity) is increased to make batteries of different capacities charge at approximately the same rate. This technology can also support high-speed static balancing at the same time, adding effective balancing time. The unique pulse technology is very obvious in terms of stable attenuation of battery capacity. The battery balancer used in this example adopts the latest developed bidirectional synchronous rectification technology [3].

4 Thirteen-series 102450 lipo battery pack discharge experiment

The experimental battery pack is shown in Figure 1. It is assembled by disassembling and selecting scrapped 102450 lipo battery packs, which is equivalent to cascade utilization. All of them are 18650 lithium batteries. The longest shelf time is more than 8 years. The initial voltage of the disassembled batteries is only a few volts to three volts. Most of them are in a scrapped state. The original single-cell design capacity is 2200mAh to 2500mAh. Most of the batteries have heavy leakage and serious heat when charging.

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

The battery with the words "head power supply" on the far right of the experimental table is only responsible for the power supply of the high-precision voltage head under the 7# and 13# batteries, and does not participate in the charge and discharge experiment. Other voltage heads are powered by cascade. Each battery has a high-precision voltage meter under it, which displays the current voltage of the battery above in real time. The experimental platform (the platform has been modified to facilitate battery replacement) supports a maximum of 2 parallel 14 series 18650 battery experiments. This example only connects 13 series of batteries with a nominal voltage of 48 volts.

4.1 Conventional discharge experiment

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

During this period, the current voltage of each battery is recorded every 10 minutes until a battery 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 recorded video, the background color represents the highest and lowest voltages in the group, the same below). The capacity of the 6# battery is the smallest. The corresponding voltage of each battery at the end of discharge is shown in Figure 3. The bar chart of the remaining power of each battery at the end of regular discharge is shown in Figure 4. The remaining voltage curve of each battery at the end of regular discharge is shown in Figure 5.

Table 11 3-series 102450 lipo battery group regular discharge data table

From the discharge measurement data, it can be seen that when the discharge reaches 33 minutes, the 6# battery reaches the discharge cut-off voltage and stops discharging. At this time, the power of the 2# battery is about to be discharged, but the other 11 batteries still have a lot of power that has not been released. The voltage at the end of discharge explains everything, especially the 1# and 7# batteries still have a lot of power that has not been released and cannot be used. This situation is especially like a power 102450 lipo battery pack equipped with a BMS battery management system with single cell discharge protection.

Although the 6# battery is protected from over-discharge, the capacity of most batteries in the battery pack is not utilized and released, resulting in serious capacity waste. In addition, the maximum voltage difference data measured every ten minutes also shows that the maximum voltage difference gradually increases as the discharge progresses.

This means that the consistency 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 seriously decayed. 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 the voltage is at the lowest in the middle and late stages, and it continues until the end of discharge.

When the discharge stops, the voltage of all batteries has begun to rebound normally, but the voltage rebound speed of the 2# and 6# batteries is much faster than that of other batteries, and soon rises to about 3.9V, further confirming that the 2# and 6# batteries are seriously decayed.

4.2 Balanced discharge experiment

Various applications and practices have shown that the practical significance of balanced discharge is greater than balanced charging. Balanced discharge can reflect the actual available capacity of the battery pack. Whether in theory or practice, 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 is, the greater the difference in actual discharge capacity.

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

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

4.3 Balanced discharge cycle expansion experiment

Based on the very satisfactory experimental data obtained from the first balanced discharge, in order to verify whether the balanced discharge has universal characteristics, the author continued to conduct more than 100 balanced charge and discharge cycle experiments on the experimental battery pack while maintaining the equalizer prototype. The cycle experiment results show that after the high-efficiency equalizer is involved, the safe discharge time and discharge capacity of the battery pack are very close, which is significantly better than ordinary charge and discharge. The temperature rise of the 2# and 6# batteries with severe attenuation is basically the same as that of other batteries, or even slightly lower. The reduction in temperature rise has positive significance for preventing thermal runaway.

5 Comparative analysis of discharge experiments

The two discharge methods have basically the same initial conditions, but the discharge results are very different. The only difference is that the connection of the equalizer is maintained during the balanced discharge. The following analysis is conducted by comparing the voltage performance and actual discharge time of the attenuated battery in the two discharge methods.

5.1 Analysis of voltage performance of decayed batteries

In the conventional discharge experiment, although the voltage of the 6# battery was at a high level at the beginning of the discharge, the voltage began to be at the lowest state after 10 minutes of discharge until the end of the discharge. The voltage of the 2# battery followed closely, also showing a serious decay state. The voltages of the 1# and 7# batteries with the least decay were always at the highest state.

In the balanced discharge experiment, before the end of the discharge, the voltage of the 11# battery was always at the lowest, not the 6# battery, nor the 2# battery. The important reason is that the intervention of the equalizer automatically changed the actual discharge current of each battery, significantly reducing the actual discharge current of the 2# and 6# batteries. In the conventional discharge experiment, the voltages of the 1# and 7# batteries were always at the highest state until the end of the discharge, showing that the capacity of these two batteries is the largest.

Since they are close to 2# and 6# batteries, they automatically supply a large amount of power to the severely attenuated 2# and 6# batteries during the balanced discharge. While increasing their own discharge current, they reduce the actual output current and voltage drop rate of 2# and 6# batteries, and the voltage of the entire battery group shows an approximately synchronous drop.

By comparing Figure 5 and Figure 7, it can be clearly found that at the end of the balanced discharge, the battery voltage consistency is significantly better than the conventional discharge. This consistency improvement runs through the entire discharge period of the battery pack, which is very beneficial to the safe and efficient operation of the battery pack.

5.2 Analysis of the maximum voltage difference change

In conventional discharge, before discharge, the maximum voltage difference between batteries is only 0.026V, with good consistency, and the overall voltage is slightly higher than the initial voltage of the balanced discharge experiment. As the discharge progresses, the maximum voltage difference gradually expands. By 30 minutes of discharge, 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 balanced 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. Under such unfavorable conditions, the intervention of the equalizer completely changes the subsequent discharge state. Before the end of discharge, the maximum voltage difference is always in a small state, far lower than the maximum voltage difference of conventional discharge, and the overall state is gradually shrinking, which is completely opposite to conventional discharge. It only starts to increase slowly at the end of discharge, but the increase is very small.

The important reason is that at the end of discharge, the dischargeable capacity of the battery drops sharply. Even for undecayed batteries, the remaining available power is very small, and it is impossible to quickly supply more power to the decaying battery through the equalizer. According to previous experimental data, for lithium batteries, at a discharge rate of about 1C, 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 the high-speed equalization function normally. If the protection power is set at 10%, the maximum voltage difference is about 0.08V, and the 6# battery with the lowest capacity still has residual power, and the consistency is very ideal.

5.3 Comparative analysis of battery pack discharge time and discharge capacity

Without the use of 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 the 6# battery. Except for the 2# battery with the second smallest capacity, the other 11 batteries still have a large amount of effective power that cannot be released and function. The capacity is seriously wasted and the utilization rate is very low. Compared with balanced discharge, the capacity utilization rate is only 47.8%.

After using the battery equalizer, under the condition of relatively low initial voltage, the effective discharge time reached 69 minutes, and the effective discharge capacity was 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 efficiency of the larger capacity battery.The capacity utilization rate of the battery is increased, thereby greatly extending the actual discharge time.

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 is increased, which is Although the 6# battery is seriously attenuated, the discharge time is still very long.

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 increases, the heat generated due to internal resistance increases significantly, and the temperature rises the fastest. Theory and practice have proved that the increase in temperature rise aggravates the attenuation speed. 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. significant changes.

6Conclusion

Through comparative experiments and data analysis of conventional discharge and equalization discharge of 13 strings of 48-volt scrapped 102450 lipo battery packs, this article can draw a conclusion that high-efficiency battery equalizers can fully utilize and regulate battery power, stabilize and extend the effective discharge time of the battery pack, and have obvious uses. , the 102450 lipo 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 102450 lipo battery packs include echelon-used battery packs. A large number of scrapped battery packs can be reused after being dismantled and screened into groups, 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, important research direction: battery balancing technology.

Original title: Comparative experiment and analysis of conventional discharge and equalization discharge of 13 strings of 48-volt scrapped 102450 lipo battery packs

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