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What is the management of lithium-ion power battery consistency?

What is the management of lithium-ion power battery consistency?

The consistency of lithium-ion power batteries means that after the cells are used in groups, the voltage, internal resistance, capacity and other parameters of each cell are not exactly the same due to the influence of factors such as production and use environment. The performance parameters such as charge-discharge capacity and cycle life of a lithium-ion power battery are generally determined by the worst-performing monomer in the battery. Therefore, the consistency of the battery pack plays an extremely important role in its performance and cycle life. The impact of consistency on battery life will be discussed and ways to improve consistency will be suggested.

1. The impact of consistency on the life of the power battery pack
The consistency of lithium-ion power batteries mainly includes voltage consistency, capacity consistency and internal resistance consistency. As the power battery usage time increases, the degree of inconsistency will gradually increase. The most intuitive reflection is that the degree of inconsistency of the cell voltages in the battery pack increases. There are two main reasons for the poor consistency of lithium-ion power batteries: one is the production and manufacturing reasons. Due to slight differences in electrode plate thickness, chemical activity, microporosity, etc., there are some differences in parameters such as internal resistance and capacity of single cells. The second is the inconsistency in the use process. Lithium-ion power batteries have complex operating conditions, long-term work under harsh conditions such as high-rate charge and discharge current, vibration, etc., coupled with the structural layout of the battery system and the design of the heat dissipation system, which lead to the temperature, self-discharge degree, and electrolyte activity of each battery cell. There are differences, and the inconsistency of lithium-ion power battery packs gradually increases as the number of uses increases. It can be seen that the inconsistency of lithium-ion power batteries is inevitable. Figure 1 shows the causes of battery pack inconsistency and its transmission process.

What is the management of lithium-ion power battery consistency?
Figure 1 Causes and transmission process of battery pack inconsistency

As the inconsistency of the power battery pack increases, the performance and life of the battery pack are seriously affected. Scholars from various countries have already made some research results on the impact of inconsistency on the life of power batteries. Wang Zhenpo et al. [71] proposed a formula for calculating the remaining capacity of the battery pack after n times of use under the influence of inconsistency:
C(n)=fn(△C)(1-nP/N)C0

In the formula, C(n) represents the remaining capacity of the battery pack after n times of use; f(△C) represents the maximum value of the damage coefficient of the battery charge and discharge capacity during each charge and discharge process, which is a positive number less than 1; N represents the battery pack The service life of the battery pack; P indicates the specified capacity decay percentage at the end of the battery pack life; C0 indicates the initial capacity of the battery pack.

If f(△C) takes the maximum value of 0.999, the end of battery life is defined as the capacity decay of 20%. Assuming that there are three single cells with cycle life of 300 times, 600 times, and 1200 times, respectively, according to the formula, the cycle life when they are used in groups can be calculated as shown in Table 1.

sampleMonomer cycle life (times)Battery pack cycle life (times)
1#300132
2#600167
3#1200191
Table 1 Relationship between single cycle life and battery pack cycle life caused by inconsistency

According to the calculation results, the cycle life of the lithium-ion power battery pack is much lower than that of the corresponding monomer. Due to the inconsistency, the cycle life of the single cell is doubled, and the life of the battery pack can only be improved by dozens of times. If the battery pack is not repaired and maintained in time, the life of the battery pack can only reach a fraction of the life of the single unit. The lithium-ion power battery used in the demonstration operation of Beijing’s public transport has a single cell life of more than 1,000 times. The capacity of the power battery system applied to the vehicle will be seriously attenuated after 150 times of charging and discharging, and the capacity of some cells has been lower than 80% of rated capacity.

2. Measures to improve battery consistency
The cycle life of the battery pack is increased by increasing the cycle life of the lithium-ion power battery cells, which is ineffective and expensive. By optimizing the charging and discharging method of the battery pack, reducing the inconsistency caused by charging and discharging, and regularly repairing and maintaining the battery pack during use, the electrical performance of the lithium-ion power battery pack can be effectively guaranteed and the service life of the battery pack can be improved. Combined with the research on the life characteristics of lithium-ion power batteries for vehicles and the actual use of vehicles, the following measures can be taken to prevent the expansion of inconsistencies in the use of battery systems.

(1) To ensure the delivery quality of lithium-ion power battery cells, the initial voltage of each cell needs to be consistent, and the same batch of cells must be correlated with voltage, internal resistance and other data before leaving the factory to ensure the same batch of cells. performance as consistent as possible. The battery cells of the same batch, the same specification, and the same type must be selected when assembling the battery.

(2) Adopt practical battery balancing system and energy management system. At present, the most effective and practical equalization method is to equalize the voltage of each cell during the charging process of the battery pack, so that the cell voltage is as consistent as possible, and equalization management is realized from the source. Charging is terminated when the cell voltage reaches the charge cut-off voltage. Charge equalization is to use an active or passive equalization method to make the voltage of each cell consistent before charging is terminated, and the passive equalization method is currently used more. The principle of passive balancing is shown in Figure 2. Each cell is connected to a load resistor and controlled by a switch. According to the result of the cell voltage detection in the battery system, the balance management system closes the switch connected to the cell with the faster voltage rise during charging, thereby maintaining the consistency of the cell voltage and improving the electrical performance of the entire battery pack. Passive equalization is carried out by means of heat dissipation, the discharge current is generally controlled at about 0.1A, and the charging equalization takes several hours to complete. Active charge equalization requires an energy storage element (capacitor, magnetic field, etc.) to transfer energy between cells. The equalizing current is large and the power consumption is small, and no special cooling measures are required, which is beneficial to improve the consistency of the battery pack. However, this method has a complex structure and high cost.

What is the management of lithium-ion power battery consistency?
Figure 2 Schematic diagram of passive charge equalization

The purpose of the battery management system (BMS) is to avoid premature failure of battery cells due to excessive use, so that the main electrical performance of the battery pack can reach and maintain the performance level of poor cells. Its main task is to prevent overcharge and overdischarge. , which provides status information such as voltage, current, temperature, and remaining power. Using thermal resistance, semiconductor refrigeration device for temperature control, etc., and controlling the charging and discharging state of the power battery pack through BMS can effectively increase the cruising range of the vehicle, prolong the service life of the battery system, and at the same time ensure the safety and reliability of the battery pack during use. Sex is important.

(3) Strengthen the maintenance and maintenance of the battery pack during use. During the use of the battery pack, it is necessary to avoid the contamination of the battery poles by water and dust as much as possible, to ensure a good working environment for the battery pack, and to avoid excessive use as much as possible. The power battery pack should be maintained regularly, and the cells with poor performance should be replaced or adjusted in time through the analysis of parameters such as the voltage of each single cell of the battery pack. The power battery pack is charged with a small current at regular intervals to promote its balance and performance recovery.

How to optimize the air cooling and cooling system of the lithium-ion power battery system?

How to optimize the air cooling and cooling system of the lithium-ion power battery system?

1. Flow field design of battery pack thermal management system
The rate of heat dissipation per unit area of ​​the battery pack to the heat transfer medium is expressed as
·Q=h(Tbat-Tamb)

Among them, h represents the convective heat transfer coefficient on the surface of the battery pack, and the subscripts bat and amb represent the surface of the battery pack and the heat transfer medium, respectively.

First, the design of the flow field determines the order in which the heat transfer medium flows through different positions of the battery pack, which will affect the value of the Tbat-Tamb term, thereby affecting the local heat dissipation rate at different positions. Second, the design of the flow field determines the flow velocity of the heat transfer medium at different locations, and the flow velocity will affect the h term of the local convective heat transfer coefficient. Third, the design of the flow field determines the local shape of the flow channel, which will also affect the value of the local convective heat transfer coefficient h. Therefore, the rationality of the flow field design has a significant impact on the thermal management effect of the battery pack.

(1) Path design of flow field – serial flow channel and parallel flow channel. According to the passage of the heat transfer medium inside the battery pack, the flow field can be divided into serial flow channel type and parallel flow channel type, as shown in Figure 1. In the serial flow channel design, the heat transfer medium passes through each single cell or battery module in strict order, while in the parallel flow channel design, the heat transfer medium enters the battery pack box and passes through the parallel flow channels. Divide the current through different battery sub-modules in parallel. For serial runner designs, the battery modules behind the runners will not be able to dissipate heat effectively because the medium will gradually be heated in the serial runners. It has been pointed out that the parallel flow channel design results in better temperature uniformity at different locations of the battery pack compared to the serial flow channel.

How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 1. Fluid design of serial and parallel runners

(2) Velocity design of flow field—speed regulation and pressure regulation of parallel flow channels. For the parallel flow channel design, the flow rates of different flow channels must be as uniform as possible to reduce the non-uniformity of temperature at different positions inside the battery pack. Two methods to ensure uniform flow rate: speed regulation method and pressure regulation method, and the optimal combination of the two methods is given. The speed regulation method refers to reducing the width of each channel in turn in the direction of increasing the number of parallel channels to adjust the flow resistance of the heat transfer medium, so that the heat transfer medium can redistribute its flow according to the resistance of each channel, so as to achieve the purpose of adjusting the flow rate distribution. The pressure regulation method changes the pressure difference on both sides of different channels by changing the inclination angle of the inlet and outlet collector plates, thereby indirectly adjusting the flow rates of different channels.

The thermal management system of the lithium-ion power battery system is mainly divided into air cooling and liquid cooling according to the different cooling media. Among them, liquid cooling has better cooling effect, but it needs to arrange special pipes, has many parts, complicated control and high cost. The gas cooling system has low heat transfer rate and low volumetric efficiency, but is widely adopted due to its simple design, simple control and low cost. Due to the small convective heat transfer coefficient of gas, it is more difficult to use gas to heat or cool battery systems than liquids. Therefore, the design of gas cooling systems should be optimized to the greatest extent possible for battery packs. Taking the development of a power battery cooling system for an electric vehicle as an example, the optimization scheme of the air-cooled cooling system is proposed, and the final optimization scheme is determined by the simulation results.

2. Problems and solutions of cooling system
The air-cooled cooling system of an electric vehicle power battery is shown in Figure 2. There are two main problems in this cooling system: one is that the temperature difference between the battery modules is too large; the other is that the pressure loss is too large, and the structure of the air channel needs to be optimized. The main reasons for these two problems are: the arrangement of the battery modules is asymmetric, and the air flow between the battery modules is inconsistent; the battery module adopts a double-layer structure, which generates heat accumulation; there is a sudden contraction or expansion in the air channel. The cross section changes suddenly, the structure does not have enough corner radius at the corner, and the air cannot transition smoothly. In order to solve these two problems, the air-cooled heat dissipation system of the battery system is optimized: the battery module is arranged in a single-layer structure, the structure of the air channel is arranged symmetrically, and the cross-section is changed by using a small shrinkage angle and multiple cross-sections. , and design a large corner radius at the corner. There are two types of improved schemes: scheme one, the air inlet of the air channel is set at the top left side, the air outlet is set at the bottom end of the right side, and the air inlet and outlet are located on both sides; scheme two, the air inlet port of the air channel is located at the top left side, The air outlet is at the bottom left side, and the air inlet and outlet are at the left end. In the two schemes, the battery modules are arranged in a single-layer symmetrical arrangement, and a large rounded transition is designed at the turn of the air inlet, and a small-angle contraction is used to reduce the pressure loss. The improved scheme is shown in Figure 3 and Figure 4.

How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 2 The structure diagram of the heat dissipation system of the original lithium-ion power battery system
How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 3 Cooling system optimization scheme 1
How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 4 Cooling system optimization scheme 2

3. Evaluation indicators of air-cooled cooling system
In the design process of the air cooling system of the battery pack, it is necessary to evaluate it with relevant indicators to determine whether the optimization scheme is feasible. The main indicators are the maximum temperature difference of the system, the maximum temperature of the system, and the pressure difference between the inlet and outlet. The maximum temperature difference of the system refers to the difference between the highest temperature and the lowest temperature of all the cells in the lithium-ion power battery system, which reflects the uniformity of the cooling system and ensures that the cooling effect of each cell is consistent. The maximum temperature of the system refers to the maximum temperature of all cells in the lithium-ion power battery system, which can represent the cooling effect of the cooling system to a certain extent. The inlet and outlet pressure difference refers to the pressure difference between the air inlet and the air outlet in the air cooling system, which is closely related to the structure of the air flow channel of the cooling system.

4. Simulation analysis
The flow field and thermal field simulation are carried out for the two optimization schemes respectively, and the temperature cloud map, pressure cloud map and velocity cloud map are formed, as shown in Figure 5, Figure 6, and Figure 7. By comparing and analyzing the simulation results of flow, pressure, temperature, etc., the advantages and disadvantages of the two schemes are judged according to the evaluation indicators of the cooling system, and the one with better cooling effect is selected. The key parameters of the two optimization schemes are compared in Table 1. From the data in Table 1, it can be seen that the maximum temperature difference of the system, the maximum temperature of the system, and the flow uniformity of the scheme 2 are better than those of the scheme 1, but the pressure difference between the inlet and the outlet of the scheme 2 is slightly different. Therefore, the second solution with the same side design of the air inlet and outlet has a better heat dissipation effect, and further optimization design can be made on the basis of the second solution to obtain a better heat dissipation effect.

SchemeMaximum temperature difference (K)Maximum temperature (K)Inlet and outlet pressure difference (Pa)Flow unevenness (%)
Scheme 114.2326.311.880.08604
Scheme 213325.811.890.06196
Table 1 Comparison of the main evaluation indicators of the two optimization schemes
How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 5. Scheme 1 temperature cloud map
How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 6 Scheme 1 pressure cloud map
How to optimize the air cooling and cooling system of the lithium-ion power battery system?
Figure 7 Scheme 1 Velocity Cloud Map
What effect does temperature have on the life of lithium-ion power batteries?

What effect does temperature have on the life of lithium-ion power batteries?

Lithium-ion power battery is an electrochemical battery based on Li+ concentration difference. The level of ambient temperature during operation directly affects the activity of positive and negative electrode materials and electrolyte, and has an important impact on its life. The electrical performance and service life of the same lithium-ion power battery at different operating temperatures are very different. Generally, the lithium-ion power battery can exert its maximum efficiency at a room temperature of about 25 °C. When the ambient temperature is too low, the activity of the electrolyte is affected, and the internal resistance increases significantly, resulting in difficult battery charging, reduced power, reduced usable capacity, and impaired battery life. When used at high temperature, the heat dissipation of the battery system will be affected. When the internal temperature of the battery exceeds the limit temperature, the internal chemical balance will be destroyed, resulting in corrosion and aging of battery materials, seriously aggravating the battery life decay process, and causing the battery to fail prematurely.

Figure 1 shows the relationship between the capacity retention rate and the number of cycles of a lithium-ion power battery cell at different temperatures. In the cyclic charge-discharge test, the discharge system is: 1C constant current discharge to voltage 2.5V; charging system: 1C constant current charge to 3.7V, transfer to constant voltage 3.7V to charge until the current drops to 1/30C, stop charging, and complete the charging process. After charging, let it stand for 1h, and then re-charge and discharge test. The remaining capacity of the monomer was measured every 20 charge-discharge cycles completed.

What effect does temperature have on the life of lithium-ion power batteries?
Figure 1 The relationship between the capacity retention rate of a lithium-ion power battery and the number of cycles at different temperatures

It can be seen from Figure 1 that when the high temperature is 40~60°C, the battery decays faster as the temperature increases, especially when the battery is at an ambient temperature of 60°C, the battery discharge capacity decays to 80.63 after 20 charge-discharge cycles. %. When the low temperature is -10~10℃, with the decrease of the ambient temperature, the battery attenuation speed is accelerated, and the battery capacity attenuation speed is significantly accelerated in the -10℃ environment. Moreover, it can be seen that the high temperature environment has a greater impact on the life attenuation than the low temperature environment, which is more detrimental to the life of the battery. The temperature of the battery increases rapidly in the high temperature environment. When the charge and discharge test is carried out in the environment of 40°C, the temperature of the battery increases by 20°C after 7 cycles of charge and discharge.

In order to protect the lithium-ion battery and improve the service life of the battery, the ambient temperature of the lithium-ion power battery should be controlled within the range of 0~40 °C, and it is forbidden to work in a high temperature environment above 50 °C. In order to further ensure that the battery capacity decay rate is within a certain range, the working temperature of the battery should preferably be controlled at 0~25℃.

When the lithium-ion power battery cells are connected in series and parallel to form a power battery pack, the temperature field of the power battery pack is not a simple superposition of the temperature field of the single cell, and the temperature distribution of the battery pack is not as uniform as that of the battery cells. The stability is also not as good as the monomer. The non-uniformity of the temperature distribution of the battery pack leads to inconsistent cell activity at different positions inside the battery pack, thereby aggravating the expansion of the inconsistency. Therefore, the power battery system needs to design a special thermal management system to ensure that the battery works in an appropriate temperature range and the uniformity of the battery temperature distribution.