The cause of the uneven lithium battery and the most practical method of matching, I will tell you all

The reason why the lithium battery pack requires a protective board, BMS management system (electrical management system and thermal management system), one important reason is to respond to the inconsistency of the battery cell and the module. Imagine that if each battery is exactly the same (capacity, voltage, internal resistance, self -discharge …), and during the use process, it is like a Swiss watch. The management system is so complicated. At least, the single voltage does not need to be measured, the equilibrium function does not need to be done, the temperature sensor, the entire battery pack has one.

The inconsistencies between single batteries often cause problems such as capacity attenuation and short life during the cycle. The selection of batteries that are consistent as possible are used to form a group, which is of great significance for the promotion and application of lithium -ion batteries in power batteries.

However, the reality is that each battery is different from birth. They will become more different during use.

1 Inconsistent analysis

1.1 Definition of inconsistency

The inconsistency of the lithium -ion battery pack refers to the difference in voltage, capacity, internal resistance, life, temperature effects, and self -discharge rates after the battery pack of a single battery set of the same specification model.

After the single battery is manufactured, there is a certain difference in initial performance. With the use of batteries, these performance differences have been accumulated. At the same time, due to the incomplete use environment of each single battery in the battery pack, the inconsistency of the single battery has gradually enlarged, thereby accelerating the performance attenuation of the battery, and eventually triggered The battery pack has failed prematurely.

1.2 expression of inconsistency

The inconsistency of lithium -ion batteries is mainly manifested in two aspects: the differences between the performance parameters of the battery (battery capacity, internal resistance, and self -discharge rate, etc.) and the difference in battery loading status (SOC).

Studies such as Dai Haifeng have found that the difference between the capacity between batteries is close to the distribution of Will, and the degree of discreteness of internal resistance is more significant, and the internal resistance of the same battery battery generally meets the rules of normal distribution. Self -discharge self -discharge The rate also presents approximately normal distribution. The SOC characterizes the battery’s load state. It is the ratio of the battery’s remaining capacity to the rated capacity. The solution is that due to the inconsistency of the battery, the battery capacity attenuation rate is different, resulting in the maximum available capacity between batteries. The SOC change rate of a small battery is faster than the battery with a capacity larger than the capacity.

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2 The cause of the inconsistency of the single cells

2.1 Production process

There are many reasons for inconsistencies in lithium -ion batteries, mainly during the manufacturing process and use process. Each link of the manufacturing process, such as the uniformity of the slurry, the density of the coating time, and the control of the surface tension during the ingredients.

The first reason is that the instability of the accuracy of the material accuracy determined by the basic industrial level brings inconsistencies in the performance of the final product. Using different batches of positive electrode, negative electrode, and electrolyte, the battery cells produced are generally not mixed. Even though the parameters in the sorting process are very consistent, the division players basically cannot reflect the state of the battery cell after a period of time, so the current processing method is to avoid mixed use.

In the whole process, the consistency of each step is very important, but the most difficult to ensure consistency is the coating process. The thickness and uniformity of the coating and the activity of the material are not easy to control. The main process of difference. The differences in the manufacturing process can only be made up in the sorting process.

Some researchers have studied the effects of lithium -ion battery manufacturing process on the consistency of batteries, and focused on the effects of lithium -ion battery production and production of lithium -ion battery system on the battery consistency. During the use of the battery, Xie Jiao and others believe that the connection method and structural parts/devices, use conditions and environment will affect the consistency of the battery pack. Because the energy consumed by each connection point is inconsistent, the performance of each component or structural part and the aging rate is also inconsistent, so the impact on the battery is not consistent. In addition, due to the different positions of each single battery in the battery, different temperatures, and different performance attenuation, these will enlarge the inconsistencies of monomer batteries.

2.2 Use process

The position of the unit in the entire battery package is different. The single body wrapped in the center of the module is the same as the outer layer of the module. The heat dissipation conditions are huge;

The relative position of the module set flowing copper row can not bring inconsistency of the monomer heat environment. The copper row is a hot conductor, and the heat dissipation capacity is higher than the battery cell. Compared with the position of the copper row of the set flow, the battery can cause different heat dissipation conditions.

Studies have shown that in the process of work, the temperature of temperature will have the most significant impact on the inconsistency of the battery cells, which makes the battery cells from inconsistent inconsistent inconsistent.

Different thermal environments are superimposed together, resulting in different working temperature conditions of the single. High -temperature work causes degradation, rising internal resistance after degradation, and returning to increase the battery temperature rising. The difference in the heat environment is the beginning of this negative feedback.

2.3 Static process

When the car is not applicable, the battery pack is in a natural temperature field. What has an impact is the relative position of the battery cells, resulting in different thermal environment.

Each cell is different from the distance from the battery packet, and the degree of influence of the external temperature changes will be different. Before reaching the thermal balance, the temperature conditions of different battles are different.

If the battery core is transported to another assembly location, the environment is generally required to be 25 ° C, and the delivery test is performed after 24 hours of static.

During the static process, there are different trends in the differential state and the changes in the performance parameters of the battery cells, such as self -discharge.

3 Module inconsistencies cause

3.1 Manufacturing process

First of all, the inconsistency of the battery cells will inevitably be transmitted to the module. The importance of the battery spanning will be reflected on the module after forming for the first time.

Secondly, the battery cells are welded, held, and connected together to form a module. The inconsistent crafts during processing will inevitably lead to inconsistency between the module and the module. The most intuitive representation is the internal resistance of the module.

The inconsistency of the welding process is extremely easy to cause differences in welding resistance. The number of battles internal resistance is a few or a dozen millions of euros. The welding is slightly inconsistent with the resistance. It is not a small problem for lithium battery modules.

Again, the interior of the module and the module or the high -voltage wire, its own size, connection method, and surface processing will also affect the consistency of the module, which is mainly reflected in the internal resistance of the module. The screw connection, pressure connection or welding, in addition to considering the processing of processing, must not consider the impact of connection resistance.

3.2 Module use process

3.2.1 Internal resistance caused by the manufacturing process, reaching the environment of the module application will be reflected in the inconsistency of the heat and the impact of the voltage of the battery end.

The resistance value of uneven resistance causes uneven thermal and affects voltage collection. The battery management system collects the end voltage of each battery. The section here can be composed of several single cells.

The voltage collection method is to welded the two sampling points of the voltage measurement circuit or connected to the two ends of the battery with screws. If the long wire connection between the module and the module, you need to connect to both ends of the wires. The voltage collection point, the length of the wire can affect the voltage collection result. It is conceivable that the inconsistency of a solder joint or screw connection effect will inevitably bring inconsistency of the voltage collection result.

Dynamic battery end voltage collection process, the battery cell can be simplified into a theoretical voltage source and an electrical model with a resistance series. When there is a working current flowing in the circuit, the voltage measured by the management system is not equal to the opening voltage of the battery. The measuring process is the difference between the voltage source and the internal resistance end voltage; the charging process, the measurement value is the sum of the voltage source and the internal resistance end voltage.

3.2.2 Different equipment positions of the battery pack make the ambient temperature affect the consistency of the internal module. Different from the external environment and the heat source conditions, the temperature conditions of different position modules in the battery pack will be brought.

3.2.3 Different connection methods, in use, the impact on the consistency of the module is also different. The welding connection point, if the surface treatment is not done well, oxidation and corrosion occurs, the resistance will naturally increase. With no rust of welding joints, the resistance will be different from the rust -corrosive solder joints. The relative effects of the internal resistance of the welding joint and the internal resistance of the cells also change.

If a screw connection is used, the anti -loose screw is generally required. During the driving process of the car, the vibration of various frequencies will be generated. The screws responsible for the electrical connection are loosely removed, which causes internal resistance to change, and the system will cause the system to call and even power off.

4 How to improve battery consistency

4.1 Control of the production process

The control of the production process is mainly from two aspects: raw materials and production processes. Try to choose the same batch of raw materials as much as possible to ensure the consistency of the size and performance of raw materials. The production process should strictly regulate the entire production process, such as ensuring that the slurry is stirred and placed for a long time, and the speed of the coating machine is controlled to ensure the thickness and uniformity of the coating. , Control the amount of liquid injection and transformation, tolerance, storage conditions, etc.

4.2 Control of the matching process

The control process control mainly refers to the sorting of the battery. The battery pack uses a unified specification and model battery, and the battery voltage, capacity, internal resistance, etc. should be measured to ensure the consistency of the initial performance of the battery.

4.3 Control of use and maintenance process

Real -time monitoring of the battery. Consistency screening of the battery when mating can ensure the consistency of the initial use of the battery pack. During the use process, the battery monitors real -time monitoring, which can be observed in real time to observe the consistency problems during the use process. However, due to poor consistency, the monitoring circuit will cut off the discharge circuit, so the performance will be reduced. The balance point between the two must be found. It can also be adjusted or replaced in time by real -time monitoring of extreme parameter batteries to ensure that the inconsistency of the battery pack will not expand over time.

Introduce a balanced management system. The battery is intelligently managed by an appropriate balanced strategy and a balanced circuit. At present, common equilibrium strategies include an external voltage -based balanced strategy, a SOC -based equilibrium strategy, and a capacity -based equilibrium strategy. The equilibrium circuit can be divided into passive equilibrium and active equilibrium according to energy consumption. Active equilibrium can achieve non -destructive energy flow between batteries, which is a hot spot at home and abroad. The commonly used methods in active balance include battery bypass, switching capacitors, switching inductance methods, DC/DC transformation methods, etc.

Perform the battery thermal management. In addition to maintaining the working temperature of the battery group as much as possible to the optimal range, the battery group should also try to ensure that the temperature conditions between the batteries are consistent, so as to effectively ensure the performance consistency between the batteries. Use reasonable control strategies. When the output power allows, minimize the battery discharge depth as much as possible, and at the same time, avoid the over charging of the battery, which can extend the cycle life of the battery pack. Strengthen the maintenance of the battery pack. For a certain period of time, the battery pack is charged with small current maintenance, and we should also pay attention to cleaning.

5 Power lithium ion battery configuration method

5.1 voltage configuration method

The voltage configuration method can be divided into static voltage configuration method and dynamic voltage configuration method. The static voltage configuration method is also called the air load configuration method. Without the load, only consider the battery itself, measure the self -discharge rate stored by the screened monomer battery for dozens of days after the static statement, and the different storage period under the full charge state. The opening voltage of the inner battery is the simplest operation, but it is not accurate. The voltage situation of the dynamic voltage configuration method inspects the load, but it is not accurate to consider factors such as load changes.

5.2 Static capacity configuration method

Under the set conditions, the battery is charged and discharged, the capacity is calculated from the discharge current and discharge time, and the battery is matched according to the capacity. This method is simple and easy, but it can only reflect the same conditions of the battery in specific conditions. It cannot indicate the complete working characteristics of the battery. It has certain limitations, but it is also the most direct and effective method.

5.3 Internal resistance matching group method

Mainly consider the internal resistance of a single battery. This method can achieve rapid measurement, but because the internal resistance of the battery will change with the discharge process, the accurate measurement of internal resistance is difficult.

5.4 Multi -parameter matching method

At the same time, considering a comprehensive assessment of battery groups such as capacity, internal resistance, voltage, self -discharge rate, etc., the battery pack with better consistency can be selected. However, the premise of this method is that the single parameters should be accurate when sorting and spending too long at the same time.

5.5 Dynamic characteristic configuration method

The dynamic characteristic configuration method is to use the battery’s charging and discharge characteristic curve to divide the battery to match. The charging and discharge curve can reflect most of the characteristics of the battery, and the use of dynamic characteristics to ensure the consistency of various performance indicators of the battery. There are many dynamic feature configuration methods, and computer programs are usually implemented. In addition, the use of batteries to be reduced by battery is not conducive to the reduction of battery pack costs. The determination of the standard curve or benchmark curve is also a difficult point in its implementation process.

6 Conclusion

(1) The causes of battery inconsistency are mainly two aspects of the manufacturing and use of batteries.

(2) Measures to improve battery consistency are mainly in the following three aspects: strict control from the production process from both sides of raw materials and production processes; adopt a more scientific sorting method, and choose batteries with the same initial performance as much as possible for partnerships; During the battery use and maintenance process, real -time monitoring of the battery, introducing a balanced management system, using reasonable control strategies, heat management of batteries, and also strengthening the maintenance of the battery pack.

(3) During the battery configuration, the single parameter mating method does not have the actual application value due to too few factors. Multi -parameter grouping method and dynamic characteristics configuration method is relatively comprehensive, and methods such as electrochemical anti -spectrum method have also made certain progress.

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