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At present, the large-scale application of lithium-ion batteries in new energy vehicles, smart grids and other fields is increasing year by year, but at present, the inconsistency of battery parameters is the key factor affecting the service life of the battery pack. Although the improvement of thermal management level ensures the safe operation of the battery pack to some extent, the improvement of the consistency level of batteries is still an important technical factor affecting the large-scale use of lithium-ion batteries.

Through the simulation of a 10-series 10-parallel battery pack, we clarified the influence of temperature distribution in the battery pack on its performance and cycle life. The lower the average temperature is, the higher the temperature non-uniformity is, and the higher the non-uniformity of the discharge depth of the single cell in the battery pack is; The higher the average temperature, the higher the temperature non-uniformity and the shorter the cycle life of the battery pack. It is worth noting that uneven temperature distribution will lead to uneven current distribution between parallel branches, which will worsen the consistency of single cell aging rate.

In terms of thermal management of battery pack, we propose a fast method to estimate the flow field and temperature field inside the parallel air-cooled battery pack. The method is composed of a flow resistance network model and a transient heat transfer model, which prevents the computational fluid dynamics method from being used to simulate a large battery pack, and ensures a high estimation accuracy. Using this method, we investigated the influence of non-uniform flow field on the temperature uniformity in the battery pack, and discussed the structural parameters of the air cooling system with the goal of improving the temperature uniformity, and gave a feasible parameter configuration method.

Due to the difference in the parameter values of the same type and specification of battery in terms of voltage, internal resistance, capacity and other aspects, its performance indicators often fail to reach the original level of single battery when used in electric vehicles, which seriously affects its application in electric vehicles.

The consistency of lithium-ion battery refers to the consistency of initial performance indicators of single battery used in groups, including capacity, impedance, electrical characteristics of electrodes, electrical connection, temperature characteristics, decay rate, etc. The inconsistency of the above factors will directly affect the difference of output electrical parameters in operation.

The inconsistency of lithium-ion battery pack or the discrete phenomenon of battery pack refers to the voltage, charge, capacity, decay rate, internal resistance and its change rate with time, life, temperature effect, self-discharge rate and its change rate with time after the single battery of the same specification and model is composed of battery pack.

After the single battery is manufactured, it has certain performance differences. The initial inconsistency is accumulated with the continuous charging and discharging cycle of the battery during use, resulting in greater differences in the status of individual cells (SOC, voltage, etc.); The use environment in the battery pack is also different for each single battery. This leads to the gradual amplification of the inconsistency of the single battery in the process of use, which in some cases accelerates the degradation of the performance of some single batteries, and eventually leads to premature failure of the battery pack.

The reasons for the inconsistency are divided in chronological order. The inconsistency of the single cell in the battery pack is mainly reflected in two aspects: the process problems and the uneven material in the manufacturing process make the activation degree and thickness of the active material of the battery plate, the micropore rate, the connecting strip, the separator, etc. have very small differences, resulting in the incomplete consistency of the internal structure and material; The influence of electrolyte density, temperature and ventilation conditions, self-discharge degree and charging and discharging process of each battery in the battery pack during loading and use.

Analysis of the differences in capacity, internal resistance and self-discharge of the same type of battery delivered from the same batch

In view of the causes of these inconsistencies, can some measures be taken to completely eliminate the inconsistencies in the battery pack: many people think that the battery inconsistency is a problem of the production process, and some people think it is a problem of the matching process, and the battery inconsistency can be completely eliminated through process control measures such as SPC.

However, practice has proved that even if the process of batching, live pulp, coating, cutting, rolling and other processes are strictly controlled, the standard deviation between batch products can only be reduced, and the inconsistency cannot be eliminated. If there are many random factors affecting a random variable, and the influence of each factor alone cannot play a decisive role, and the influence of these factors can be superimposed, then the random variable obeys normal distribution, and the characteristic parameters have standard deviationσ And mean valueμ. The voltage value of the battery charging and discharging process is a comprehensive reflection of the thermodynamic and dynamic state of the battery. It is not only affected by the technological conditions of each process in the battery production process, but also by the current, temperature, time and accidental factors in the battery charging and discharging process. Therefore, the voltage value of each battery in the battery pack cannot be completely the same.

Improvement measures

1. Production process measures The battery company should control the consistency of various raw materials; The rheology monitoring of the slurry shall not be used for a long time to ensure that the rheology of the slurry is the same during coating; Monitoring of coating parameters, especially the lithium iron phosphate slurry. Due to the fine particles of lithium iron phosphate and the poor processing performance of the slurry, attention should be paid to slow down the speed of the coating machine during coating; Reasonable detection of slurry viscosity; Visual inspection of the pole piece; Remove the defective pole piece; Polar piece weighing; Comparison of battery quality before and after liquid injection; Formation temperature; Humidity control; Formulate standards for various raw materials, and inspect and store raw materials in strict accordance with the standards; Consistency control of production process; Fine control the consistency of production process; Carry out strict statistical process control (SPC) for the process; Ensure that each process is within the specified tolerance range; Ensure that the process capability index follows the normal distribution law of production parameters.

2. The measures in the matching process ensure that the battery pack adopts the same specification and model, and ensure the consistency of the battery quality, especially the initial voltage. The screening condition is voltage; Internal resistance; Battery formation data; There are many methods for consistency evaluation. At present, the most commonly used methods are range coefficient method, standard deviation coefficient method and threshold method. Combined with cluster analysis, the shape, distance and area of the battery charge-discharge curve formed by each detection point within the time interval are used to classify scientifically, so as to determine the consistency of the battery. Based on the capacity or voltage threshold, the shape of the charge-discharge curve, the distance between the curves, and the area around the curve are calculated, and the parameters that can reflect the consistency of the curve are selected for judgment. In the process of charging and discharging, select the cells with close curves, small relative distance, small area enclosed by the curve, and small difference between groups to match, so as to achieve the optimal uniform matching.

3. From the perspective of the battery management system, the battery balance management detects the single battery parameters during the use of the battery pack, especially the voltage distribution during the stopping or driving process of the electric vehicle, grasps the development law of the single battery inconsistency in the battery pack, and timely adjusts or replaces the extreme parameter batteries to ensure that the battery pack parameter inconsistency does not increase with the use time. Prevent the battery from overcharging, and try to prevent the battery from deep discharge. Ensure a good use environment for the battery pack, try to ensure constant temperature, reduce vibration, and ensure that water, dust, etc. pollute the battery pole. At the same time, from the perspective of energy management and strategy, introduce a practical battery energy management and balance system, formulate a reasonable battery balance strategy, and actively intervene and reduce battery inconsistency.

4. Battery thermal management During the use of the battery, the difference of internal resistance, battery layout and other factors will lead to the difference of its own temperature and ambient temperature during the charging and discharging process, which will directly lead to the difference of its output performance. The purpose of battery thermal management is to keep the operating temperature of the battery pack within the optimal operating temperature range of the battery. Ensure the consistency of temperature conditions between batteries, so as to ensure the consistency of battery service parameters. (The life of the battery is different at different temperature states, and the degradation rate will double every 10 ℃ rise in temperature)

5. In terms of energy management, the control strategy should minimize the depth of battery discharge when the output power is allowed. The consistency of lithium-ion battery under the condition of deep discharge will become worse, and the life of battery pack will also be reduced. Try to prevent the battery from overcharging while avoiding deep discharge. The equalization circuit in the system can prevent overcharging of individual batteries, and properly reduce the charging termination voltage to extend the cycle life of the battery pack.

6. During the routine maintenance during other use, the battery with low capacity in the measurement shall be charged separately for maintenance to restore its performance. The battery pack shall be charged with low current maintenance at intervals to promote the balance and performance recovery of the battery pack itself. In terms of use environment, ensure the good use environment of the battery pack is consistent, reduce vibration, and prevent water, dust, etc. from polluting the battery pole. This content is generally difficult to apply to the vehicle, and it also needs battery system and vehicle controller to realize.

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