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Six ways to improve battery pack consistency

- Nov 29, 2018 -

Lithium ion batteries at present in the field of new energy vehicles, such as smart grid large-scale application in increase year by year, but the battery parameters of inconsistencies are the key factors affecting the service life of the battery pack, while the thermal management level of ascension to some extent to ensure the safe operation of the battery pack, but to improve the consistency of the battery is still the important technical factors affecting the mass use of lithium batteries.

Through the simulation of a 10-tandem 10-parallel battery, the effect of temperature distribution in the battery on its performance and cycle life is illustrated. The lower the average temperature, the higher the degree of temperature inhomogeneity, and the higher the inconsistency of discharge depth of single battery in the battery pack; The higher the average temperature, the more uneven the temperature, the shorter the battery cycle life. It is worth noting that uneven temperature distribution will lead to uneven current distribution between parallel branches, thus worsening the uniformity of single battery aging rate.

In the aspect of battery thermal management, we propose a fast method to estimate the flow field and temperature field inside the parallel air-cooled battery. This method is composed of flow resistance network model and transient heat transfer model, which avoids the problem that computational fluid dynamics (CFD) method is used to simulate large battery pack with too much computation, and ensures high estimation accuracy.

By using this method, we investigate the influence of the non-uniform flow field on the temperature uniformity in the battery pack, and with the aim of improving the temperature uniformity, discuss the structural parameters of the air cooling system, and give a feasible parameter configuration scheme.

Due to the difference in the parameters of the same type and specification battery in voltage, internal resistance, capacity and other aspects, the performance index of the battery used in electric vehicles is often not up to the original level of the single battery, which seriously affects its application in electric vehicles.

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

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

After the single battery is manufactured, it has certain performance differences. The initial inconsistency accumulates with the continuous charging and discharging cycles in the use of the battery, resulting in greater differences in the state of each single battery (SOC, voltage, etc.). The operating environment of the battery pack is different for each individual battery. This leads to the gradual amplification of the inconsistencies in the use of single cells, which in some cases accelerates the attenuation of the performance of some single cells and ultimately leads to premature failure of the battery pack.

Inconsistencies reason from the time order, divided into single cells in the battery inconsistency is mainly manifested in two aspects: the problem on the process and material in the manufacturing process of uneven, makes the battery plate activation degree of active material and thickness, pore rate, even a very small differences, partition, etc, to produce on the internal structure and material is not completely consistent; During loading and using, the electrolyte density, temperature and ventilation conditions, self-discharge degree and charging and discharging process of each battery in the battery pack are affected by differences.

Analysis of the reasons for the differences in capacity, internal resistance and self-discharge of the same type of battery produced in 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 inconsistencies in the battery pack are caused by the production process, while others think that the inconsistencies in the battery pack are caused by the assembly process. SPC and other process control measures can completely eliminate the inconsistencies in the battery pack. However, practice has proved that even if the process of batching, slurry, coating, cutting and roller pressing is strictly controlled, the standard deviation between batch products can only be reduced, and inconsistency cannot be eliminated. If there are many factors influencing 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 parameters of the characteristic have standard deviation and mean microns. Battery charging and discharging process of the voltage value is the comprehensive reflection of battery state of thermodynamics and kinetics, both by the influence of battery production process conditions in various processes, and the battery charge and discharge current, temperature, time, and in the process of the accidental factors in the use process resulting in individual batteries in the battery voltage value can't be exactly the same.

Improvement measures

1. Production process measures

Battery enterprises control the consistency of various raw materials; The rheological monitoring of slurry, do not use the slurry for a long time, ensure that the slurry is the same rheological coating; Coating parameter monitoring, especially lithium iron phosphate slurry, because the lithium iron phosphate particles are relatively fine, slurry processing performance is poor, in the coating should pay attention to slow down the speed of the coating machine; Reasonable determination of viscosity of slurry; Inspection of the appearance of the polar plate; Remove the defective pole; Pole plate weighing; Comparison of battery quality before and after liquid injection; Temperature of formation; Humidity control;

To formulate standards for various raw materials, inspect and store raw materials in strict accordance with the standards; Consistent control of production process; Fine control of the consistency of production process; Strict statistical process control (SPC) is carried out in the process; Ensure that each process is within the specified tolerance range; Ensure process capability index to follow normal production parameter distribution.

2. Measures for the allocation process

Ensure that the battery pack adopts uniform specifications and models, ensure the quality of the battery factory especially the consistency of the initial voltage, screening conditions: voltage; Internal resistance. Battery conversion data; There are many methods for consistency evaluation. The most commonly used methods are range coefficient method, standard deviation coefficient method and threshold method. Combined with clustering analysis, the shape, distance and area of the charging and discharging curve of the battery formed by each detection point within the set time interval are scientifically classified to determine the consistency of the battery.

On the basis of capacity or voltage threshold, the shape of charge-discharge curve, the distance between curves and the surrounding area of curves are calculated, and the parameters that can reflect the consistency of curves are selected for judgment. In the process of charging and discharging, the curve is relatively close, the relative distance is relatively small, the area enclosed by the curve is small, and the cells with small differences between groups are allocated, so as to achieve the optimal uniform allocation.

3. Balanced battery management

From the point of view of battery management system, in the process of the battery pack using testing single battery parameters, especially the electric cars stopped or driving voltage distribution in the process, master single cell inconsistency development regularity in battery, the battery parameters of extreme adjust or change, in order to make sure the battery parameters not consistency increases with time. Avoid battery overcharge and deep discharge.

Ensure the battery pack good service environment, as far as possible to ensure constant temperature, reduce vibration, ensure water, dust and other pollution of the battery pole pole. At the same time, from the energy management and strategy, the introduction of practical battery pack energy management and balance system, develop a reasonable battery balance strategy, active intervention and reduce the battery inconsistency.

4. Battery thermal management

In the process of battery use, the difference in internal resistance, battery layout and other factors will lead to the difference in its own temperature and environmental temperature in the charging and discharging process, which will directly lead to the difference in its output performance. The function of battery thermal management is to keep the working temperature of the battery set within the optimal working temperature range of the battery. Ensure the consistency of temperature conditions between batteries, so as to ensure the consistency of battery parameters. (battery in different temperature condition is different, the life of a temperature rise every 10 ℃ its degradation speed is double)

5. Control strategy

In terms of energy management, the discharge depth of the battery should be minimized as long as the output power is allowed. The consistency of li-ion battery in the condition of deep discharge becomes worse, and the battery life will be reduced. Try to prevent the battery deep discharge at the same time, avoid the battery overcharge. With the balanced circuit in the system, the overcharge of individual batteries can be prevented, and the charging termination voltage can be appropriately reduced to extend the cycle life of the battery pack.

6. Other use procedures

During daily maintenance, the battery with low capacity in the measurement is charged separately for maintenance to restore its performance. Small current maintenance charge is carried out on the battery pack at a certain interval to promote the balance and performance recovery of the battery pack. In terms of the operating environment, it is necessary to ensure the good operating environment of the battery pack, reduce vibration and avoid water, dust and other pollution of the battery pole pole. This content, generally difficult to use in the car, to use the battery system and the vehicle controller to achieve.