Editor's note
    Lithium-ion battery consistency refers to the consistency of the initial performance indicators of the single battery used in groups, including: capacity, impedance, electrical characteristics of the electrode, electrical connection, temperature characteristics, decay rate, etc. The inconsistency of the above factors will directly affect the difference of the output electrical parameters in operation.
    At present, the large-scale application of lithium-ion batteries in new energy vehicles, smart grid and other fields is increasing year by year, but the current inconsistency of battery parameters is the key factor affecting the service life of battery packs. Although the improvement of thermal management level ensures the safe operation of battery packs to some extent, However, to improve the consistency level of the battery is still an important technical influence factor for the large-scale use of lithium batteries.
    The influence of temperature distribution on the performance and cycle life of a 10 series 10 parallel battery pack was studied by simulating the battery pack. The lower the average temperature is, the higher the degree of temperature non-uniformity is, and the higher the discontinuity of single battery depth is. The higher the average temperature, the higher the degree of temperature heterogeneity, and the shorter the cycle life of the battery pack. It is worth noting that the uneven temperature distribution will cause the uneven distribution of current between the parallel branches, thus worsening the consistency of the aging rate of a single battery.
    Lithium-ion battery consistency refers to the consistency of the initial performance indicators of the single battery used in groups, including: capacity, impedance, electrical characteristics of the electrode, electrical connection, temperature characteristics, decay rate, etc. The inconsistency of the above factors will directly affect the difference of the output electrical parameters in operation.
    The inconsistency of the lithium-ion battery pack or the dispersion of the battery pack refers to the voltage, charge, capacity, decay rate, internal resistance and its rate of change with time, life, temperature effect, self-discharge rate and its rate of change with time after the single battery of the same specification and model is composed of the battery pack.
    After the single battery is manufactured, there are certain performance differences in itself. The initial inconsistency accumulates with the continuous charge and discharge cycle of the battery during use, resulting in greater differences in the state of each individual battery (SOC, voltage, etc.); The operating environment in the battery pack is also different for each individual battery. This results in the gradual amplification of the inconsistencies of the single battery during use, which in some cases accelerates the deterioration of the performance of some single batteries, and eventually causes the premature failure of the battery pack.
    The reason for the inconsistency is divided from the chronological order, the inconsistency of the single battery in the battery pack is mainly reflected in two aspects: the process problems in the manufacturing process and the uneven material, so that the activation degree and thickness of the active material of the battery plate are very small differences, resulting in the incomplete consistency of the internal structure and material; When loading and using, the difference of electrolyte density, temperature and ventilation conditions, self-discharge degree and charge and discharge process of each battery pack is affected.

    Analysis of the differences in capacity, internal resistance and self-discharge of the same type of battery from the same batch factory In view of the causes of these inconsistencies, whether the inconsistencies in the battery pack can be completely eliminated through some measures: Many people think that the battery inconsistency is a problem of the production process, and some people think that it is a problem of the group process, and the inconsistency of the battery can be completely eliminated through process control measures such as SPC.
    However, practice has proved that even if the process of batching, sizing, coating, cutting, rolling and so on is strictly controlled, it only reduces the standard deviation between batch products, but cannot eliminate the inconsistency. 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 follows a normal distribution, and the characteristic parameters have the standard deviation σ and the mean μ.
    The voltage value of the battery charging and discharging process is a comprehensive reflection of the thermodynamic and dynamic state of the battery, which is not only affected by the process conditions of the battery production process, but also affected by the current, temperature, time and accidental factors during the battery charging and discharging process, so the voltage value of each battery in the battery pack cannot be exactly the same.

Improvement measure
    1, the production process measures, battery enterprises control the consistency of various raw materials; Rheological monitoring of the slurry, do not shelving the slurry for a long time, to ensure that the rheological properties of the slurry are the same when coating; Coating parameters monitoring, especially lithium iron phosphate slurry, because lithium iron phosphate particles are fine, the processing performance of the slurry is poor, should pay attention to slow down the speed of the coating machine during coating; Reasonable detection of slurry viscosity; The appearance inspection of the polar plate; Remove defective pole film; Pole plate weighing; Comparison of battery quality before and after injection; Formation temperature; Humidity control; Formulate the standards of various raw materials, and strictly inspect and store the raw materials according to the standards; Consistent control of production process; Fine control of the consistency of the production process; Strict statistical process control (SPC) is carried out in the process. Ensure that each process is within specified tolerances; Ensure that the process capability index follows the normal distribution of production parameters.
    2, the measures of the assembly process, to ensure that the battery pack uses a unified specification, model battery, to ensure the quality of the battery, especially the consistency of the initial voltage, screening conditions: voltage; Internal resistance; Battery formation data; There are many consistency evaluation methods, the most commonly used at present are range coefficient method, standard deviation coefficient method and threshold method. Combined with cluster analysis, the shape, distance and area of the battery charge and discharge curve formed by each detection point in 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 and discharge curve, the distance between curves and the area enclosed by 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 curves are close, the relative distance is small, the area enclosed by the curves is small, and the difference between groups is small, so as to achieve the optimal uniform group.
    3, battery balanced management, from the perspective of battery management system, in the use of the battery pack to detect single battery parameters, especially the electric vehicle stop or driving voltage distribution, master the single battery in the battery pack inconsistent development law, the extreme parameter battery timely adjustment or replacement, to ensure that the battery pack parameter inconsistency does not increase with the use of time. Avoid overcharging and deep discharge of the battery.
    Ensure a good use environment of the battery pack, try to ensure constant temperature, reduce vibration, and ensure that water, dust and other pollution of the battery pole. At the same time, from the aspect of energy management and strategy, the practical battery pack energy management and balance system is introduced to formulate a reasonable battery balance strategy, and actively intervene and reduce the inconsistency of the battery.
    4, battery thermal management During the use of the battery, the internal resistance, battery layout and other factors, will appear in the charge and discharge process of its own temperature and environmental temperature differences, which will directly lead to differences in its output performance. The function of battery thermal management is to keep the operating temperature of the battery pack within the optimal operating temperature range of the battery. Ensure that the temperature conditions between batteries are consistent, so as to ensure the consistency of battery parameters. (The life of the battery is different at different temperatures, and the degradation rate doubles for every 10 ° C increase in temperature)
    5, control strategy In terms of energy management, the output power allows to minimize the depth of battery discharge. The consistency of lithium-ion batteries under deep discharge conditions becomes worse, and the life of the battery pack will also be reduced. Try to prevent the battery from deep discharge at the same time, avoid overcharging the battery. The equalization circuit in the system can prevent the overcharging of individual batteries, and appropriately reduce the charging termination voltage, which can extend the cycle life of the battery pack.
    6. In the daily maintenance process of other use processes, the battery with low capacity in the measurement is charged separately for maintenance to restore its performance. The battery pack is charged with small current maintenance at intervals to promote the balance and performance recovery of the battery pack itself. In terms of use environment, ensure a good use environment of the battery pack, reduce vibration, and avoid water, dust and other pollution of the battery pole. This content is generally difficult to use on the car, and the battery system and vehicle controller are also needed to achieve it.
    Conclusion: Improving the consistency of battery is a systematic project, which needs to be considered in the design, production, quality control, application and maintenance of battery. Degree and ventilation conditions, degree of self-discharge and charge and discharge process and other differences.