The consistency of battery cells is important for power battery pack

The current large-scale application of lithium-ion batteries in new energy vehicles, smart grids and other fields is increasing year by year, but the current inconsistency of battery parameters is a key factor affecting the service life of battery packs, although the improvement of thermal management level is to some extent The safe operation of the battery pack is guaranteed, but it is still an important technical influence factor for the large-scale use of lithium batteries for improving the consistency of the battery.

By simulating a 10-series 10-parallel battery pack, the influence of the temperature distribution in the battery pack on its performance and cycle life is clarified. The lower the average temperature, the higher the degree of temperature unevenness, the higher the inconsistency of the discharge depth of the single cells in the battery pack; the higher the average temperature, the higher the degree of temperature unevenness, the shorter the cycle life of the battery pack. It is worth noting that the uneven temperature distribution will cause uneven current distribution among the parallel branches, thereby worsening the consistency of the aging rate of the single cell.

Lithium-ion battery consistency refers to the consistency of the initial performance indicators of a group of single cells, including: capacity, impedance, electrical characteristics of electrodes, electrical connections, temperature characteristics, decay rate, etc. The inconsistency of the above factors will directly affect the difference in output electrical parameters during operation.

The inconsistency of the lithium-ion battery pack or the discrete phenomenon of the battery pack refers to the voltage, charge capacity, capacity, decay rate, internal resistance and its rate of change over time, life, and temperature of the single battery of the same specification and type to form the battery pack Influence, self-discharge rate and its rate of change with time.

After a single battery is manufactured, there are certain performance differences. The initial inconsistency is accumulated with the continuous charge and discharge cycles of the battery during use, resulting in greater differences in the state of each cell (SOC, voltage, etc.); the use environment within the battery pack is also not for each cell Exactly the same. This causes the inconsistency of the single cells to gradually increase during use, which in some cases accelerates the degradation of the performance of some single cells 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 cells in the battery pack is mainly reflected in two aspects: the process problems in the manufacturing process and the unevenness of the material, so that the activation degree and thickness of the active material of the battery plate and the micropores There are very slight differences in the rate, connecting strips, separators, etc., resulting in incomplete consistency in internal structure and materials; when used in vehicles, the electrolyte density, temperature and ventilation conditions of each battery in the battery pack, the degree of self-discharge And the impact of differences in charging and discharging process.

Analysis of the difference in capacity, internal resistance and self-discharge of the same type of batteries shipped from the same batch. For the reasons for these inconsistencies, can some measures be used 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 that it is a problem of the assembly process, through SPC and other process control measures can be completely eliminate battery inconsistencies. However, practice has proved that even strict control of batching, slurry, coating, cutting, rolling and other processes only reduces the standard deviation between batches of products, but cannot eliminate inconsistencies. If there are many random factors that affect a random variable, and the influence of each of these factors 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 parameters of the characteristics have standard deviation σ and Mean μ. The voltage value during the battery charging and discharging process is a comprehensive reflection of the battery's thermodynamic and dynamic state, which is not only affected by the process conditions of each process in the battery production process, but also by the current, temperature, time and accidental factors during the battery charging and discharging process Therefore, the voltage value of each battery in the battery pack cannot be exactly the same.

Improvement measures

1. Production process measures, battery companies control the consistency of various raw materials; slurry rheology monitoring, do not leave the slurry for a long time to ensure that the slurry rheology is the same when coating; coating parameter monitoring, especially phosphoric acid Lithium iron slurry, due to the fine particles of lithium iron phosphate, the processing performance of the slurry is poor, you should pay attention to slow down the speed of the coating machine when coating; the reasonable detection of the viscosity of the slurry; the appearance inspection of the pole piece; Defective pole piece; pole piece weighing; comparison of battery quality before and after injection; formation temperature; humidity control; formulating various raw material standards, strictly inspecting and storing raw materials in accordance with the standard; consistent control of production process; production Fine-tune the consistency of the process; strict statistical process control (SPC) of the process; ensure that each process is within the specified tolerance range; ensure the process capability index so that it follows the normal production parameter distribution law.

2. Measures in the assembly process to ensure that the battery pack uses uniform specifications and types of batteries to ensure the quality of the battery factory, especially the consistency of the initial voltage. The screening conditions are: voltage; internal resistance; battery formation data; there are many consistency evaluation methods. Frequently used 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 within a set time interval are used for scientific classification to determine the consistency of the battery. Based on the capacity or voltage threshold, the shape of the charge and discharge curves, the distance between the curves, the area taken by the curve, etc. are calculated, and the parameters that reflect the consistency of the curves are selected for judgment. In the process of selecting charge and discharge, the curves are close, the relative distance is small, the area enclosed by the curve is small, and the cells with small differences between the groups are grouped to achieve the optimal consistent grouping.

The battery capacity testing equipment and automatic battery cell sorting machine produced by WinAck Battery can greatly improve the cell consistency. In addition, the highly automated battery pack assembly line integrates assembly and testing, which is controlled from the assembly production stage to ensure the consistency of the battery pack.

3.Battery balancing management, from the perspective of the battery management system, detects the parameters of single cells during the use of the battery pack, especially the voltage distribution during the stopping or driving of the electric vehicle, and masters the inconsistent development rules of single cells in the battery pack. Make timely adjustments or replacements to ensure that the battery pack parameter inconsistency does not increase with time of use. Avoid overcharging the battery and try to prevent the battery from deep discharge. Ensure a good use environment of the battery pack, try to ensure a constant temperature, reduce vibration, and ensure that the poles of the battery are contaminated by water and dust. At the same time, from the perspective of energy management and strategy, a practical battery pack energy management and balancing system is introduced to formulate a reasonable battery balancing strategy to actively intervene and reduce battery inconsistency.

4.Battery thermal management: During battery use, differences in internal resistance, battery layout and other factors will cause differences in their own temperature and ambient temperature during charging and discharging, which will directly lead to differences in their output performance. The role 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 use parameters. (The life of the battery is different in different temperature states, and its degradation rate doubles every time the temperature rises by 10°C).

5. In terms of energy management, the control strategy minimizes the depth of battery discharge when the output power allows. The consistency of lithium-ion batteries under deep discharge conditions deteriorates, and the life of the battery pack also decreases. Try to prevent the battery from deep discharging and avoid overcharging the battery. Equipped with an equalization circuit in the system can prevent overcharging of individual batteries, appropriately reduce the charge termination voltage, and extend the cycle life of the battery pack.

6. During other routine maintenance procedures, the battery with low capacity in the measurement is separately maintained and charged to restore its performance. The battery pack is charged with small current maintenance at regular intervals to promote the battery pack's own balance and performance recovery. In terms of use environment, ensure that the battery pack has a good use environment consistent, reduce vibration, and avoid contamination of the battery poles with water and dust. This content is generally difficult to use in a car, and the battery system and the vehicle controller are required to use it.

Conclusion: Improving the consistency of the battery is a systematic project, which needs to be considered in many aspects such as battery design, production, quality control, application, and maintenance.