Editor's note
Lithium-ion battery consistency refers to the consistency of the initial performance indexes of the individual cells used in the group, including: capacity, impedance, electrical characteristics of the electrodes, electrical connections, temperature characteristics, decay speed, and the like. The inconsistency of the above factors will directly affect the difference in output electrical parameters during operation.
The large-scale application of lithium-ion batteries in new energy vehicles and smart grids 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 level of thermal management is improved to some extent. The safe operation of the battery pack is guaranteed, but the level of consistency of the battery is still an important technical factor for the large-scale use of lithium batteries.
The simulation of a 10-string 10-and-battery battery illustrates the effect of temperature distribution within the battery pack on its performance and cycle life. The lower the average temperature, the higher the temperature unevenness, and the higher the inconsistency of the discharge depth of the single cells in the battery pack; the higher the average temperature, the higher the temperature unevenness, and the shorter the cycle life of the battery pack. It is worth noting that the uneven temperature distribution leads to uneven current distribution between the parallel branches, which deteriorates the uniformity of the aging rate of the single cells.
Lithium-ion battery consistency refers to the consistency of the initial performance indexes of the individual cells used in the group, including: capacity, impedance, electrical characteristics of the electrodes, electrical connections, temperature characteristics, decay speed, and the like. 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, decay rate, internal resistance, and time-varying rate, life, and temperature of the battery pack of the same specification. Impact, self-discharge rate and its rate of change over time.
After the single cell is manufactured, there is a certain performance difference. The initial inconsistency is accumulated as the battery continues to charge and discharge cycles during use, resulting in a greater difference in the state of each cell (SOC, voltage, etc.); the use environment within the battery pack is not for each cell. Do the same. This leads to the inconsistency of the single cells being gradually amplified during use, which in some cases accelerates the attenuation of certain single cell performance and eventually causes the battery pack to fail prematurely.
The reason for the inconsistency is divided by chronological order. The inconsistency of the single cells in the battery pack is mainly reflected in two aspects: the technical 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 slight differences in rate, strips, partitions, etc., resulting in incomplete consistency of internal structure and material; electrolyte density, temperature and ventilation conditions, self-discharge degree of each battery in the battery pack when used in loading And the effects of differences in charging and discharging processes.
Analysis of the differences in capacity, internal resistance and self-discharge of the same type of battery manufactured in the same batch. For the reasons of these inconsistencies, whether the inconsistency in the battery pack can be completely eliminated by some measures: Many people think that the battery inconsistency is the production process. The problem is also considered to be a problem in the matching process. The process control measures such as SPC can completely eliminate the inconsistency of the battery.
However, practice has proved that even strict control of the process of batching, pulping, coating, cutting, rolling, etc., only reduces the standard deviation between batch products, and can not eliminate the inconsistency. If there are many random factors affecting a random variable, and the influence of each of them can't play a decisive role alone, and the influence of these factors can be superimposed, then the random variable obeys the normal distribution, and the parameters of the characteristic have the standard deviation σ and 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. It is affected by the process conditions of each process in the battery production process, and is affected by the current, temperature, time and accidental factors in the process of battery charging and discharging. The effect is therefore that the voltage values ​​of the individual cells in the battery pack may not be exactly the same.
Improvement measures
1. Production process measures, the battery enterprise controls the consistency of various raw materials; the rheology monitoring of the slurry, the slurry is not left for a long time, and the rheology of the slurry is the same when coating; the coating parameter monitoring, especially It is a lithium iron phosphate slurry. Since the lithium iron phosphate particles are fine, the processing property of the slurry is poor. At the time of coating, attention should be paid to slowing down the speed of the coater; reasonable detection of the viscosity of the slurry; inspection of the appearance of the pole piece; Elimination of pole pieces with flaws; pole piece weighing; comparison of battery quality before and after liquid injection; formation temperature; humidity control; standardization of various raw materials, inspection and storage of raw materials in strict accordance with standards; consistency control of production process; Fine control of the consistency of the production process; strict statistical process control (SPC) of the process; ensure that each process is within the specified tolerances; ensure that the process capability index, so that it follows the normal distribution of production parameters.
2. The measures of the matching process ensure that the battery pack adopts the battery of uniform specification and model to ensure the consistency of the factory quality of the battery, especially the initial voltage. The screening conditions are: voltage; internal resistance; battery formation data; there are many methods for consistency evaluation. The most commonly used methods are the range coefficient method, the standard deviation coefficient method, and the 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 the capacity or voltage threshold, the shape of the charge and discharge curve, the distance between the curves, the area around the curve, etc. are calculated, and the parameters that can reflect the curve are selected for determination. During the charging and discharging process, the curves are close, the relative distance is small, the area around the curve is small, and the cells with small differences between the groups are matched to achieve optimal matching.
3, battery balance management, from the perspective of the battery management system, in the use of the battery pack to detect the parameters of the battery, especially the voltage distribution during the stop or driving of the electric vehicle, master the development law of the inconsistency of the single battery in the battery pack, to the extreme The parameter battery is adjusted or replaced in time to ensure that the battery group parameter inconsistency does not increase with the use time. Avoid overcharging the battery and try to prevent the battery from being deeply discharged.
Ensure a good use environment of the battery pack, try to ensure constant temperature, reduce vibration, and ensure that water, dust and other contaminated battery poles. At the same time, from the energy management and strategy, the practical battery management and equalization system is introduced, and a reasonable battery balancing strategy is established to actively intervene and reduce battery inconsistency.
4. Battery thermal management During the use of the battery, the difference of internal resistance, battery arrangement and other factors will cause differences in self-temperature and ambient temperature during charging and discharging, which will directly lead to differences in output performance. The battery thermal management function 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 the batteries are consistent, thus ensuring the consistency of battery usage parameters. (The life of the battery is different at different temperature conditions, and the degradation rate doubles for every 10 °C increase in temperature)
5. Control strategy In the aspect of energy management, if the output power is allowed, try to reduce the battery discharge depth. Lithium-ion batteries have poor consistency under deep discharge conditions, and the life of the battery pack is also reduced. Try to prevent the battery from being over-charged while avoiding deep discharge of the battery. The system has an equalization circuit to prevent overcharging of individual batteries, and appropriately reduce the charge termination voltage, which can extend the cycle life of the battery pack.
6. Other use process During the routine maintenance process, the battery with low capacity in the measurement is separately maintained and charged to restore its performance. The battery pack is subjected to small current maintenance charging at intervals to promote the balance and performance recovery of the battery pack itself. In terms of the use environment, ensure that the battery pack has a good environment for use, reduce vibration, and avoid water, dust, etc. from contaminating the battery pole. This content is generally difficult to use in the car, and the battery system and the vehicle controller are also required to be used.
Conclusion: Improving the consistency of the battery is a systematic project, which requires the consideration of battery design, production, quality control, application and maintenance.
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