Battery pack Inconsistency analysis
Inconsistency analysis
1.1 The meaning of inconsistency
Inconsistency of lithium-ion batteries means that there are some differences in voltage, capacity, internal resistance, life, temperature effect, self-discharge rate and other parameters after the single batteries of the same specification and model form a battery pack.
After the single battery manufactured, there are some differences in its initial performance. With the use of batteries, these performance differences accumulate continuously, and at the same time, because the use environment of each single battery in the battery pack is not exactly the same, the inconsistency of the single battery gradually enlarged, thus accelerating the performance attenuation of the battery and eventually causing the premature failure of the battery pack.
1.2 The performance of inconsistency
Inconsistency of lithium-ion batteries mainly manifested in two aspects: the difference of battery performance parameters (battery capacity, internal resistance and self-discharge rate, etc.) and the difference of battery state of charge (SOC).
We found the difference distribution capacity between cells is close to Weil distribution.but the dispersion degree of internal resistance is more significant than capacity, and the internal resistance of the same batch of batteries generally meets the normal distribution law, and the self-discharge rate also presents an approximate normal distribution. SOC represents the state of charge of the battery, which is the ratio of the remaining capacity of the battery to the rated capacity. It believed that the maximum available capacity of the batteries is different due to the inconsistency of the batteries and the different capacity decay rates. The SOC of the battery with small capacity changes faster than that of the battery with large capacity, and reaches the cut-off voltage faster when charging and discharging.
1.3 Causes of inconsistency
There are many reasons for the inconsistency of lithium-ion batteries, especially in the manufacturing process and the use process. Every link in the manufacturing process, such as the uniformity of slurry during batching, the control of surface density and surface tension during coating, etc., will cause differences in the performance of single cells.
After studying the influence of the manufacturing process of lithium-ion battery on the consistency of the battery, the influence of the production process of lithium-ion battery with water-based binder system on the consistency of the battery emphatically studied. In the process of using the battery, we think the connection mode, structural parts/devices, working conditions and environment will affect the consistency of the battery pack. Because the energy consumed by each connection point is inconsistent, the performance and aging rate of each component or structure are also inconsistent, so the impact on the battery is also inconsistent. In addition, due to the different position, temperature and performance attenuation of each single cell in the battery, the inconsistency of single cells will amplified.
Second, methods to improve battery consistency
2.1 Control of production process
The control of production process is important from two aspects: raw materials and production technology. As for raw materials, try to select the same batch of raw materials to ensure the consistency of particle size and performance of raw materials. The whole process should strictly controlled. such as ensuring the slurry to be evenly stirred and placed for a short time, controlling the speed of the coating machine to ensure the coating thickness and uniformity, inspecting the appearance of the pole pieces, weighing and grading, controlling the liquid injection amount and the conditions of formation, volume separation and storage, etc.
Through the research on the preparation technology of lithium-ion battery, the key technologies that have a great influence on the consistency of lithium-ion battery are determined, including mixing, coating, rolling, winding/laminating, liquid injection and formation, and the relationship between each key process parameter and battery performance deeply studied and analyzed.
2.2 Control of matching process
The control of the assembly process mainly refers to the sorting of batteries. The battery pack adopts batteries with uniform specifications and models, and the voltage, capacity and internal resistance of the batteries should measure to ensure the consistency of the initial performance of the batteries.
Through the research, it found that the voltage difference of single cells is an important factor affecting the consistency of each single cell at the end of battery charging and discharging, while the internal resistance difference of single cells leads to a great difference in the voltage platform of each single cell during battery charging and discharging. We analyzed the influence of DCR, an important factor in parallel battery, and the influence of capacity, an important factor in series battery, on the battery pack by studying the inconsistency of single cells in lithium ion series-parallel battery pack, so as to provide the necessary basis for the combined battery pack. Through the research on the influence of discharge rate on the consistency of battery assembly, Chen Ping and others found that with the increase of discharge rate, the inconsistency of batteries amplified, and the effect of eliminating bad batteries achieved.
2.3 Control of use and maintenance process
Monitor the battery in real time. Consistency screening of the batteries during matching can ensure the consistency of the battery pack at the initial stage of use. When the battery monitored in real time, the consistency problem can observed in real time. However, when the consistency is poor, the monitoring circuit will cut off the charging and discharging circuit, so the performance will reduced. We must find a balance between the two. The battery with extreme parameters can also adjusted or replaced in time through real-time monitoring to ensure that the inconsistency of the battery pack will not expand with time.
Introduce a balanced management system. The battery intelligently managed by adopting appropriate equalization strategies and equalization circuits. At present, common equalization, strategies include external voltage-based equalization strategy, SOC-based equalization strategy and capacity-based equalization strategy. The equalization circuit can divided into passive equalization and active equalization according to the energy consumption mode. Among them, active balancing can realize lossless energy flow between batteries, which is a hot research topic at home and abroad. The commonly used methods in active equalization include battery bypass, switched capacitor, switched inductance, DC/DC conversion etc.
Thermal management of the battery. In addition to keeping the working temperature of the battery pack in the optimal range as far as possible, the thermal management of the batteries should also try to ensure the consistency of temperature conditions among the batteries, thus effectively ensuring the consistency of performance among the batteries. Adopt reasonable control strategy. When the output power allows, the battery discharge depth should reduced as much as possible, and at the same time, the battery overcharge should prevented, which can prolong the cycle life of the battery pack. Strengthen the maintenance of battery packs. Carry out low-current maintenance charging on the battery pack at intervals, and pay attention to cleaning.
