What are the factors that lead to inconsistent parameters of Ni-MH battery packs and modules?

(1) Factors that lead to inconsistent parameters of Ni-MH battery packs

The inconsistent parameters of Ni-MH battery packs are a cumulative process. The longer the time, the greater the difference in parameters between Ni-MH battery packs. The reason is that during the use of lithium-ion battery modules, the inconsistency of lithium-ion battery cell parameters is gradually amplified by the influence of environmental factors, which leads to the accelerated attenuation of the performance of some Ni-MH battery packs and eventually the end of the life of lithium-ion battery modules. Factors that affect the inconsistent parameters of Ni-MH battery packs are:

1) During the manufacturing process of Ni-MH battery packs, the instability of the manufacturing process, material precision and purity leads to inconsistent performance parameters of the final product. Ni-MH battery packs processed with positive electrodes, negative electrodes and electrolytes from different batches are generally not mixed. Even if the parameters of Ni-MH battery packs are very consistent during the sorting process, the state of Ni-MH battery packs after a period of use in the future cannot be guaranteed. Therefore, the current solution is to be cautious about mixing Ni-MH battery packs processed with different batches of positive electrodes, negative electrodes and electrolytes.

Consistency is affected by many factors such as processing procedures

2) The processing technology of Ni-MH battery packs is relatively complicated. During the entire processing process, the consistency of each process is very important, but the most difficult to ensure consistency is the coating process. The coating thickness and uniformity and material activity are not easy to strictly control by mechanical means, which is an important process that causes differences in Ni-MH battery packs. Differences in the manufacturing process can only be compensated in the sorting process. In the processing of Ni-MH battery packs, even if the process of ingredients, active slurry, coating, cutting, rolling and other processes are strictly controlled, it only reduces the standard deviation between batches of products, but cannot eliminate inconsistency.

(2) Factors leading to inconsistent parameters of lithium-ion battery modules

The main structures of Ni-MH battery packs include positive electrode, positive electrode current collector, negative electrode, negative electrode current collector, diaphragm and electrolyte. The inconsistency of the performance of Ni-MH battery packs can ultimately be traced back to these components. The inconsistency of Ni-MH battery packs will inevitably be transmitted to the lithium-ion battery module, and the importance of lithium-ion battery cell sorting will be reflected in the formed lithium-ion battery module.

The single Ni-MH battery packs are welded, clamped, connected in series and parallel, and connected together to form a lithium-ion battery module. The inconsistent process in this processing process will inevitably lead to inconsistency between lithium-ion battery modules. The most intuitive manifestation is the inconsistent internal resistance of lithium-ion battery modules.

1) In the process of assembling lithium-ion battery modules, inconsistent welding processes can easily cause differences in welding resistance. The internal resistance of Ni-MH battery packs is on the order of several or dozens of milliohms. Slight inconsistency in welding resistance during the welding process is unacceptable for lithium-ion battery modules.

2) The size, connection method and surface treatment of the copper busbar or high-voltage wire used for connecting the internal and external modules of lithium-ion battery modules will also affect the consistency of lithium-ion battery modules, which is mainly reflected in the internal resistance of lithium-ion battery modules.

3) The lithium-ion battery modules with different connection methods (screw connection, crimping or welding) have different effects on the consistency of lithium-ion battery modules during use. In addition to considering the processability of production, the influence of connection resistance should also be considered. For the connection points using welding, if the surface treatment is not done well and oxidation corrosion occurs, its resistance will naturally increase. The resistance of the solder joints without rust will be different from that of the solder joints with rust.

(3) Factors that cause inconsistent parameters of lithium-ion battery modules after they are put into use

After the lithium-ion battery modules are put into use, the parameters of each single lithium-ion battery cell in the lithium-ion battery module are affected by differences in electrolyte density, temperature and ventilation conditions, self-discharge degree and charging and discharging process. Because the positions of Ni-MH battery packs in the lithium-ion battery module are different, the heat dissipation conditions of the Ni-MH battery packs wrapped in the center of the lithium-ion battery module and the single Ni-MH battery packs in the outermost layer of the lithium-ion battery module are very different. Before reaching thermal equilibrium, the temperature conditions of different single Ni-MH battery packs are different.

1) If the lithium-ion battery pack is installed in different positions, the influence of different ambient temperatures on the consistency of the lithium-ion battery module is also different. Due to different distances from the external environment and heat sources, the temperature conditions of lithium-ion battery modules at different positions in the lithium-ion battery pack will be different.

2) The different positions of the current collecting copper bars of Ni-MH battery packs will cause different heat dissipation conditions between them, resulting in inconsistency of the thermal environment temperature of the Ni-MH battery packs.

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