In recent years, the automotive logistics industry and the supply and demand market have steadily developed. Overall, the entire industry has entered a period of stable and rapid development. The development of electric vehicles is a national trend, and electric vehicles are widely regarded as the main direction for the transformation and development of future automotive energy and power systems.
The mainstream battery equipped for pure electric logistics vehicles today is mainly large capacity square batteries, which have complex production processes and high production costs. The production volume of small capacity cylindrical batteries is large, and their applications are more widespread. Their production and replacement costs are relatively low.
Considering its strong advantages such as simple manufacturing process, stable performance, high yield, and low cost, small capacity cylindrical batteries will be a better choice. However, due to the limitations of small capacity cylindrical batteries, which have small individual capacities (1.5-2.5 Ah), in order to meet the energy and voltage requirements of pure electric logistics vehicles, a large number of battery cells need to be combined in series and parallel, resulting in issues related to battery consistency and connection reliability. This article mainly conducts theoretical research on the large-scale grouping of small capacity cylindrical lithium-ion power batteries, and tests the cycling performance and vibration resistance of battery module samples.
1 Theoretical research on lithium-ion power batteries
1.1 Theoretical Research on Battery Consistency
The inconsistency of batteries refers to the differences in parameters such as voltage, capacity, and internal resistance self discharge rate between batteries of the same specification and model. The generation of inconsistency mainly occurs in the process of product manufacturing.
Due to process issues and uneven materials, there are very small differences inside the battery. This type of inconsistency in the internal structure and material of the battery can make it impossible for the capacity, internal resistance, and other factors of the same batch of batteries of the same type to be completely consistent.
The adverse effects of increased battery pack inconsistency on the performance of power batteries are as follows:
(1) Reduced usable capacity of battery pack
Due to inconsistent voltage or capacity of the battery pack, and due to the short board effect, during charging and discharging, some individual cells first meet the cut-off conditions, while other batteries do not meet the cut-off conditions. In order to prevent overcharging or discharging of the battery, the battery management system intervenes, resulting in a decrease in the actual usable capacity of the battery pack.
(2) Reduced cycle life of battery pack
According to the barrel effect, the cycle life of a battery pack is determined by the cycle life of the module with the shortest lifespan.
For pure electric logistics vehicles, due to the low energy of small cylindrical lithium-ion power batteries, the entire battery system requires a large number of individual batteries to be connected in series or parallel, with a quantity of over 3000, which inevitably makes the consistency of the batteries difficult to control.
The consistency of battery packs is relative, while inconsistency is absolute. The inconsistency of batteries has already occurred during the production stage, and in the application process, certain measures need to be taken to alleviate the trend or speed of battery inconsistency expansion. Based on the application experience and experimental research of power batteries, the following six measures are often adopted to ensure that the battery pack life gradually approaches the service life of individual batteries.
1) Improve the level of battery manufacturing technology, ensure the quality of the battery before leaving the factory, especially the consistency of the initial voltage. Before leaving the factory for the same batch of batteries, parameter correlation analysis is conducted based on voltage, internal resistance, and battery formation data to select batteries with good correlation, in order to ensure that the performance of the same batch of batteries is as consistent as possible.
2) When assembling power batteries, it is important to ensure that the battery pack is of the same type, specification, and model.
3) During the use of the battery pack, detect the parameters of the single battery, especially the voltage distribution under dynamic and static conditions (when the electric vehicle is stopped or running), grasp the development law of the inconsistency of the single battery in the battery pack, and adjust or replace the extreme parameter batteries in a timely manner to ensure that the inconsistency of the battery pack parameters does not increase with usage time.
4) Perform low current maintenance charging on the battery pack at regular intervals to promote the balance and performance recovery of the battery pack itself.
5) Ensure a good usage environment for the battery pack, try to ensure a uniform temperature field of the battery pack, reduce vibration, and avoid water, dust, and other pollution of the battery poles.
6) Adopting a battery pack balancing system for intelligent management of battery pack charging and discharging.
1.2 Research on Connection Consistency Theory
For pure electric logistics vehicles, more than 3000 battery cells are often required to be connected in series, parallel, or hybrid to meet the demand for high voltage and high capacity, which inevitably increases the number of welding points and leads to connection consistency issues. If there is detachment or false soldering, it will cause an increase in the internal resistance of the connection.
On the one hand, it will have an impact on the charging and discharging of the battery system. On the other hand, during vehicle operation, it will cause heating at the connection point, which in severe cases will lead to safety issues. Therefore, theoretically speaking, the fewer welding points, the higher the consistency of the system’s connection.
1.3 Research on heat dissipation theory
The heat exchange between the battery and the outside world is carried out through the surface of the battery. The larger the battery, the smaller the proportion of surface area, and the less easily the heat generated inside the battery is dissipated to the outside. Moreover, the larger the battery, the more energy it contains, and the greater the danger of instantaneous release.
Therefore, for the sake of battery safety, in battery design, it is necessary to determine the maximum volume, minimum proportion of specific surface area, and appropriate battery shape of this type of battery based on the thermal parameters of the selected material.
For example, for cylindrical batteries, when used as power batteries, when the diameter exceeds a certain value (around 30mm), the current distribution, temperature distribution, heat transfer, etc. inside the battery will be greatly restricted, which seriously affects the battery life and brings safety issues in high current applications.
By using small capacity batteries in parallel, on the one hand, the conductivity of the battery pack is improved, because using small batteries in parallel connection increases the conductive area of the conductive connection invisibly.