Cylindrical battery pack – design and structure introduction

 Among the three cell types of power batteries, although the cylindrical cell does not occupy the largest market share, its commercial standardization is the most mature due to its wide application in the consumer goods market. After years of precipitation, its technology is stable and has the best consistency.

Cylindrical batteries are small in size and are very suitable for battery packs with irregular spaces, making full use of the corner space. Cylindrical cells made of ternary materials have an energy density of 210~250Wh/kg. Large-scale standardized batteries make cylindrical battery packs also have the premise of automated production.

This article provides a detailed and comprehensive overview of cylindrical battery pack, covering all aspects of their design, construction, and manufacturing process.

Basic structure of cylindrical battery pack

In the cylindrical battery pack design, the battery pack structure is various. It is mainly determined according to the needs of customers and models, which ultimately leads to different manufacturing processes of battery packs.

Cylindrical battery packs are generally composed of battery cells, upper and lower brackets, busbars (some are also called connecting pieces), sampling harnesses, insulating plates and other main components, as shown in the figure below：

Cylindrical battery pack structure design

Cylindrical battery packs, such as the 18650 battery pack, are designed to fix multiple cylindrical batteries at designated positions to ensure that excessive displacement does not occur under reasonable vibration and shock conditions.

The position of the cell is determined by the cell bracket. In extreme cases, the cell bracket may be deformed. In order to maintain the distance between the cells, a high-temperature-resistant, low-quality telecom spacer is generally designed separately.

Inside the cylindrical battery pack, parallel connection is relatively easy to realize, as long as a busbar is connected to one pole of the battery cell, but to achieve uniform current density distribution and uniform thermal field is the place to test the level of engineers.

Generally, try to design a relatively symmetrical structure, but the vicinity of the inlet and outlet of the cylindrical battery pack is always different from the uniform arrangement of other cells, so it is the key point of design simulation.

Cylindrical battery pack heat dissipation

Battery thermal management is equally important in cylindrical battery packs. The PCM heat dissipation structure of the battery pack and the application of phase change materials can be used in conjunction with liquid cooling or independently.

Standalone applications can be arranged in a variety of ways. The PCM sheet can be attached to the outside of the battery module to assist in heat dissipation. As shown below：

Cylindrical battery pack manufacturing process

The manufacturing process of cylindrical battery packs is a complex multi-step process

1) Cell sorting. When designing the module process, it is necessary to consider the consistency of the electrical performance of the module to ensure that the overall performance of the Pack meets or meets the requirements of the entire vehicle.

2) Insert the battery into the lower bracket. Inserting the battery into the lower bracket refers to inserting the battery into the battery positioning hole of the lower bracket.

3) Battery polarity judgment. Battery polarity judgment refers to checking whether the polarity of the battery meets the requirements of the document, which is a safety inspection.

4) Cover the stand. Covering the bracket refers to covering the upper bracket on the cell and fixing the cell in the bracket.

5) Module spacing detection. The module spacing detection refers to the detection of the distance between the end face of the battery pole and the surface of the bracket.

6) Wash. Plasma cleaning is a kind of dry cleaning, which mainly relies on the “activation” of active ions in the plasma to achieve the purpose of removing stains on the surface of objects.

7) Busbar installation. Busbar mounting refers to fixing the busbar to the module for resistance spot welding.
8) Resistance welding. Resistance welding refers to welding the bus bar and the pole surface of the battery core together by means of resistance welding.

9) Welding inspection. Check and confirm the welding effect again by manually inspecting the appearance and manually picking up the busbar.

10) Glue. Glue generally has two purposes in the application of modules: one is to fix the battery cell, and the other is to transfer the heat of the battery cell and the module through the thermally conductive adhesive.

11) Cover the insulation board. Covering the insulation board means to insulate and protect the busbar of the module.

12) Module EOL test. EOL test (end of line) (generally also called end of line test) is a key link in quality control in the production process, mainly for testing the special characteristics of cylindrical battery packs.

13) Transfer to Pack assembly or storage. Modules that pass the EOL test are transferred to the Pack assembly process or put into storage according to regulations. During the transfer process, the cylindrical battery pack needs to be insulated and protected to prevent the module from falling.

Precautions for cylindrical battery pack

When designing a module process flow, the following points generally need to be considered:
● Safety: product safety and safe production;

● Electrical performance: capacity, voltage, internal resistance, consistency of performance;

● Production tempo: The higher the tempo, the greater the production capacity;

● Dimensions: external dimensions and fixed dimensions;

● Process route: refers to the selection and determination of key processes;

● Cost: An element that needs to be considered in both product design and process design.

Through the above analysis, it is not enough to design the process flow of the cylindrical battery pack well, and it also needs to be supported by a complete production system in order to manufacture products that satisfy customers.

Summary

In summary, cylindrical battery packs are a mature technology with broad application prospects. As the battery swap system becomes more and more perfect in today’s world, the research and development of cylindrical battery packs will continue to mature, thereby bringing innovative and improved products that meet customer needs.