When it comes to battery modules, two other words often come to mind: battery cell, battery pack. In fact, battery is a generic term for all three, while battery cell, battery module and battery pack are different forms of batteries in different stages of application. The smallest of these units is the battery cell, several cells can form a module, several modules can form a battery pack by adding BMS and other management systems. Therefore, we can understand the battery module as an intermediate product between the battery cell and the battery pack. When multiple battery cells are packaged together in the same housing frame and linked to the outside through a uniform boundary, this makes up a battery module. It consists of a series-parallel combination of cells, the structure of which must play a role in supporting, fixing and protecting the cells. Whether it is able to fix the cell position and protect it from performance-damaging deformation, how to meet the temperature control of the cell, etc., will be the criteria for judging the merits of the battery module.
The lithium battery modules currently on the market, most of them are modules for electric vehicles. Many lithium battery manufacturers will launch some fixed parameters of module products according to market needs, also known as standard module. When the standard module can not meet the needs of customers, it is necessary to customize the lithium battery module according to different needs, also known as customized battery module.
Through the above, we have a general understanding of the battery module. Here we will introduce battery modules in more detail by comparing battery cells, battery packs and pack formation process.
Battery cell vs module
As the smallest unit of the battery cell, its performance directly affects the performance of the battery module. Therefore, choosing the right battery cell is the first step in forming a battery pack. Battery cell formation usually follows the “barrel principle”, that is, the lowest capacity cell in a group of cells determines the capacity of the whole group of batteries. This requires that the selected cell type, model consistency, capacity, internal resistance, voltage difference is not more than 2%.
Li-ion battery cells can be divided into soft pack, cylindrical and square according to the structure：
Soft pack
cylindrical
square
Advantages
The external structure has a small impact on the cell, the material used for the package has a small mass, and the energy density of the battery is high
High process maturity, mature shell structure, low process manufacturing cost.
High protection for the cell, can ensure the express conduction of internal heat by reducing the thickness of the single cell, and the safety performance of the cell is greatly improved. High capacity of individual cells, less cells required for battery pack.
Disadvantages
The sealing process of large-capacity batteries is more difficult and relatively less reliable, and the mechanical strength of the aluminum-plastic composite encapsulation film used is low, and the life of the aluminum-plastic composite film restricts the service life of the battery
Uneven distribution of current density on the collector, resulting in inconsistent response of each internal part. The heat generated inside the battery cell is difficult to be released quickly, and the accumulation will cause a safety hazard to the battery.
Complex internal structure, high production process requirements. The energy density of the cells was low in the early days, and there has been a major breakthrough in this area in recent years.
For safety reasons, the current market of electric vehicle lithium batteries, mainly used in the cylindrical structure and square structure. Cylindrical cells have very low individual capacity. Therefore, for a battery pack of the same voltage and capacity, far more cylindrical cells are required than square cells. In general, a small portion of capacity is lost through the combination of series and parallel connections. The greater the number of battery cells, the greater the capacity loss. The number of batteries is large, and when the battery inside the battery box heats up, the heat is not easily dissipated. This causes uneven temperature between the whole battery and different discharge characteristics, which will lead to the degradation of battery performance for a long time. Therefore, there will be more choices of square lithium-ion cells in the market.
Formation process
After battery cells, we then look at the process of connecting the battery cell. At present, there are two main types of assembly process: bolt-on and weld-on:
bolt-on
weld-on
Advantages
1. Simple and easy connection, low cost.
2. Easy to assemble and disassemble, convenient for maintenance
1. Good consistency and low contact resistance.
2. The contact resistance is stable because the contact surface is closed and not easily oxidized.
Disadvantages
1. Surface is prone to corrosion.
2. Influenced by preload force, surface quality, unstable contact resistance
1. High equipment cost and high initial investment.
As an electric vehicle battery module, it must be in a constant state of bumps and vibrations. From the advantages and disadvantages of the above connection methods, we can see that if you use bolts to connect the battery cells, it will leave hidden problems in use. Therefore, with the availability of laser welding machines, the second connection method is now mostly used. Or you can buy the battery modules directly to use.
Battery module vs pack
As we said before, a battery pack can be composed of several modules plus a management system such as BMS. So some people may ask, why not just use the cells to make the battery pack? This allows more space to enhance the capacity and improve the range. In fact, for some small battery packs(e.g. 12v 100ah energy storage battery pack, etc.), we can do so, not only to reduce the weight but also to reduce the size. But as an electric vehicle battery, it is necessary to take into account both energy, volume and weight, and finally cost and safety. Electric vehicle battery packs are very high voltage and require many more cells, usually from a bunch of cells connected in series. If one cell has a problem it will make the whole unusable, and it is troublesome to replace the cell in this case. And how to fix these cells to cope with the vibration environment of the car is also a problem.
8 modules in a battery pack(battery system)
The cells form a module, which modularizes and standardizes it. This is nothing more than a small loss of capacity and range. It not only improves the overall structural strength, but also improves the heat dissipation efficiency, reduces the risk of thermal runaway, facilitates the later replacement of individual cells, and has high ladder utilization rate in the later stage. Ensures later use and maintenance. So the significance of the module is the minimal failure unit and the platform-level modular design. Standardized modules, mainly to facilitate the advantages of both standardized production and installation and fixing. Adopt modular production, easy to weld, module transport, improve production efficiency, improve the structural strength between cells, and good anti-vibration performance. This is also the reason why most car companies choose to use battery modules at present.
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