1. Prismatic Housing: Lithium Batteries' "Safeguard" and "Space Manager"
Why do lithium batteries favor prismatic housings? This stems from their dual core functions. First, as a physical shield, they protect against external impact and compression, preventing damage to the internal battery cells and separators that could lead to short circuits. Second, their neat rectangular structure maximizes battery pack space, enabling high-density battery packs like building blocks-critical for electric vehicles striving for long driving ranges.
Furthermore, the prismatic housing also fulfills the important tasks of heat dissipation and sealing. The metal's thermal conductivity effectively dissipates heat generated by the battery cells, while the precise sealing design prevents electrolyte leakage and protects the cells from moisture and dust. It can be said that the quality of the prismatic housing directly determines the lifespan and safety of lithium batteries.
2. Choosing the right material is half the battle for the housing.
The prismatic housing's strength comes from high-quality materials. Currently, the mainstream choices are aluminum alloy and stainless steel, each with its own strengths and suited to different application scenarios.
(I) Aluminum Alloy: The "Golden Choice" for Lightweight and Cost-Effectiveness
In the field of new energy vehicle power batteries, 3003-H14 aluminum alloy is the undisputed "leading player." With a density of only 2.7g/cm³, it's over 60% lighter than stainless steel, effectively reducing the overall weight of the battery pack. The addition of manganese enhances its ductility, allowing it to withstand the "test" of repeated stretching and forming, achieving a tensile strength exceeding 140MPa-equivalent to a fingernail-sized area withstanding a pressure of 1.4kg.
However, aluminum alloy surfaces are susceptible to oxidation and have poor corrosion resistance, necessitating a "surface upgrade." This requires an anodizing process, which forms a 5-10 micron oxide film on the surface. This film acts like a protective layer, protecting against corrosion from the electrolyte. Some high-end products also undergo a passivation treatment to further enhance weather resistance.
(II) Stainless Steel: A "Hard-Core" for Extreme Environments
For applications requiring high-strength protection, such as energy storage power stations and specialized equipment, stainless steel materials such as 1020 steel are highly favored. Its tensile strength exceeds 450 MPa, more than three times that of aluminum alloys, allowing it to withstand severe impact and high temperatures. However, stainless steel also has its drawbacks: its density, as high as 7.85 g/cm³, increases the overall weight of the battery, and its cost is approximately 30% higher than that of aluminum alloys. Therefore, it is rarely used in weight-sensitive automotive power batteries.
To address stainless steel's oxidation resistance, factories employ nickel or zinc plating to create a metallic coating on the surface, enhancing corrosion resistance and optimizing welding performance.
3. Core Process: The Transformation from Flat Plate to Cavity
Transforming a flat sheet of metal into a neatly shaped square housing requires multiple meticulously crafted processes, including stamping, stretching, assembly, and welding, each of which demands meticulous precision.
(1) Stamping: Defining the "basic contour" of the housing
Stamping is the first step in housing manufacturing, essentially shaping the metal sheet. Workers feed the cut metal sheet into a servo stamping machine, where high-precision dies (with tolerances controlled to within ±0.01mm, equivalent to 1/5 the diameter of a human hair) apply pressure to stamp out the housing's bottom and sidewalls in one go.
Key to this step are die precision and punching pressure: The die surface roughness must be controlled below Ra0.8 to ensure a smooth, burr-free surface. Burrs can later pierce the cell separator and cause a short circuit. The punching pressure must be adjusted based on the sheet thickness, generally between 50 and 200 kN. Too little pressure results in incomplete shaping, while too much can cause the sheet to crack.
(2) Deep Drawing: Key to making the housing "stand upright"
The stamped prototype undergoes a deep drawing process to create a three-dimensional cavity structure. This is like "blowing a balloon," but the process is much more precise: a multi-station stretching machine is used to gradually deepen the sheet, with each stretch depth controlled at 5-10mm. After 3-5 stretches, the desired shell depth is achieved.
During the stretching process, there are three key parameters that cannot be incorrectly adjusted: the stretching speed should be maintained at 5-10mm/s. Too fast can easily cause material tearing, while too slow results in poor efficiency. The die gap should be set at 1.1-1.2 times the sheet thickness to ensure smooth material flow. A special lithium-based lubricant should also be applied to reduce friction between the die and the sheet. These details directly determine the uniformity of the shell wall thickness (tolerance must be within ±5%).
(3) Cell Assembly: The "Technology Showdown" of Lamination and Winding
After the shell is formed, the "core component"-the battery cell-must be installed. This step uses two main processes: lamination and winding, each tailored to different performance requirements.
Lamination: The "Exquisite Choice" for High-End Products
The lamination process is like stacking playing cards, stacking the positive and negative electrodes and separators layer by layer in the order "positive electrode - separator - negative electrode - separator." This process offers advantages such as a more regular cell structure, reduced internal stress, and a longer cycle life (up to 2,000 cycles or more). Its energy density is 5%-10% higher than the winding process, making it widely used in high-end digital batteries and solid-state batteries.
However, lamination requires extremely high precision: a vision alignment system is required to ensure that the positional deviation of each electrode does not exceed ±50μm (equivalent to half a human hair). Otherwise, current distribution will be uneven, affecting battery performance. Currently, the most advanced high-speed lamination machines can stack up to 300 cells per minute, equivalent to five cells per second.
ACEY-SSM-C battery stacking machine can be applied to the stacking process of lithium-ion battery pole pieces. The process of manual film loading, subsequent pole piece position correction and lamination is completed automatically, which has the characteristics of high lamination efficiency and high accuracy.
Winding: The "Speed King" of Mass Production Efficiency
The winding process is like rolling toilet paper: the electrode sheets and separator are spirally rolled into a cylindrical cell, which is then placed into a rectangular casing to shape it. Its greatest advantage is high production efficiency. A single production line can produce up to 100,000 cells per day, making it ideal for large-scale production of electric vehicle power batteries.
The core of winding is controlling "tension": the tension must be maintained between 10-20N. Uneven tension can cause cell deformation, separator wrinkling, and even short circuits. Therefore, high-end winding machines are equipped with a dynamic tension compensation system to ensure that tension fluctuations do not exceed ±1N. Infrared detection also monitors electrode alignment in real time (edge deviation ≤ ±100μm).
ACEY-SWM-P battery winding machine can be applied to the winding process of square batteries. The equipment is designed for isolation film roll material feeding, pole piece type feeding, manual film feeding, automatic film pulling, pin insertion and winding. The winding needle adopts a trapezoidal pair-to-plug combination design, and the width of the winding cell can be adjusted within a certain range.
(4) Sealing Welding: Building a Solid "Safety Line"
After the battery cell is installed in the casing, the top cover must be installed and sealed with a weld. This is a critical step to prevent electrolyte leakage and isolate external impurities. Currently, there are two mainstream welding technologies: laser welding and resistance welding.
Laser Welding: A Master of Precision Sealing
Laser welding is like a delicate scalpel, using a 50-100W pulsed laser to create a 0.3-0.5mm wide weld at the joint between the housing and the top cover-roughly the thickness of a human hair. This welding method has a small heat-affected zone, preventing damage to the internal battery cells. It also provides exceptional sealing, with a helium leak detector measuring the leakage rate to ≤1×10⁻⁹mbar・L/s (equivalent to less than 1 drop of leakage per year).
Resistance Welding: The Efficiency Key to Mass Production
Resistance welding is a "fast spot welding gun" method. Electrodes apply a pressure of 50-100N and a current of 5-10kA, using the heat generated by the current to melt and fuse the welded parts. Its high welding speed (only 0.1-0.3 seconds per point) makes it suitable for mass production of large-sized housings. However, it requires strict current waveform control-excessive current can easily burn through the housing, while too little current can result in cold welds.
4. Key Equipment: The "Hardcore Support" of Automated Production Lines
The precision manufacturing of square housings relies on a series of high-end equipment. These devices act as the "hands and feet" of the production line, ensuring the accuracy and efficiency of each process.
(1) Molding Equipment: The "Mold Masters" that Shape Housings
Servo Stamping Machine: Equipped with a displacement sensor with a resolution of 0.001mm, it monitors the stamping pressure and position in real time, achieving closed-loop control-like a "craftsman with eyes," ensuring the precise dimensions of each housing.
Multi-Station Stretching Machine: Integrated with a robotic automatic loading and unloading system, the stretching cycle per station is ≤5 seconds, improving efficiency by over 30% compared to manual loading and unloading, while also eliminating errors caused by manual operation.
(2) Assembly Equipment: Building a "Master Assembly Tool" for Battery Cells
High-Speed Stacking Machine: Driven by a linear motor, it achieves a positioning accuracy of ±25μm and automatically corrects electrode sheet deviations during stacking to ensure neat stacking.
High-Precision Winding Machine: Suitable for electrode sheets with a width of 50-300mm, it uses a servo motor to control the winding speed and is equipped with a electrode sheet defect detection system that automatically stops the machine if scratches or damage are detected.
(3) Inspection Equipment: Safeguarding Quality
- Coordinate Measuring Machine: Fully inspects the housing's dimensions, including length, width, and height (tolerance ±0.1mm), and wall thickness uniformity (≤5%), ensuring each housing meets assembly requirements.
- X-ray Inspection: Penetrates the housing to clearly see the internal battery cell assembly, inspecting electrode alignment (required ≥99%) and diaphragm damage to prevent internal short circuit risks.
- Compression Testing Machine: Applying a pressure of 100-500N to the housing, inspecting deformation to ≤0.5mm, ensuring protection against collisions and crushing.
5. Quality Control: A "Safeguard Net" Throughout the Entire Process
Lithium battery safety requires no compromise. Therefore, the manufacturing of square housings undergoes comprehensive quality control from raw materials to finished product, with checkpoints established at every stage.
(1) Raw Material Inspection: Keeping a Close Eye on the "Entry"
When raw materials enter the factory, aluminum alloys and stainless steel must be tested for composition (such as manganese and nickel content), mechanical properties (tensile strength and ductility), and surface smoothness. If the raw materials have defects, even the most sophisticated processing cannot correct them.
(2) Process Control: Keeping an Eye on Every Step
- Die Calibration: Stamping and drawing dies are calibrated every four hours to prevent die wear and dimensional deviations in the housing.
- First-Part Inspection: Before each production shift, the first finished product undergoes a comprehensive inspection of its dimensions, appearance, and sealing. Only qualified products can be put into mass production.
- Patrol Inspection: Quality inspectors randomly select 10 products for testing every hour to promptly identify process fluctuations.
(3) Finished Product Testing: Strengthening "Factory Pass"
- Before leaving the factory, finished products must undergo a series of rigorous tests:
- Drop test: Free drop from a height of 1.5 meters onto a hardwood floor, with no damage to the housing and a cell performance fluctuation of ≤5%;
- High-temperature storage test: Storage at 60°C for 72 hours, with no bulging or leakage in the housing and a change in electrolyte conductivity of ≤3%;
- Cycle life test: 1000 cycles at 1C charge and discharge conditions, with a capacity retention rate of ≥85%. Only housings that pass all these tests are approved for use.