Detecting the Difference: EV Battery Inspection Challenges

Pouch, Prismatic, Cylindrical, and Beyond

Consumers have more choices than ever when selecting EVs, pushing manufacturers to innovate and stand out in an increasingly crowded market. As a result, the EV industry is focused on reducing costs and increasing throughput to drive more adoption and make vehicles more accessible. At the same time, battery producers and automakers must ensure EVs meet rigid safety requirements and performance standards.

Since batteries can account for up to 40% of an EV's price tag, they're a logical starting point for cost reduction, and automation using machine vision is essential for bringing down battery production costs. Manufacturers and OEMs use the technology to inspect batteries for defects, guide cells into modules, verify assemblies, measure features, and identify parts. But the type of battery being assembled can create different challenges for automation.

Images from left to right: machine vision systems measure the width of electrode coating, detect defects on a cylindrical battery, guide cells into a module housing, and identify codes on prismatic batteries.

Pouch, prismatic, and cylindrical EV battery cells serve the same function: manufacturers assemble them into EV battery modules and packs. But understanding the unique properties and manufacturing obstacles - and how to solve those obstacles with machine vision - is critical to lowering EV costs.

All lithium-ion EV batteries start with electrode manufacturing, where battery manufacturers pour a mixture of carbon, graphite, and other elements onto metal sheets to form anodes and cathodes. That's where many of the manufacturing similarities end, and distinct benefits and manufacturing challenges emerge.

Pouch Cells

Manufacturing process: Separator material and electrode-coated anodes and cathodes are cut into sheets and placed in a pouch made from multilayer aluminum composite foil. Next, the battery manufacturer seals the edges of the foil, welds the tabs together, and fills the pouch with liquid electrolyte. Finally, the cell goes through various aging and degassing procedures in a process called cell formation.

Benefits: Some manufacturers advocate pouch batteries for their durability and compact and uniform size compared to other geometries. Pouch batteries are generally manufactured through cell sheet stacking or "Z-folding," where a robot picks up a single anode or cathode sheet, places it on top of separator material, and then places the next cathode or anode on top. Proponents of pouch cells say the stacking method enables more even heat distribution than winding, which is used in cylindrical and some prismatic geometries. Since pouch cells lack a rigid casing, they are more economical to manufacture and have good energy density.

Drawbacks: Depending on the battery system's design and requirements, some of the advantages of pouch battery geometries can also be detrimental. While the lack of casing might reduce manufacturing costs and weight and improve energy density, pouch cells can swell up to 10% in volume after a certain number of charge cycles. Some pouch module designs require aluminum heat transfer plates between each pouch cell to transfer heat away from the pouches. Depending on the module's design and number of cells, these thermal management systems can offset some of the benefits of pouch cells.

Unique challenges: Pouch-style EV batteries are metallic and flexible, creating a lot of specular glare and wrinkles that make defect detection difficult. Letting a flaw slip through the inspection process can compromise battery performance and integrity. However, mistaking acceptable cosmetic blemishes for critical flaws on the surface of a pouch can result in higher scrap rates and costs. The difference between a flaw such as a scratch and an acceptable blemish can be a few microns.

Solutions: Detecting defects on pouch cells is too complicated for rule-based machine vision solutions; rules must account for hundreds of variables and iterations of defects and acceptable anomalies. In addition, users often require different specifications and designs on short notice; changing vendors, manufacturing techniques, or materials can introduce new problems and defects.

To tackle those challenges, Cognex created machine vision software powered by AI, VisionPro Deep Learning, that learns what "good" and "bad" batteries are by analyzing thousands of labeled images. Combined with machine vision systems, VisionPro Deep Learning automates battery inspections, reducing costs and inspection times while preserving battery quality. Computational imaging technology can provide even better inspection results, illuminating the surface of a pouch battery from various angles to reveal subtle defects and features.

Prismatic Cells

Manufacturing process: Prismatic cells are manufactured by winding electrode-coated anodes, cathodes, and separator material into a single layer or by stacking them together. Once arranged, cell sheets are inserted into a rectangular metal housing. Generally, stacked cell sheets release more energy and perform better, while rolled or winded cell sheets contain more energy and translate to longer service life.

Benefits: Since prismatic battery cells are rectangular, they exhibit better energy density in modules and packs: there's no wasted space between cells. Some battery manufacturers use lithium-iron-phosphate chemistry in their prismatic cells, which involves more widely available and cost-effective materials than lithium-ion batteries. And because prismatic cells are larger than other geometries, they require fewer electrical connections and wiring harnesses, which can reduce opportunities for manufacturing defects.

Disadvantages: There's a limited number of standard sizes of prismatic battery cells. Different battery systems can require unique cell specifications and dimensions, which can complicate large-scale production and increase costs. And while the uniform shape and large size of prismatic batteries increase their energy density at the module and pack level, this design can cause thermal buildup - there's nowhere for the heat to go because the cells are in close contact.

Unique challenges: Manufacturing a prismatic EV battery cell involves a lot of welding. Manufacturers weld the metallic housing around the cell sheets and weld a top panel or lid on top of the housing. Since prismatic batteries are stacked close together, the lid must have room to expand and contract as the cells heat up and cool down. Inspecting welding seams on the top and side panels before the cell is installed in a module is essential.

Solutions: Flaws in prismatic battery weld seams, such as underpowered and overpowered welds, can look almost identical to acceptable anomalies, and come in all shapes and sizes. Accounting for all the iterations of a defective weld and separating those from similar acceptable anomalies is too complex for rule-based machine vision.

Cognex VisionPro Deep Learning in conjunction with 2D and 3D machine vision systems are designed to detect such subtle defects. VisionPro Deep Learning's defect detection and classification tools are trained on a wide range of "good" and "defective" weld variations, enabling the solution to identify and classify functional flaws from acceptable cosmetic blemishes.

Cylindrical cells

Manufacturing process: Unlike pouch or prismatic batteries, cylindrical EV battery cells are formed only by winding sheets of electrode-coated anodes, cathodes, and separator material together into a single layer. Then, manufacturers insert resulting cylinder or "jelly roll" into the casing, fill it with liquid electrolyte, and seal the cell.

Benefits: Cylindrical EV battery cells are favored by numerous EV manufacturers, including Tesla, Rivian, and Lucid, and for good reason. Companies have been manufacturing cylindrical batteries for decades, and standardized sizes have emerged over the years, making them more economical to produce than prismatic and pouch battery geometries. Depending on the size of the battery and manufacturing process, winding or rolling electrodes can be faster, at 200 parts per minute (PPM), compared to stacking or "Z-folding" at 10 PPM, according to some estimates. Cylinders are also inherently more mechanically robust than other shapes and can accommodate gas buildup using thinner battery walls, making them a safer option.

Disadvantages: When cylindrical battery cells are placed into a module, there's a gap between each cell. The space between each cell reduces thermal buildup but results in a lot of unoccupied space in the module, reducing energy density. Also, according to some manufacturers, cylindrical batteries don't perform as well as other geometries in colder environments.

Unique challenges: The design of a cylindrical battery cell hinders traceability. Electric vehicle battery plants put data such as manufacturing processes, material origin, and lot numbers in a code etched directly on the battery, called a direct part mark (DPM). This information is important to manage recalls more effectively, comply with numerous regulations and sustainability goals, and isolate issues in the production process.

However, reading DPMs on cylindrical batteries is a unique challenge - codes can be located anywhere on the sides of the battery. And since the batteries are round and metallic, codes are often distorted, deformed, and challenging for machines to read.

Solutions: Cognex image-based barcode readers use advanced image formation and decoding technology to overcome these challenges. High Dynamic Range Plus (HDR+) increases localized contrast to create a more uniform image and enables Cognex barcode readers to cover a greater field of view to read codes on different sides of the cylindrical battery. 2DMax with PowerGrid is an algorithm that combines textural and geometric data to read low-quality DPMs, even if they are deformed or distorted.

New Shapes and Sizes: Blade and Solid-State Batteries

There is no objective "best" EV battery cell. Each geometry comes with its advantages, disadvantages, and unique manufacturing challenges. Companies are experimenting with new battery manufacturing techniques, designs, and chemistries, which bring new production problems.

BYD, a leading EV manufacturer and battery producer, introduced its "blade" battery in 2020. The design eliminates the need for modules, with the battery system comprising long, thin, rectangular battery cells installed directly into a pack. The Chinese automaker claims the blade battery saves 50% more space than other lithium iron phosphate batteries. Tesla began using blade batteries in some models in 2023, and in early 2024, BorgWarner Inc., a tier-1 automotive supplier to Ford, GM, and Stellantis, signed a deal to use BYD's blade battery.

BYD's blade battery, introduced in 2020, claims to have better energy density and safety features compared to other EV battery geometries. Several automakers and original equipment manufacturers (OEMs) are interested in the technology. (Image source: BYD)

More than a dozen automakers and battery manufacturers are also exploring solid-state battery technology. Unlike traditional batteries, which use liquid electrolytes as a membrane to conduct ions, solid-state batteries use solid electrolytes. Benefits of solid-state batteries include better energy storage, faster charging, and safer EVs. The technology is still evolving, though, and critics cite high production costs, lack of validation, and other hurdles solid-state batteries must clear before they're suitable for mass production.

Machine vision will always be a helpful tool for EV battery manufacturing, whatever the shape of the cell. As battery technologies and manufacturing techniques evolve, the ability to separate defects from acceptable anomalies, and track and trace components will become more critical to manufacturers.

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Tags:Vision - Inspection,Electric Vehicles,Edge Learning,2D Vision Systems,AI,Deep Learning,3D Vision Systems