Ever wonder why some lithium batteries are safer than others? The physical form factor of a battery cell plays a crucial role in its overall safety. In this deep dive, we’ll explore the engineering considerations, testing protocols, & design innovations that are essential for creating safer, more reliable battery packs.
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The safety of lithium-ion batteries, particularly in micro-mobility applications like e-scooters and e-bikes, is a topic of increasing importance, as we have seen over the past couple of years based on consistent reporting about fire outbreaks related to these batteries. While these batteries offer exceptional performance, understanding the factors that influence their stability is crucial for ensuring safe and reliable operation. One critical aspect that significantly impacts battery safety is the cell form factor, which refers to the physical shape and construction of individual battery cells.
Three Main Cell Formats:
There are three primary cell formats used in lithium-ion batteries:
- Cylindrical cells: Resembling the familiar AA batteries, these cells are enclosed in a rigid metal casing.
- Pouch cells: These cells are encased in a flexible aluminum-plastic film, giving them a flatter, rectangular shape.
- Prismatic cells: These cells have a hard outer casing and come in various rectangular shapes.
Each of these form factors has its own set of advantages and disadvantages, and their safety profiles vary depending on the design and how they are integrated into a battery pack.
Cell Form Factor and Safety:
Cylindrical Cells: The Safest Option
Cylindrical cells are generally regarded as the safest option among the three formats. This is primarily due to their robust metal casing, which provides excellent mechanical protection and helps prevent internal damage. The cylindrical shape also allows for consistent spacing between cells in a battery pack, facilitating efficient heat dissipation. This is crucial for preventing overheating, which is a major trigger for thermal runaway events, a dangerous condition where the battery enters an uncontrollable self-heating state.
Pouch Cells: The Highest Safety Risk
Pouch cells, while offering advantages in terms of flexibility and packageability, tend to pose the highest safety risk. Their flexible packaging makes them more vulnerable to physical damage, such as punctures or tears. Additionally, their compact design often leads to denser packing within a battery pack, hindering effective heat dissipation. This can result in localized hot spots, increasing the likelihood of thermal runaway.
Prismatic Cells: The Middle Ground
Prismatic cells fall somewhere in between cylindrical and pouch cells in terms of safety. Their more rigid structure compared to pouch cells offers better protection, and they typically have some spacing between cells to facilitate cooling. However, they may not dissipate heat as effectively as cylindrical cells, especially in high-power applications.
By understanding the relationship between cell form factor & safety, & considering the various factors that influence battery stability, we can make informed decisions when choosing battery technologies for different applications.
Cell Size Matters
Regardless of the form factor, larger cells generally present greater safety concerns. This is because they contain more active material, leading to a higher potential for energy release during a failure event like thermal runaway. Consequently, larger cells require more complex thermal management systems and safety precautions to mitigate risks. Beyond the cell form factor, several other factors are crucial for ensuring battery safety:
- Thermal Management: Efficient heat dissipation is key for preventing thermal runaway. This involves carefully designing the battery pack with appropriate spacing between cells and implementing cooling systems tailored to the application’s power demands. This also entails other design considerations, like the inclusion of intumescent materials in the module as well.
- Battery Management System (BMS): A sophisticated BMS is essential for monitoring cell voltages, temperatures, and currents, ensuring safe operating limits, and preventing overcharging or over-discharging, which can mitigate risk and safety issues.
- Mechanical Stability: Cells need to be securely fixed within the battery pack to prevent movement and potential damage from vibrations or impacts.
UL regulations concerning lithium-ion batteries test for optimal thermal stability and mechanical stability, but despite city ordinances and other laws coming online in the US, some battery products are not certified to current standards, or even worse, some battery manufacturers purchase certification through unscrupulous means or even acquire fake certification labels. One way to see if a battery is certified is through a search in the UL product sourcing and certification database: https://www.ul.com/software/product-sourcing-and-certifications-database
Continuous Advancements in Battery Safety
The field of battery technology is constantly evolving, and product design and manufacturers are continually striving to improve the safety of all cell chemistries. Research and development efforts focus on safer cathode materials, such as Lithium Iron Phosphate (LFP), which exhibit higher thermal stability than other chemistries like Lithium Nickel Manganese Cobalt Oxide (NMC), for example. Separators with enhanced thermal and mechanical properties, such as ceramic coating, can prevent internal short circuits and improve overall safety. Also, incorporating additives into the electrolyte can enhance thermal stability, reduce flammability, and improve the battery’s resistance to abuse conditions.
While the design of all the cell materials is critical to all performance metrics, including safety, the overall safety of a battery system also relies on the control and integration of peripheral but essential components for energy storage. The first layer of these components is the cell casing, often called the cell form factor.
By understanding the relationship between cell form factor and safety, and considering the various factors that influence battery stability, we can make informed decisions when choosing battery technologies for different applications. Continuous advancements in battery design, materials, and battery management systems hold the promise of safer and more reliable energy storage solutions for the future. And perhaps most importantly, rigorous battery testing that adheres to standards and regulations is critical to ensuring the safety and performance of these technologies across applications and diverse operating conditions. In the end, a holistic approach that combines innovative design, meticulous testing, and adherence to safety standards and regulations is key to unlocking the full potential of battery cells, no matter what the form factor, while safeguarding usage across industries and everyday life.