Acculon Energy

Understanding Cell Form Factors & Cell Sizes in Lithium-Ion Battery Pack Design

Li-ion batteries, coming in all shapes & sizes, have revolutionized the way we power portable electronics, electric vehicles, & renewable energy systems. In this post, we will explore the significance of different cell formats & their implications on battery pack performance.

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Lithium-ion cells are the building blocks of battery packs, and they are available in various form factors and sizes. The three primary components of a lithium-ion cell are the cathode and anode, separated by an electrolyte. These parts are stacked together and placed in one of a few packages: cylindrical, pouch, or hard case prismatic. Each packaging type has a variety of sizes, the cell size refers to its physical dimensions and capacity, often measured in ampere-hours (Ah) or milliampere-hours (mAh). There is no single packaging type that is superior to others, each has its own advantages and disadvantages. Usually, cell manufacturers focus on one or two types of cell formats in manufacturing.

Common Cell Formats and Sizes

Cylindrical cells have their electrodes rolled up like a jelly roll and placed inside a cylindrical case. These cells are relatively small, and dimensionally stable during operation.  

18650 Cells: 18650 cells are among the most widely used lithium-ion cell sizes. They measure 18mm in diameter and 65mm in length, hence the name. Capacity ranges from 1000mAh up to 3500mAh. These cells are used in laptops, flashlights, e-cigarettes, and some pioneer electric vehicle applications.

21700 Cells: 21700 cells are a newer and bigger option. They measure 21mm in diameter and 70mm in length. These cells typically have higher capacity, ranging from 3000mAh to 5700-5800mAh. You can find these cells in electric vehicles, power tools, and high-performance flashlights.

26650/26700/32700: Further evolution of the 18650/21700 size, with higher diameters. Usually found for LFP chemistry.

46XX Cells: The hyped 4680 cell and several similar-sized large-format cells labeled as 4695, 46115, and others—often abbreviated by the 46XX abbreviation—are upcoming automotive application-focused cell formats. Unlike the mere size progression seen in the shift from 18650 to 21700 cells, these cells significantly advance the connection between the electrode stack and the case through a “tabless” design, improving manufacturability and reducing costs. However, the augmented energy capacity of these cells raises safety concerns. Cells employing high-nickel layered oxide cathodes of this caliber are not readily accessible, as cell manufacturers tend to carefully vet customers for these energy-dense, large-form-factor cells. In contrast, 4680 LFP cells have recently become available on the open market.

60140/60280/66160 etc. Some large sizes of cylindrical format are present on the market, with LTO and LFP chemistry. 

When designing application-specific battery packs, considering cell size in conjunction with factors such as energy density, power output, thermal management, safety, & cost, can help you make an informed choice
that aligns with your battery program’s goals & requirements.

Pouch Cells:
Pouch cells feature an electrode stack packaged with a thin flexible multilayer film material. The lack of a rigid casing enables a higher energy density, making pouch-type Li-ion batteries ideal for space- and weight-sensitive applications such as portable electronic devices. Most of the pouch cells are prismatic in shape, but even cylindrical pouch cells exist. The range of cell sizes varies widely, from the miniature few tenths of milliamp-hours found in earbuds to over a hundred amp-hours in the pouch cells of electric vehicles. However, the need for pack-level management of mechanical stiffness and the requirement to accommodate cell expansion during operation complicates the design of large packs using this cell format. Lack of built-in safety features, such as cell vents and current interception devices (CIDs), normally present in hard-cased Li-ion cells, requires special attention for large pack designs as well.

Prismatic Cells: Prismatic cells feature a hard case, usually made of aluminum. The prismatic cell’s shape makes volume utilization high when a pack is made of these cells. Most modern prismatic cells are tenth to hundreds of Ah capacity mostly found in automotive and stationary storage applications. Large cell size and effective cell-to-pack packaging simplify pack design and manufacturing, driving costs down, but cooling and safety must be effectively managed to ensure safe and reliable battery operation.

Considerations for Cell Size in Battery Pack Design

With different options available for designing battery packs, what determines cell choice? There are several factors that come into play when designing application-specific battery packs. 

One of the biggest considerations when determining cell size is energy density. Larger cells typically offer higher energy density, meaning more energy storage per unit volume. However, larger cells may become difficult to cool due to their size, necessitating more complex thermal management systems. One important consideration is the ratio of pack voltage to capacity. In some modern applications, there is a tendency to use smaller cells connected in series to keep the pack size small while increasing the voltage.   Finally, cell size can impact the safety of a battery pack. It is much easier to manage the thermal runaway propagation from cell to cell with smaller cells compared to larger ones.

All in all, different cell sizes offer unique advantages and disadvantages, making it essential to carefully consider the specific requirements of your application when designing the right energy storage solution. Whether you’re designing a battery pack for an off-road industrial vehicle or a stationary energy storage system, understanding the implications of cell size is key to achieving optimal results. By considering cell size in conjunction with factors such as energy density, power output, thermal management, safety, and cost, you can make an informed choice that aligns with your battery program’s goals and requirements.