Sep. 18, 2024
A secondary lithium battery operates similarly to other primary batteries, providing power to devices (a process known as discharging). However, unlike primary batteries, it can be recharged and reused. For a detailed comparison between SLA (sealed lead acid) and lithium batteries, you can refer to our guide here. In this blog, we will explore lithium battery cells in more depth, discussing their configurations, practical applications, and how their construction makes them well-suited for specific use cases.
When you take off the top of a lithium battery pack, you’ll first notice the individual cells and a circuit board of some kind. There are three types of cells that are used in lithium batteries: cylindrical, prismatic, and pouch cells. For the purpose of this blog, all cells are lithium iron phosphate (LiFePO4) and 3.2 volts (V).
A cylindrical cell resembles a traditional household battery, like a AA battery, which inspired its shape when these cells first entered the market in the mid-1990s. Cylindrical lithium cells are available in various widths, lengths, and amp-hour ratings, and can be classified as either energy or power cells. These cells are versatile, used in both large and small battery packs with different capacities and voltages. However, cylindrical cells are particularly ideal for applications requiring smaller Ah batteries, such as power tools, drones, children's toys, and medical equipment, where space and weight are critical to performance.
If you consider the compartments where batteries are placed, most are square-shaped, which inspired the prismatic cell form factor. Prismatic cells, like the ones found inside laptops, offer higher capacity in a compact footprint and are rectangular in shape. Available as both power and energy cells, prismatic cells can be used in batteries designed to match sealed lead acid battery dimensions. Though larger than cylindrical cells, prismatic cells provide more amp-hours per cell by having a higher lithium content by volume, enabling larger battery pack configurations and single-cell options. This makes them a popular choice for building larger battery packs and a top option for energy storage devices.
As the name suggests, a pouch cell consists of an aluminum foil pouch that encases lithium iron phosphate polymer chemistry, with two terminal tabs extending from one end. This form factor allows for the highest lithium content by volume and is designed to be placed directly into its application without needing a cell case. Due to the use of lithium polymer (powder), pouch cells offer higher power density compared to other cell types, thanks to their compact construction and size.
In addition to choosing the form factor of a lithium cell, you'll also need to decide between a lithium power cell or a lithium energy cell. A power cell, as the name suggests, is designed to deliver high power, while an energy cell is designed to deliver high energy. But what exactly does this mean, and how do power cells and energy cells differ?
All types of cells cycle, but the depth and speed of cycling vary (see battery C ratings). Power cells are designed to deliver high current loads over short periods of time with intermittent intervals, making them ideal for high-rate applications like starters or power tools that generate high loads or torque. On the other hand, energy cells are designed to deliver sustained, continuous current over extended periods, making them better suited for motive cyclic applications like scooters or e-bikes. While all lithium cells work well in cyclic applications, the cycle length varies—power tools might run for about an hour before needing a charge, while a scooter user would expect longer run times.
Once you've selected the cell type for your lithium battery, you'll need to determine the amp-hours (Ah) and voltage required for your application. Additionally, you'll need to consider the necessary amperage.
For instance, to build a 125 Ah, 12.8V battery using 25 Ah, 3.2V prismatic cells, you'll need a 4S5P configuration. This means arranging the cells into 4 master packs of 5 cells in parallel (5P), then connecting the 4 master packs in series (4S), resulting in a total of 20 cells. The parallel connection increases amp-hour capacity, while the series connection increases voltage. Learn more about connecting batteries in series or parallel.
The different lithium cell form factors exist for two reasons: one is to provide various sizes, shapes, and flexibility levels for different battery designs. The other is to offer flexibility in capacity and voltage, allowing you to choose between building a battery with many cylindrical cells or fewer prismatic cells, depending on your needs.
Additionally, as mentioned earlier, the type of application must be considered. For instance, while it's possible to use lithium energy cells to build a starter battery, it’s more effective to use power cells, as they deliver higher power suited to such applications. Similar to lead acid batteries, a lithium battery won’t last as long if it's not used for its intended purpose—whether cyclic, starter, or high-rate applications.
As you can see, building a lithium battery requires careful consideration of various factors. From the intended application and physical size constraints to voltage and amp-hour requirements, understanding the lithium configuration options will help you create a more efficient battery. If you have any questions on this topic, please feel free to contact us.
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