What are the disadvantages of a prismatic cell?

Due to the vast shapes of electronics and other applications that function on battery power, battery cells also come in different shapes to fit the application. Two common types of shapes are prismatic cells and cylindrical cells. Both offer specific qualities to the application, whether you are looking for cost-effective batteries that are easy to mass produce or batteries with a higher capacity range.

Yet keep in mind that these two batteries have some functional differences. Selecting the right cell can greatly influence the design of your product, the available power, and what may occur if there is a cell failure.

Battery Cell Shapes and Sizes

Both cells have distinct shapes that can accommodate different devices. Cylindrical cells are long and round, much like the batteries found in toys, remote controls, and other devices. It's constructed by encasing electrodes that are wound tightly. Then they are placed into a specially designed metal can. This shape offers smaller sizes.

Example of cylindrical and prismatic battery cells.

Cylindrical cells are the most common cell shapes that are used in our daily lives and for various applications. This round shape allows for both the electrolyte and the internal pressure to be evenly distributed. So, there is a less likely chance of leaks or cell bloating.

Prismatic cells have flat and rectangular shapes. They have everything inside sandwiched into layers and are encased in steel or aluminum. An insulated film covers the outside of the can. This shape allows for the battery to save space inside the application as prismatic cells are commonly found in electric and hybrid vehicles. The shape of prismatic cells is quite larger than cylindrical cells.

One concern with prismatic cell shapes involves the pointed corners. These corners could experience more stress such as shocks and vibrations based on the application, making them weaker if not protected effectively in an enclosure. The corners also could not allow for the electrolyte to be distributed throughout, which may cause cell bloating and other deformations.

Prismatic cells come in varying sizes. There are no universal format types that every manufacturer adheres to, so they may design several specific shapes. Cylindrical cells do have universal format types between manufacturers, such as 14650, 18650, 21700, and 26500 to name a few. Because there are no universal types of prismatic cells this means that most models are custom builds, which also means the customers need to be cognizant of minimum order quantities and what certifications need to be attained.

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Voltages and Capacities

When it comes to voltages and capacities, there are key advantages and disadvantages to both cell forms. Prismatic cells are larger in size than cylindrical cells and have fewer connections in the application. So prismatic cells allow for larger capacities. For example, one lithium phosphate battery (LifePO4) in prismatic cell form has 3.2 volts 100ah.

On the other hand, cylindrical cells have more connections in the application and come in smaller sizes that allow for less energy storage. Even with the lower capacity, cylindrical cells have more voltage power. They provide faster discharge rates in per amp hours (Ah) due to requiring more connections.

For cylindrical cells to reach the same amp hours as one prismatic cell, you would need to have 18 cylindrical cells. To reach 48 volts for both battery cell shapes, you would require 18 prismatic cells and 16 groups of 18 cells.

So, while you would need more cylindrical cells to achieve the same capacity as prismatic cells, you have better energy efficiency with cylindrical cells. It's due to this reason that cylindrical cells are used more often in high-performance applications such as toys and electronics for longer-lasting battery life. Prismatic cell shapes are better suited for energy-intensive applications such as storage systems, medical devices, and vehicles.

Build and Lifecycle

Placing multiple cells into a battery pack requires a specific alignment for each battery shape. Cylindrical cells are stacked in several series and parallels. There may be 12 batteries aligned as three rows of four cells or four rows of three cells depending on the space available in the application. The shape of the round cells offers maximum airflow between each cell for better temperature control. The heat can dissipate quickly for better performance.

Prismatic cells are usually only stacked in a series. Due to the rectangular shape, the cells can touch one another. Unfortunately, this stacking does not allow for air to travel between the cells. The prismatic cells cannot discharge heat as quickly, instead, they can pass between the cell's walls into the next adjacent cell.

Typical prismatic cells may have lifecycles averaging around 2000 cycles. Cylindrical cells have less of a lifespan ranging between 300 to 500 cycles. An important thing to understand about battery cell lifecycles is how one bad cell can impact the rest of the cells within the pack. If one cylindrical cell goes bad, the many connections and the pack build are not impacted by the bad cell as the pack can continue to provide power as the capacity is dictated by the lowest capacity within the group. For prismatic cells, one bad cell can impact the entire battery pack based on how the cells are placed in the series.

Battery Testing, Certifications, and Costs

All battery packs no matter their shape should undergo the required testing based on their cell chemistry, industry requirements, and customer specifications. Testing and certification are typical requirements for lithium battery chemistries for both cylindrical and prismatic cells.

When it comes to costs, cylindrical cells are easier to manufacture as the technologies have been around for generations. Prismatic cells may cost more, yet the prices will also be based on the size of the cells and the volume of the order. Customers should also keep in mind that certain battery chemistries, such as lithium batteries, may have increased costs due to the testing and certification that is required compared to other nickel-based chemistries.

If trying to determine the best battery shape for your budget, this factor will depend on the application's power needs, lifecycle, and how much you want to spend. One type of battery cell is not actually better than the other. While prismatic cells offer better long-term capacity, they have higher prices. Cylindrical cells are cheaper to manufacture, have better thermal management, and are less likely to bloat, leak, or rupture. Yet cylindrical cells have lower capacities and may require larger volume purchases to obtain the same capacity as a prismatic cell.

The one thing that is most overlooked when discussing prismatic cells is that while many sizes are available, they are not an off-the-shelf products. That means that getting small quantities and samples may be difficult given the fact that most manufacturers need a minimum order amount to run off enough raw materials (anodes/cathodes) to make the production run worthwhile. Be sure to ask your supplier which sizes they run on a regular basis if your requirements are less than 5,000 pieces. If your estimated annual usage is over that amount, then it may make sense but keep in mind that if you require samples to do testing, they will usually be made in a sample lab and will have some differences from the full production models.

Another key item to be aware of when looking at using a custom-sized prismatic pack is the certifications. When getting started these cells will need to go through UL and UN 38.3 certification for safety and transportation requirements. Once the cells pass these certifications, they may need to be updated yearly, which the manufacturing site usually manages for high-volume customers. If you are right on the minimum order quantity where the factory may not need to run them again for 18-24 months, you will need to make sure that you plan for them to keep the UL file current every year otherwise it will run out and you will have to pay to get the certification back to active.

Summary

Battery pack manufacturers will state the minimum purchase requirements for both prismatic and cylindrical cells. Some companies only offer one shape of cell or battery chemistry while others offer more variety. When it comes to selecting the best cell for your application, speaking with your manufacturer can help you determine the right better power for your needs.

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