Battery Storage

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A Look at Cell Formats and how to Build a good Battery

Early batteries of the 1700s and 1800s were mostly encased in glass jars. As the batteries grew in size, jars shifted to sealed wooden containers and composite materials. There were few size standards, except perhaps the No. 6 Dry Cell named after its six inches of height. Other sizes were hand-built for specific uses. With the move to portability, sealed cylindrical cells emerged that led to standards. In around 1917, the National Institute of Standards and Technology formalized the alphabet nomenclature that is still used today. Table 1 summarizes these historic and current battery sizes.

Cylindrical Cell

The cylindrical cell continues to be one of the most widely used packaging styles for primary and secondary batteries. The advantages are ease of manufacture and good mechanical stability. The tubular cylinder can withstand high internal pressures without deforming.

Most lithium and nickel-based cylindrical cells include a positive thermal coefficient (PTC) switch. When exposed to excessive current, the normally conductive polymer heats up and becomes resistive, acting as short circuit protection. Once the short is removed, the PTC cools down and returns to conductive state.

Most cylindrical cells also feature a pressure relief mechanism. The most simplistic design utilizes a membrane seal that ruptures under high pressure. Leakage and dry-out may occur after the membrane breaks. Re-sealable vents with a spring-loaded valve are the preferred design. Some Li-ion cells connect the pressure relief valve to an electrical fuse that opens the cell if an unsafe pressure builds up. Figure 2 shows a cross section of a cylindrical cell.

Typical applications for the cylindrical cell are power tools, medical instruments and laptops. To allow variations within a given size, manufacturers use fractural cell length, such as half and three-quarter formats.

Figure 2: Cross section of a lithium-ion cylindrical cell The cylindrical cell design has good cycling ability, offers a long calendar life, is economical but is heavy and has low packaging density due to space cavities. Courtesy of Sanyo

Nickel-cadmium provided the largest variety of cell choices and some spilled over to nickel-metal-hydride, but not to lithium-ion as this chemistry established its own formats. The 18650s illustrated in Figure 3 remains one of the most popular cell packages.

Figure 3: Popular 18650 lithium-ion cell
The metallic cylinder measure 18mm in diameter and 65mm the length. The larger 26650 cell measures 26mm in diameter.
Courtesy of Cadex

In 2013, 2.55 billion 18650 cells were produced; earlier with 2.2Ah and now mostly with a capacity of 2.8Ah. Some newer 18650 Energy Cells are 3.1Ah and the capacity will grow to 3.4Ah by 2017. Cell manufacturers prepare for the 3.9Ah 18650, a format that they hope will be made available at the same cost as lower capacity versions.

The 18650 is the most optimized cell and offers the lowest cost per Wh. As consumers move to the flat designs, the 18650 is peaking and there is over-production. Batteries may eventually be made with flat cells but experts say that the 18650 will continue to lead the market. Figure 4 shows the over-supply situation that has been corrected thanks to the demand of the Tesla electric vehicles.

Figure 4: Demand and supply of the 18650.
The demand for the 18650 would have peaked in 2011 had it not been for Tesla. The switch to a flat-design in consumer products and larger format for the electric powertrain will eventually peak the 18650.
Courtesy Avicenne Energy

The larger 26650 cell with a diameter of 26mm instead of 18mm did not gain the same popularity as the 18650. The 26650 is commonly used in load-leveling systems with Li iron phosphate.

Some lead acid systems also borrow the cylindrical design. Known as the Hawker Cyclone, this cell offers improved cell stability, higher discharge currents and better temperature stability compared to the conventional prismatic design.

Even though the cylindrical cell does not fully utilize the space by creating air cavities on side-by-side placement, the 18650 has a higher energy density than a prismatic/pouch Li-ion cell. The 3Ah 18650 delivers 248Wh/kg, whereas a modern pouch cell has only 143Ah/kg. The higher energy density of the cylindrical cell compensates for its less ideal stacking characteristics. The empty space can be used for cooling to improve thermal management.

Cell disintegration cannot always be prevented but propagation can. The cylindrical concept lends itself better to stop propagation should one cell take off than is possible with the prismatic/pouch design. In addition, a cylindrical design does not change size whereas the prismatic/pouch will grow. A 5mm prismatic can expand to 8mm with use. In spite of the apparent advantages of the cylindrical design, advances are made with the pouch cell and experts predict a shift to this flat format.

Prismatic Cell

Introduced in the early 1990s, the modern prismatic cell satisfies the demand for thinner sizes. Wrapped in elegant packages resembling a box of chewing gum or a small chocolate bar, prismatic cells make optimal use of space by using the layered approach. Others designs may be wound and flattened into a pseudo-prismatic jelly. These cells are predominantly found in mobile phones, tablets and low-profile laptops and range from 800mAh to 4,000mAh. No universal format exists and each manufacturer designs its own.

Prismatic cells are also available in large formats. Packaged in welded aluminum housings, the cells deliver capacities of 20 to 30Ah and are primarily used for electric powertrains in hybrid and electric vehicles. Figure 6 shows the prismatic cell.

 

Figure 6: Cross section
of a prismatic cell

The prismatic cell improves space utilization and allows flexible design but it can be more expensive to manufacture, less efficient in thermal management and have a shorter cycle life than the cylindrical design.

Courtesy of Polystor Corporation

 

The prismatic cell requires a slightly thicker wall to compensate for decreased mechanical stability compared to the cylindrical design. Some swelling due to gas buildup is normal. Discontinue using the battery if the distortion becomes so large that it presses against the battery compartment. Bulging batteries can damage equipment.

Pouch Cell

In 1995 the pouch cell surprised the battery world with a radical new design. Rather than using a metallic cylinder and glass-to-metal electrical feed-through, conductive foil-tabs are welded to the electrodes and brought to the outside in a fully sealed way. Figure 7 illustrates a pouch cell.

Figure 7: The pouch cell

The pouch cell offers a simple, flexible and lightweight solution to battery design. Exposure to high humidity and hot temperature can shorten service life.

Courtesy of Cadex

The pouch cell makes the most efficient use of space and achieves a 90–95 percent packaging efficiency, the highest among battery packs. Eliminating the metal enclosure reduces weight but the cell needs some support in the battery compartment. The pouch pack finds applications in consumer, military and automotive applications. No standardized pouch cells exist; each manufacturer designs its own.

Pouch packs are commonly Li-polymer and serve well as Power Cells by delivery high current. The capacity is lower than Li-ion in the cylindrical package and the flat-cell may be less durable. Expect some swelling; 8–10 percent over 500 cycles is normal. Provision must be made in the battery compartment for expansion. It is best not to stack pouch cells on top of each other but to lay them flat side by side. Prevent sharp edges that can stress the pouch as they expand.

Extreme swelling is a concern but battery manufacturers insist that these batteries do not generate excess gases. Most swelling can be blamed on improper manufacturing. Users of pouch packs have reported up to three percent swelling incidents on a poor batch run. The pressure created can crack the battery cover, and in some cases break the display and electronic circuit boards. Manufacturers say that an inflated cell is safe. Discontinue using the battery and do not puncture it in close proximity to heat or fire. The escaping gases can ignite. Figure 8 shows a swollen pouch cell.

 

Figure 8: Swelling pouch cell

Swelling can occur as part of gas generation. Battery manufacturers are at odds why this happens. A 5mm (0.2”) battery in a hard shell can grow to 8mm (0.3”), more in a foil package.

Courtesy of Cadex

Pouch cells are manufactured by including a temporary “gasbag” on the side. During the first charge, gases escape into the gasbag. The gasbag is cut off and the pack is resealed as part of the finishing process. Subsequent charges should no longer produce gases. Ballooning indicates that the manufacturing process may not be fully understood. Manual labor may also contribute the cause.

The prismatic and pouch cells have the potential for greater energy than the cylindrical format but the technology to produce large formats is not yet mature. The cost per kWh is still higher than the 18650. As a comparison, the cost for the Nissan Leaf with Pouch/Prismatic cells is $455/kWh and best practice (DoE/AABC) with pouch/prismatic is $350/kWh. The lowest price per kWh is the Tesla EV with the 18650 cells. The Tesla Gen III battery goes for $290/kWh (Estimations by Greenwich Strategy).

Frequently Asked Questions about Li-ion Cells

Not all cells are made the same and true performance will only come to light after a battery pack has endured two and more years of field service. When building a pack, select a cell that meets the loading requirements and then build it a bit larger to reduce stress. Evaluating a cell by cycling does not reflect true field conditions. A serious pack maker must pay the price for a quality cell. Table 10 answers FAQs.

Question Fact
Is lithium-polymer superior? “Polymer” is a Li-ion in a pouch pack with no common definition. (See BU-206 Li-polymer Battery: Substance of Hype)
Do I get more capacity using flat than cylindrical cells? No. Prismatic/pouch cell have lower capacities than the 18650. Li-ion pouch cell = 150Ah/kg; Li-ion in 18650 up to 248Wh/kg.
Do flat cells pack easier than cylindrical? Flat cells must allow space for swelling; the 18650 does not change size. Thermal management is harder with flat cell.
Is the flat cell cheaper than the cylindrical? No. 18650 has lowest cost/Wh. The cost of flat cell pack is $350/kWh; 18650 pack is $290/kWh (EV battery pricing).
What do I need to know when buying a Li-ion cell?
  • Energy Cells are optimized for capacity, commonly used in consumer products. Cycle life is less important.
  • Power Cells deliver high currents, are rugged, have long life   but have a lower capacity may be more expensive.
Do I need cell-balancing?
  • Single-cell pack doesn’t need matching; tolerance not critical.
  • The cells for a multi-pack must be matched. Most quality cells for multi-cell design are matched.
What does the protection circuit do? The mandatory protection circuit only controls outside stress and cannot stop a disintegrating cell once in process.
What should I observe in a pack design? Isolate the cells to prevent propagation of a failing cell. Quality cells have a very low failure rate.
How are quality cell checked? Manufacturers include self-discharge test and cell matching.  Yield is 50–100%. Rejected cells are sold at barging prices.
How is the cost divided? On an 18650, 50% goes to material; 50% is manufacturing.
What are recommended 18650 makers? Panasonic, Samsung, LG Chem, E-One Moli.

Table 10: Clearing misunderstandings in the choice of Li-ion cells