Lithium Battery Knowledge

Battery Types

1. 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 has the ability to withstand internal pressures without deforming. Figure 1 shows a cross section of a cell.

Figure 1: 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.

Typical applications for the cylindrical cell are power tools, medical instruments and laptops. Nickel-cadmium offers the largest variety of cell choices, and some popular formats have spilled over to nickel-metal-hydride. To allow variations within a given size, manufacturers use fractural cell length, such as half and three-quarter formats.

The established standards for nickel-based batteries did not catch on with lithium-ion and the chemistry has established its own formats. One of the most popular cell packages is the 18650, as illustrated in Figure 2. Eighteen denotes the diameter and 65 is the length of the cell in millimeters. The Li-manganese version 18650 has a capacity of 1,200–1,500mAh; the Li-cobalt version is 2,400–3,000mAh. The larger 26650 cells have a diameter of 26mm with a length of 65mm and deliver about 3,200mAh in the manganese version. This cell format is used in power tools and some hybrid vehicles.

Figure 2: Popular 18650 lithium-ion cell

The metallic cylinder measure 18mm in diameter and 65mm the length. The larger 26650 cell measures 26mm in diameter.

Lead acid batteries come in flooded and dry formats; portable versions are packaged in a prismatic design resembling a rectangular box made of plastic. Some lead acid systems also use the cylindrical design by adapting the winding technique, and the Hawker Cyclone is in this format. It offers improved cell stability, higher discharge currents and better temperature stability than the conventional prismatic design.

Cylindrical cells include a venting mechanism that releases excess gases when pressure builds up. The more simplistic design utilizes a membrane seal that ruptures under high pressure. Leakage and subsequent dry-out may occur when the membrane breaks. The re-sealable vents with a spring-loaded valve are the preferred design. Cylindrical cells make inefficient use of space, but the air cavities that result with side-by-side placement can be used for air-cooling.

2.  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 for insulation, conductive foil tabs welded to the electrode and sealed to the pouch carry the positive and negative terminals to the outside. Figure 5 illustrates such a pouch cell.

Figure 3: 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.

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

Pouch packs are commonly Li-polymer. Its specific energy is often lower and the cell is less durable than Li-ion in the cylindrical package. Swelling or bulging as a result of gas generation during charge and discharge is a concern. Battery manufacturers insist that these batteries do not generate excess gases that can lead to swelling. Nevertheless, excess swelling can occur and most is due to faulty manufacturing, and not misuse. Some dealers have failures due to swelling of as much as three percent on certain batches. The pressure from swelling can crack a battery cover, and in some cases break the display and electronic circuit board. Manufacturers say that an inflated cell is safe. While this may be true, do not puncture a swollen cell in close proximity to heat or fire; the escaping gases can ignite. Figure 6 shows a swelled pouch cell.

Figure 4: 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.

To prevent swelling, the manufacturer adds excess film to create a “gas bag” outside the cell. During the first charge, gases escape into the gasbag, which is then cut off and the pack resealed as part of the finishing process. Expect some swelling on subsequent charges; 8 to 10 percent over 500 cycles is normal. Provision must be made in the battery compartment to allow for expansion. It is best not to stack pouch cells but to lay them flat side by side. Prevent sharp edges that could stress the pouch cell as they expand.

Summary of Packaging Advantages and Disadvantages

A cell in a cylindrical metallic case has good cycling ability, offers a long calendar life, is economical to manufacture, but is heavy and has low packaging density.

The prismatic pouch pack is light and cost-effective to manufacture. Exposure to high humidity and hot temperature can shorten the service life. A swelling factor of 8 - 10 percent over 500 cycles is normal.

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