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.
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.
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.
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.
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.