In the previous tutorial, we discussed different types of batteries. In addition to reusability and battery chemistry, the way the battery is packaged is an important factor as it must be accommodated in a device without compromising safety standards. The geometry and size of a battery also play a vital role in its application to a circuit.
Batteries have been in use since the 1700s and are now available in many shapes and sizes. The earliest battery shape was the jug design. In 1896, large F cells were introduced and followed by D cells in 1898. C cells were the first small-sized batteries introduced in 1900, followed by the still popular AA batteries introduced in 1907. Currently, batteries mainly come in the following forms:
- Cylindrical Cells
- Button cells
- Prismatic Cells
- Pouch cells
Standardization of battery size began after the introduction of cylindrical cells. In 1906, a non-governmental standards organization – the International Electrochemical Commission (IEC) – was formed to develop standards for batteries. In 1917, the National Institute of Standards and Technology (NIST) introduced an alphabetical nomenclature for standard battery sizes that is still in use today.
Battery Sizes
Batteries are available in the following sizes:
Cylindrical cells
Cylindrical cells are the most popular batteries for primary and secondary batteries and some of the easiest to manufacture. These cells have a pressure relief mechanism that allows them to withstand high internal pressures without deforming. Additional benefits include high mechanical stability, low cost, long service life and excellent cyclability. These cells do not change their shape or size, so thermal management can be done easily with adequate spacing between cells. The only disadvantage of these robust batteries is their weight and low packing density due to the space cavities.
Example of cylindrical cell batteries.
Cylindrical cells are highly reliable and typically come with a PTC switch that stops current conduction when a load draws excessive current, thus offering short circuit protection. The switch returns to the conductive state after cooling. For mechanical safety, these cells use membrane seals or reusable vents. Some cylindrical lithium-ion cells use a charge interrupt device that disconnects the cell when exposed to unsafe mechanical pressure.
Cylindrical cells are typically used for nickel-cadmium and nickel-metal hydride batteries. They are available in popular sizes like AA, AAA, AAAA, etc. and are also available in partial sizes such as 1/2 and ¾ formats. Rechargeable lithium-ion batteries in the 18650 size are also available in cylindrical cells. Cylindrical lithium-ion 18650 cells have been widely used in laptop and electric vehicle batteries, but their demand is declining as new electric vehicles will rely on flat battery designs. Still, cylindrical lithium-ion cells of size 21700, 20700 and 22700 will continue to be in demand for various consumer electronics applications.
Button cells
Button cells or coin cells are used as small-sized primary batteries intended for compact portable devices. These low-cost cells are found in motherboards, wristwatches, portable medical equipment and cordless phones. Their main advantage is their compact size, allowing them to be easily stacked in a tube to achieve higher voltages. Its main disadvantage is the lack of security openings and the high charging time. These cells are mainly used in compact devices or as a power reserve for some part of a circuit. Coin cells are also used to maintain the clock on motherboards.
Example of button cells.
Prismatic cells
Prismatic cells feature a flat rectangular design that allows for maximum space utilization. There is no standard size format for these cells and their size varies depending on the manufacturer. They are commonly used in cell phones, smartphones, notebook computers and portable electronic devices, and are now the preferred choice in electric vehicles. The main disadvantages of these cells are poor thermal management, short lifespan and cost. They are prone to swelling due to gas buildup and can expand in size over time, which poses safety risks. They can also damage the device they are housed in, due to expansion and distortion.
Example of Prismatic or Rectangular Cell Battery
Pouch cells
Bag cells are the most flexible and lightest batteries. With sealed foil electrode flaps, these cells have a flexible, bag-like casing designed to accommodate any swelling in the battery. Generally, small pocket cells can swell after a few hundred charge-discharge cycles, while large pocket cells can swell after several thousand cycles. This battery design is the lightest and allows maximum space utilization. Although designed for light loads, these cells can handle high load currents and can be stacked side by side to achieve higher voltages.
Example of Prismatic or Rectangular Cell Battery
When pouch cells are manufactured, they are provided with a temporary gas bag. The gases that accumulate in the first charge escape into the gas bag, which is subsequently removed. The pouch is then sealed and ready to be shipped. In subsequent charging cycles, a minimal amount of gas is produced, which can only accumulate after hundreds or thousands of cycles.
These cells have greater energy density; however, these are quite expensive to manufacture. There is no standard size format for these cells – they come in various sizes and the size can vary depending on the manufacturer. Bag cells are commonly used in wearable electronics (such as Bluetooth headphones, music players, and toys) and portable devices. There are also large pouch cells which are used in other applications like UPS, ESS, etc.
Future trends
Currently, cylindrical cells and button cells are the most popular batteries. However, the trend is shifting towards prismatic and pouch cells as their manufacturing cost gradually decreases. Future electric vehicles may use flat design cells (prismatic and pouch). and the same trend can be seen in batteries used in portable devices, gadgets and consumer devices. Pouch cells can be manufactured like food packaging, while rectangular cells can be manufactured more easily with improved automation systems. Manufacturers are still finding ways to improve the safety and thermal stability of dead battery designs, as the future of batteries depends on utilizing space, weight and maximizing energy density without compromising safety.
In the next article, we will discuss choosing a battery.
