05Nov 2014

That is of a rechargeable lithium-ion battery, of course….We all know that lead-acid batteries, the type you have under your hood, tend to be of a standard size, but lithium-ion batteries can come in a multitude of packaging and shapes.

One of the most common misconceptions is that polymer batteries are different. In fact, they are one of the common types of lithium-ion batteries, assembled and packaged in a flat, pouch-like shape.  Their core design is based on the standard lithium-ion chemistry. They are called “polymer” batteries because they tend to use an electrolyte that is gel-like than liquid-like. The outer package is a thin foil that holds the internal structure together. Consequently, they can be prone to damage or puncture, and are often if not always embedded within the mobile device for mechanical protection.

One of the advantages of polymer batteries is that they can be manufactured in nearly arbitrary custom dimensions or shapes. This ability to make the battery fit the mobile device (instead of the other way around) gave polymer batteries their great appeal. Polymer batteries can also be made very thin. The photograph shows a polymer cell made by Sony for use in their Xperia Z2 smartphone. It is only about 4 mm thick. The downside of polymer batteries is the lack of standardization, and consequently, higher cost of polymer batteries; each battery model has to be designed and shaped to the particular dimensions required by the manufacturer of the mobile device. A polymer battery can be nearly twice more expensive (for the same amount of stored energy) relative to their older sibling, the standard 18650 battery cell.

Three different types of rechargeable lithium-ion batteries. From left to right: Prismatic (used in a Samsung Galaxy S5), Polymer (used in a Sony Xperia Z2), and an 18650.

The 18650 cell was named with very little creativity. It comes as a standard cylinder with 18mm in diameter, and 65mm in height, hence the naming. The standard size of these cells made them immensely ubiquitous and inexpensive in the past decade. They were widely used in laptop computers but proved less practical for smartphones with thin profiles. Tesla Motors took advantage of the large-scale manufacturing and low cost of 18650s, and adopted them for use in their electric vehicles. The battery pack in a Tesla Model S contains nearly 7,000 such cells. The photograph above shows an 18650 cell with a capacity of 3,400 mAh made by Panasonic; it is similar to the one used in a Tesla vehicle. The other major manufacturers of electric vehicles have elected to use large size polymer-type batteries. Nonetheless, 18650s are here to stay. There is so much manufacturing oversupply of 18650s that their price continues to plummet, making them an attractive commodity.

The third type of cells are called prismatic. They are, at their core, very similar to the polymer cell but are packaged inside a solid case or can, typically made of an aluminum alloy. This offers added mechanical protection and the requisite safety. Mobile devices that offer replaceable batteries use prismatic cells. The photograph above shows a prismatic cell used in the Samsung Galaxy S5. Owing to the walls of the external can, they tend to be thicker than polymer batteries. 

Back to the photograph above, the keen reader might ask about the connector attached to the Sony polymer battery. It is indeed an electrical connector made using a thin flexible cable. At the tip of this cable, one can observe some circuitry that provides the necessary electronic protection for the battery. In particular, this circuitry ensures that the battery does not experience excessive voltages or excessive currents. A built-in fuse disconnects the battery should it get exposed to adverse conditions. Similar circuitry is also embedded inside the case of a prismatic cell. However, the 18650 cell is bare, i.e., does not include any such protection circuitry which must be included in an external battery management system before the battery is put to use.