The shift in the personal-computing market since Apple introduced the iPad in 2010 has been dramatic. According to the January 2013 Quarterly Mobile PC Shipment and Forecast Report from NPD DisplaySearch, “Tablet PC shipments are expected to reach more than 240 million units worldwide in 2013, easily exceeding the 207 million notebook [laptop] PCs that are projected to ship.” The same report says that in 2011, the combined market was split 75% notebooks/25% tablets, shifting to about 60% notebooks/40% tablets in 2012, and anticipated to be 45% notebooks/55% tablets in 2013.
Although not as obvious, this shift in end products also affects battery OEMs and engineers who choose and design-in batteries and multi-cell battery packs for other types of products. The notebook PC industry has, to a large extent, standardized on battery packs based on lithium-ion (Li-ion) cylindrical cell technology, with what are designated as 18650 batteries. (The 18 is the diameter in millimeters, the 65 is the length in millimeters, and the 0 denotes a cylindrical shape).
The Li-ion Legacy
In commercial use since 1999, the 18650 typically provides 3.6 V nominal with a capacity that has gradually increased to 3000 mAh. Energy density by weight is impressive at 100 to 150 Whr/kg (360 to 540 kJ/kg, for those of you with a more physics-oriented perspective), while corresponding energy density by volume is 350 to 420 Whr/L (1.25 MJ to 1.5 MJ/L). Also important to users, these batteries and packs have great cycle life, a modest self-discharge rate of 2% to 5% per month, and many safety features.
Due to the enormous volume of these cells in use, the number of viable suppliers has increased (the numerous Chinese suppliers have improved their quality) and the price per watt-hour has dropped dramatically in the past few years. In addition, the widespread availability of the 18650 cell has made it the preferred power source for many non-notebook applications such as radios, credit card readers, bar-code scanners, and even medical equipment, increasing their production volume even higher than just the notebook numbers indicate. The standard form factor means that the cells are available from many suppliers and can support the long product life cycles seen outside of the consumer electronics market.
Also important to engineers, their widespread use means that the battery pack’s performance and characteristics are well understood and field-proven through many diverse situations and circumstances. In terms of overall safety, while there were problems in some notebook PCs a few years ago with smoke and even fires (supported by credible reports of self-initiated fires, apparently due to internal battery defects), they have been largely overcome through manufacturing improvements, improved quality control, and much more sophisticated charging and monitoring circuitry. In fact, since the 18650 cell is the “flagship” product for most cell suppliers, it always is the first on the roadmap to utilize any new safety technology that gets introduced.
The Tablet’s Influence
The tablet environment is changing the situation, however. Due to their form factor and design tradeoffs, the different tablet models often use custom lithium-polymer cells designed in flat packages. Because the chemistry and internal structure is the same as for conventional Li-ion cells, the front end of the manufacturing process is essentially unchanged. All of the same automated high-throughput equipment can be used.
The process diverges from conventional cells in assembly. Lithium-polymer assembly tends to be semi-automatic, giving it the advantage of faster and less expensive conversion to new cell sizes versus the highly automated cylindrical cell process, which makes changing sizes very expensive and time-consuming. Lithium polymer batteries are predicted to become quite common, with manufacturers producing 1 billion cells per year by the end of this decade, according to NPD DisplaySearch.
Many of the design and usage “ground rules,” which are well established due to the high volume and experience with 18650-based battery packs, don’t apply to those lithium-polymer packs used in tablets for several reasons. Lithium-polymer cells are inherently more fragile and have tabs to weld to, for example.
Perhaps more important than the design challenges associated with the use of lithium-polymer cells are the supply chain issues that will be encountered as the standard cell becomes less so. The custom cells will almost inherently never reach the kind of volumes afforded by standardization. Therefore, there will be smaller price reductions and capacity improvements. Designers will need to work closely with battery vendors to get the packaging and performance they need (and a price they can accept).
The lack of a standard form factor will be extremely problematic for the designers of any portable equipment that is produced in volume too low to warrant a custom cell. Even if the designers of these products can “piggy back” on a cell designed for consumer electronics, they likely will face the potential of a cell’s end of life well before their host product.
Traditionally, designers of diverse products have always benefited from the availability of high-volume, standardized, fully characterized, and well-understood power (and other) components that were initially designed for other applications and niches—think of those AA batteries, both rechargeable and non-rechargeable, that you can get almost anywhere.
The same applies to the ubiquitous 18650 battery, whether used singly or in multi-cell packs. As the market transitions from notebook PCs to tablets and also fragments in design approach, the same benefits of volume, cost, electrical, and thermal certainty may no longer be available to the same extent.
Robin Tichy is senior marketing manager at Electrochem Solutions Inc. She has a doctorate of philosophy from the University of Texas for her work in solid-oxide fuel cells.