Ritch Russ

5/10/2001 9:04 AM EDT

Ritch Russ, Business Development Director, Display Devices and Components Business Unit, Toshiba America Electronic Components Inc., Raleigh,N.C., ritch.russ@taec.toshiba.com Portable power options multiply
Ritch Russ, Business Development Director, Display Devices and Components Business Unit, Toshiba America Electronic Components Inc., Raleigh,N.C., ritch.russ@taec.toshiba.com

Notebook computer designers face more choices than ever before when it comes to power sources. The battery market has evolved from nickel cadmium to nickel metal hydride to lithium ion batteries. Lithium polymer batteries and advanced lithium batteries are now being considered as among the most advanced solutions currently available.

With all of these choices, how should designers determine which solution is best for their applications? What features must the battery have in order to meet the growing power demands of high-end notebooks? No single battery technology is a perfect fit for all notebook-computing needs; each presents distinct advantages that de-sign engineers must consider when deciding which battery solution to use. Form factor, voltage, energy density, temperature performance, drain rate and cycle life all rank near the top of the battery design checklist.

High-end features in notebook computers place increasing demands on the power source, and can therefore be a tremendous drain on battery life. The cylindrical Li-ion battery has the highest energy density of any cell that is commercially available. Additionally, it one of the most economical, making it the primary choice for powering most notebook-computing applications.

Li-ion batteries have been in commercial production for almost 10 years. As the chemistry has matured, the Li-ion cells have become more robust and less expensive. A single Li-ion cell's voltage is 3.7 V. Key factors in the Li-ion's success are its high energy density and low self-discharge, but many other cell characteristics are being further developed to enhance the chemistry. For instance, to further reduce costs, battery manufacturers are developing batteries that will no longer require protection circuitry.

Capacity evolution
The majority of notebooks in the marketplace today use the 18650 (18 x 65-millimeter) Li-ion cell. These cells continue to push the technology envelope, spurred on by notebook manufacturers who demand that battery technology advance at a more rapid pace, relative to Moore's Law. Over the past two years, the 18650 Li-ion cell's capacity has evolved from 1,350 milliamps per hour to 1,600 and 1,800 mA-hr., to the current capacity of nearly 2,000 mA-hr. Advanced electrode designs will push the cell's capacity close to 2,200 mA-hr. within the year. Additionally, enhancements are being made to the separator and electrolyte to further increase cell capacity and robustness. However, this may not be enough to satisfy the increasing power requirements of today's high-performance notebook computers.

One of the major design obstacles facing engineers is reduced real estate within notebook computers. The market is seeing a trend toward thinner and lighter solutions, and ultraslim models, once a niche market, are now becoming a mainstream commodity. To meet the requirements for lower-profile solutions, prismatic Li-ion cells with a thickness of 8 to 10 mm have started to make headway in ultrathin and subnotebook systems. Their "aluminum can" design, which reduces overall weight, coupled with a capacity of up to 1,600 mA-hr., gives prismatic cells a mechanical advantage over cylindrical cells. However, the trade-off for designing with prismatic Li-ion is a reduced energy density when compared with its cylindrical Li-ion counterparts.

With the emergence of lithium polymer (PLB) and advanced lithium (ALB) battery technologies, many notebook designers are beginning to look for more creative mechanical solutions to power their ultraslim and high-end notebook designs. As slender as 3 mm, PLBs and ALBs provide the thinnest battery solution currently available. These batteries can be embedded between the liquid-crystal display and the outer plastics, allowing them to be used either as the main power source in a smaller system or as an additional battery to supplement the main one in large, high-end systems. Since many leading cell suppliers also manufacture LCD panels, this convergence of technologies could lead to more synergy, resulting in OEMs purchasing finished LCD, battery and top-cover units from a single source.

Stability under abuse
ALB cells operate at 3.7 V and offer the energy density of a prismatic Li-ion cell in an ultrathin aluminum-laminated foil package. ALBs have safety advantages over standard Li-ion cells, since they boast a nonflammable electrolyte structure and additional stability under abusive conditions.

But the primary advantage of ALBs over Li-ion batteries is their thin design. ALBs can be manufactured in form factors as thin as 1 mm and offer capacities up to 1,500 mA-hr. Compared with PLBs and even prismatic Li-ion, the ALB chemistry offers reduced swelling (less than 0.1 mm under severe conditions) and better low-temperature performance. The ALB cell maintains more than 50 percent residual capacity at -20 degrees C, whereas PLB has almost none below 0 degrees C.

Mass production has begun only in the last 12 months, and the chemistry still carries a price premium over its Li-ion counterparts. However, as yield rates and manufacturing capacities increase, pricing should continue to decrease. If this trend continues, ALB cell prices could eventually meet or fall below those of Li-ion cells. Since ALB technology is still fairly new to the market, its energy density is expected to continue to grow rapidly, to better meet the demands of high-end wireless systems.

PLB cell technology is nearly identical to ALB technology, except for its polymer-type electrolyte, as opposed to the ALB's liquid electrolyte. This polymer-type electrolyte provides an additional level of safety in preventing battery leakage. The PLB is still somewhat limited in its low-temperature and swelling performance, but will become more robust as mass production continues. It offers similar capacities, and the same thin form factor and aluminum-laminated foil package, as the ALB. Many applications use both PLBs and ALBs as dual power sources.

Pricing still plays a major role in the selection of a power solution for notebook computers. Although the power demands of most notebooks have increased beyond the capabilities of today's NiMH technology, there is still a need for cost-effective batteries for entry-level systems. As NiMH cells near their theoretical capacity limits and Li-ion pricing becomes increasingly cost competitive, NiMH cells may eventually disappear from notebooks altogether.

For decades, the battery industry advanced slowly, offering NiCd and sealed lead acid as the only sources of rechargeable energy. Now, within the span of nearly a decade, the leap has been made from NiMH to Li-ion, to PLB and ALB technologies. The demands placed on battery manufacturers by the notebook-computing industry continue to increase, even as new technologies are developed. The current expectation for battery capacity enhancement is five to 10 times greater than the 10 to 20 percent capacity increases now being achieved each year.

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