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Batteries drive every portable device. With such a high demand, there's an obvious emphasis on developing lighter weight, longer life cells.
Richard Nass, Editor-in-Chief
Batteries. It's the component that differentiates a portable from a non-portable system. It powers just about every system we cover here at Portable Design. For these reasons, we spend a lot of our time covering the latest batteries, battery technologies, and battery alternatives.
Some people in the portable-system industry might argue that battery technology hasn't advanced as quickly as it could or should. I often find myself in this group. But research continues at its own pace, and we're seeing some new developments along the way. For example, some new form factors are coming on-line, including batteries that are measured in microns rather than inches or millimeters. We're also closer to realizing a fuel cell for portable systems (although volume shipments are at least three years away). And super capacitors are now being asked to do more "battery-like" functions.
Currently, lithium ion (LiIon) cells are king of the heap. Just about every laptop computer now ships with a LiIon-based battery. While most of the midrange and high-end phones contain LiIon, the lower end phones still employ nickel metal hydride (NiMH), which is less expensive, but doesn't perform as well. NiMH won't go away altogether, at least not anytime soon. It'll remain in lower end applications for some time.
"LiIon came along faster than anything that we've seen in the battery industry," says Norm Allen, president and chief executive officer of PowerSmart, a producer of power-management ICs. "The reason for this is because a lot of bigplayers were willing to lose money to make it happen."
Lithium polymer is supposed to be the next successor to LiIon. And lately, there's some question as to whether the advantages offered by polymer will ever be realized.
"LiPolymer is great for market hype, but they're having a real tough time with it, relative to its cost-performance," says David Heacock, business manager for portable power products at Texas Instruments. "NiMH weighed more, but cost less. So as soon as the price of LiIon dropped, everyone went with it because it was a dramatic increase in capacity relative to the weight. When you go from LiIon to LiPolymer, you don't see that delta."
Polymer advantages
LiIon development is still occurring, including thinner cells, so the biggest advantage offered by polymer manufacturers—the ability to produce very thin cells—isn't as much of an advantage as was touted a year ago.
For example, Maxell Corp. of America claims to produce the thinnest LiIon rechargeable cell for today's cell phones. This is achieved by employing an aluminum alloy that results in a cell thickness of less than 3 mm. The cell, designated the ICS283465G, is built to a prismatic configuration. The aluminum alloy is harder than traditional aluminum owing to the addition of 4.5% magnesium.
Fig. 1. If you're looking for a thin rechargeable lithium battery, check out the solution offered by Infinite Power Solutions. This thin-film battery ranges from 15 to 18 microns in thickness and has a capacity that ranges up to 300 ?Ah/cm.
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An important structural feature of the Maxell cell is a surrounding 1.3-mm rib, which carries protective circuitry and aids in positioning. The rib serves as the base of a simplified battery pack and helps package the cell into the plastic case of the battery pack.
But if you really want to talk thin, then take a look at the rechargeable lithium battery offered by Infinite Power Solutions (IPS). The company's thin-film battery ranges from 15 to 18 microns in thickness, although that battery must be deposited onto a substrate (Fig. 1). The IPS battery has a typical capacity that cm ranges from 175 to 300 µAh/cm, depending on how the battery is constructed.
The energy density is a function of the battery's surface area and the thickness of the cathode. The capacity increases for each micron of thickness that's added to the battery.
The flexible component is suited for applications like RFID cards, or anything that requires the lighting of an LED. The self-discharge rate is just a few percent the first year, then 1% every year thereafter.
"The fact that we're rechargeable means that we have a different way of thinking about batteries," says Joe McDermott, manager of business and product development at Infinite Power Solutions. "For example, look at an application that draws 10 µA and must last for two years. They say 10 µA times 24 hours times 365 days times 2, and come up with a capacity that's in the order of milliamp-hours."
McDermott continues, "We tell people, Look at what your duty cycle is for three days. If it's realistic that you can recharge the battery every three days, then all you need is 24 hours times 10 µAh times three days. Now, I can get by with a 750- to 800-µAh rechargeable battery. And we can recharge with microamps, rather than milliamps."
The goal for the IPS battery is to be in limited production by mid 2002. Full-scale production will occur the following year.
Large and flat
As for the more traditional LiPolymer cells, they're now finding a home in applications requiring a large flat cell, in some cases as large as 4 by 8 in., although 4 by 4 in. will be the "sweet spot" for that technology. Even with such large length and width measurements, the height can still be kept to just a few millimeters.
Valence Technology offers a polymer-based cell that's built with a stacked construction, which lends itself to being manufactured in very large footprints. The company also produces LiIon phosphate cells (rather than the more traditional LiIon cobalt oxide). The phosphate, which is used as the cathode material, is a lower cost material than the cobalt cathode material.
Valence claims that the phosphate chemistry offers better thermal stability, which enables the manufacture of larger cells. Stability results from eliminating the thermal runaway characteristics associated with cobalt. Moreover, it has a higher level of "environmental friendliness."
According to George Adamson, vice president of research and development at Valence, "We're going through the certification process now, but we have every reason to believe that batteries made with LiIon phosphate will be landfillable. There are no heavy metals, no materials in the batteries that would be of concern in terms of landfills."
The polymer cell is typically packaged in a hermetic foil bag, as opposed to the LiIon cell that's inserted into a hermetic can with a gas vent. The polymer cell is "degassed" before the package is sealed. The foil bag is lighter than the metal can, but not as rugged. But the pack is then placed into a housing inside the system.
The thinness of the polymer construction comes from stacking anode-cathode-air bi-cells almost like sheets of paper. Then the bi-cells can be cut to any reasonable dimension, from very small to 4 by 8 in. Because a LiIon cell is wound, it doesn't lend itself to being used in large formats.
Slightly different
The LiIon cell produced by Toshiba America Electronic Components is similar to traditional LiIon cells, but the cobalt in the cell is replaced with a nickel manganese oxide. Whereas the cobalt-based cells deliver a 3.6-V output, the nickel manganese oxide model offers 2.4 V. While that may seem to be a negative, it's offset by the energy density offered by nickel manganese, which runs about 600 Wh/l, and which compares favorably with cobalt's 450 to 480 Wh/l. What's more, the nickel manganese is less expensive than the cobalt.
Click here to enlarge image
Fig. 2. A zinc-air cell can be used in a fuel-cell-like configuration. In this design, the zinc-air cartridge is replaced to gain a new charge.
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Toshiba also boasts about the safety characteristics of the nickel manganese oxide technology. Ritch Russ, director of business development for battery products at Toshiba, says, "It gives us a truly safe cell technology, which means that we can eliminate the protection circuit that's currently required, further reducing cost."
Toshiba is one of a handful of battery manufacturers that's also spending its R&D dollars on fuel-cell development. It claims that it will have a direct methanol fuel cell to sell within three years. Methanol is the fuel of choice because of its high energy density, although some work on hydrogen-based fuel cells is also occurring.
One important question that still must be answered is, Where do end users purchase the methanol to their fuel cells? It's not like you can go down to the corner convenient store and buy methanol. It may be up to the system or phone manufacturer to provide it, or provide a means for end users to obtain it.
There's also an environmental issue to be determined. If the fuel for the fuel cell is available in a cartridge (like a cigarette lighter), what happens when they're empty. Do you just throw it away, or can it be refilled? And if the replacement cartridge is used, will it fit to a standard connector, or will each manufacturer use a proprietary connector, so you'd have to buy a Nokia cartridge to fit your Nokia phone, for example.
Motorola is one of the leaders in fuel-cell R&D, both in terms of developing actual cells as well as getting the word out on how the fuel cell should be deployed. In fact, the company has put together a specification for other manufacturers, to try to spur on the market. Its specification, which admittedly is a liquid document, shows cell makers where they need to be if they want to compete, and how a cell should react to the outside world. If all the cell makers remain closely in line, the overall acceptance level should grow significantly.
"If they want to enter into this small portable energy market, they need to follow these guidelines," says Ed Decker, a staff engineer at Motorola's Energy Systems Group. "It puts them on the right track as to what the expectations are for their products. The technology still has a lot of unanswered questions as to how these things will perform, from performance and safety standpoints."
Battery replacements, eventually
While most fuel-cell developers see their products as eventual battery replacements, this will probably be a two-step process. The first incarnation will have the fuel cell serve as a battery charger, as a desktop or multi-unit charger that you can deploy in the field. Then, as it evolves and gets down to an amenable form factor, it can serve as the battery replacement.
A technology that can be considered a distant relative of the fuel cell is the zinc-air battery. This chemistry has long-term potential, but like most technologies, it comes with its tradeoffs. For the zinc-air, the tradeoff is between power generation and dry out. In other words, more power can be delivered if you send a lot of air through the cell. But the more air that's sent through, the faster the cell dries out (the faster the charge is lost). There has been some work on a membrane that lets air in, but doesn't let the water vapor in or out. But it's not far enough along to be used in a commercial product.
Because of how it's deployed, zinc-air is looked at more as a primary (non-rechargeable) power source. Its ability to be cycled is limited. Zinc-air is used today in products like hearing aids and other low-load applications that require long life.
One vendor has combined the zinc-air chemistry with a fuel-cell-like technique to produce a quasi zinc-air fuel cell. Electric Fuel's zinc-air micro fuel cell is aimed at consumer electronic applications. The company claims that the cells outperform competing hydrogen, methanol, and ethanol fuel cells currently under development for similar applications.
Available in the second half of next year, the fuel cells offer an energy density of 400 Wh/kg. In one configuration, the cells have a capacity of 30 Ah, measure just 3 by 3 in., and weigh about 3 oz. (Fig. 2).
Is more better?
Another way to get higher power from existing technologies is by changing the way in which the cells are implemented. For example, using more cells in series can give you higher voltages. But that's mostly used in very tightly controlled applications.
Today, most commercial notebook computers use four or fewer cells in series, resulting in a maximum 16.8 V. Running at a higher voltage than that makes it difficult on the discharge side of the equation because the latest microprocessors, say, one running at 1.2 GHz, is looking for 40 A at 1.2 V. Lithium-based cells don't necessarily operate very well at those high rates.
To answer the need for high current and low voltage, you can increase the number of cells, reducing the effective discharge per cell. But again, that increases the voltage, putting a heavier burden on the front-end capacitors and FETs. In such a situation, more attention must be paid to the power conversion subsystem.
Putting the cells in parallel might give you a higher current, but the efficiency that's achieved by each cell is lower. Power is being dissipated in places like connectors, which could have an impedance as high as 50 mΩ. By keeping the current down, power conversion can be done right at the processor, thereby raising the efficiency. Another downside to the higher current is that the grounding issues become more prominent.
Electric Fuel
New York, NY
(212) 529-9200
www.electric-fuel.com
Infinite Power Solutions
Littleton, CO
(303) 285-5108
www.infinitepowersolutions.com
Maxell Corp. of America
Fair Lawn, NJ
(201) 794-5900
www.maxell.com
Motorola Energy Systems Group
Lawrenceville, Georgia
(770) 338-3742
www.motorola.com/ies/ESG
PowerSmart
Shelton, CT
(203) 925-1340
www.powersmart.com
Texas Instruments
Dallas, TX
(800) 336-5236
www.ti.com
Toshiba America
Electronic Components
Irvine, CA
(949) 455-2000
www.toshiba.com/taec
Valence Technology
Austin, TX
(512) 527-2908
www.valence.com
Portable Design January, 2002
Author(s) :
Richard Nass
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