New chemistries linked to advanced power management systems are producing much more efficient batteries
Gil Bassak
In the continuing quest to extend the operating time of their mobile computers, manufacturers are finally getting smart--smart batteries, that is. And more powerful batteries, too.
Thanks to sophisticated electronics and power management subsystems, these new rechargeable batteries promise to live longer, better lives. Equally important, they can predict their own useful operating time with unsurpassed accuracy, ending the guesswork that has long plagued users. Smart batteries began appearing in notebooks from Compaq and IBM in 1993, and they're coming soon to many other systems.
In the past, you judged batteries by sheer electrical capacity alone. The stronger the batte
ry, the longer it could run between charges. Obtaining a longer battery life boiled down to increasing the battery's size or reformulating its chemistry. To be sure, advances in chemistry continue to bear fruit. But the performance of existing battery technologies can be significantly improved by integrating smart electronics that report the battery's precise charging status, temperature, age, and chemistry.
Armed with this information, a mobile system can accurately predict how much longer the battery will last and adjust its power management tactics to squeeze more work out of the battery's remaining capacity. You see a "fuel gauge" that shows almost exactly how much battery time remains, and you have more options for extending that time.
When a smart battery finally runs out of juice, it also knows how much recharging it needs. This extends the overall life of the battery and keeps the mobile computer running at maximum operating capacity.
Starting from Strength
Intelligence notwithstanding, a battery's fundamental strength starts with its chemistry. For that reason, battery makers are hard at work inventing and refining better rechargeable cells. And that work is paying off. Eclipsing the once-dominant nickel-cadmium batteries are NiMH (nickel-metal-hydride) and lithium-ion cells. Lead-acid, which is a popular chemistry for rechargeable batteries in other devices, has a doubtful future in mobile computing. Rechargeable alkaline batteries probably won't play a major role in laptop computers, but they could prove useful in smaller devices.
For years, nickel-cadmium has been synonymous with rechargeable batteries for portable tools and electronics. But because cadmium can be hazardous if the spent batteries are not disposed of properly, nickel-cadmium batteries have lost ground to cells built with the newer NiMH chemistry. NiMH batteries create fewer disposal problems and have at least 20 percent more
volumetric energy density
(i.e., energy for a giv
en volume). NiMH batteries enjoy a service life that's about 40 percent longer than nickel-cadmium batteries.
What's more, nickel-cadmium is a mature technology, with faint prospects for further advancement. NiMH batteries, which were introduced six years ago, should continue to improve for years to come. For example, new sponge-metal NiMH cells from Panasonic are said to deliver up to 150 percent more power than comparable nickel-cadmium batteries.
These strengths have not been lost on systems designers, who are adopting NiMH cells as the battery of choice for today's mobile computers. For power tools and other consumer devices, nickel-cadmium batteries are still a mainstay. They are less prone than NiMH batteries to damage from high charge and discharge rates.
Lithium-ion batteries are gaining favor, too. They deliver about 50 percent more volumetric energy density than NiMH batteries and about 80 percent more
gravimetric energy density
(i.e., energy per unit of weight). Also,
lithium-ion cells have a low rate of self-discharge: 10 percent per month, compared to 25 percent or more per month for NiMH batteries. And lithium-ion cells do not suffer from the so-called memory effect that shortens the operational lives of nickel-cadmium and (to a lesser extent) NiMH batteries.
The memory effect occurs when a battery is repeatedly, but not fully, discharged. Over time, the battery begins to "remember" those partial cycles, causing the output voltage to drop well before the battery is fully drained. To prevent this from happening, most nickel-cadmium and NiMH batteries should be fully discharged before recharging.
Although lithium-ion batteries are the rising star in mobile computing, a potential problem is that lithium is highly reactive, posing safety concerns. Early versions were sometimes known to ignite or explode. In addition, some lithium-ion cells lose their capacity after repeated charge-and-discharge cycles. Still, lithium-ion's high energy density makes it an appea
ling mobile technology. These batteries are showing up in such notebooks as the Latitude XP from Dell, the HiNote from Digital Equipment, and the T3400CT Portege from Toshiba.
For smaller devices such as palmtops and PDAs (personal digital assistants), rechargeable alkaline batteries show some promise. They fall somewhere between NiMH and lithium-ion batteries in terms of volumetric and gravimetric energy densities. Available in AA and AAA sizes, they are well suited to the moderate power requirements of palmtops and PDAs. Rechargeable alkaline batteries can work alongside standard alkaline cells, have a five-year shelf life, and--like lithium-ion batteries--have no memory effect.
Looking ahead, two more battery chemistries are appearing on the horizon: lithium-polymer and zinc-air. Both have been under development for years and have commercial potential.
It is claimed that lithium-polymer cells offer twice the gravimetric energy density and 50 percent more volumetric energy density than
lithium-ion batteries. Such high energy densities help offset lithium-polymer batteries' main drawback: a useful life of only 150 discharge cycles. In contrast, nickel-cadmium batteries can last up to 500 cycles; NiMH batteries, 300 to 500 cycles; and lithium-ion batteries, 500 to 800 cycles.
Zinc-air cells have two to three times as much gravimetric energy density as nickel-cadmium and NiMH chemistries, and about one-and-a-half times the gravimetric energy density of lithium-ion cells. In terms of volumetric energy density, however, they lag behind most other types of batteries. That's because a zinc-air cell requires more airflow, so it tends to be rather boxy and bulky. Engineers find it more difficult to fit zinc-air batteries into the tight quarters and sleek styling of today's laptops.
As with lithium-polymer batteries, zinc-air cells suffer from limited discharge cycles--if fully discharged, they'll endure from 25 to 50 cycles. And recharging can take as long as 10 hours. Still, zinc-air'
s high energy capacity yields longer run times between charges, so fewer cycles are needed over the battery's service life. In terms of total useful life, therefore, zinc-air is competitive with the other types of cells.
AER Energy Resources recently announced that it is developing a zinc-air battery for Hewlett-Packard's color OmniBook 600. AER says the battery will run from 10 to 15 hours per charge and withstand about 50 cycles if fully discharged. However, because most users will probably recharge the battery before it is fully discharged, they'll get as many as 200 cycles, according to AER. And there's no penalty for recharging a partially discharged battery, because zinc-air cells don't exhibit the memory effect.
Getting Smart
To make the most of a battery's inherent capacity, engineers are moving beyond chemistry to intelligent electronics. These
smart batteries
integrate microcontrollers that monitor and communicate information about the b
attery's past and present operating states. This information includes output voltage, temperature, and current drain (both instantaneous and average).
Having these details, a smart battery can accurately predict its operating life and recharging time under a given load. The computer's power management software can read this data to display a fuel gauge that's accurate to within 1 percent or 2 percent.
As a result, you get the most work out of your machine without getting caught with your voltage down and vital files unsaved. You can even set alarms that sound when the remaining operating time reaches, say, 5 minutes.
In contrast, the fuel gauges commonly displayed by today's mobile computers give only a rough indication of the remaining operating time. They draw their conclusions from output voltage alone, a measurement that is skewed by complex interactions among such variables as the battery temperature, electrical load, construction of the cells, type of chemistry, and usage history of
the battery. As a result, today's systems warn you that power is low and force a shutdown before the battery is fully depleted. As much as 20 percent of the battery's capacity may still remain.
Smart batteries can also figure out how much time they need to reach a full charge and how much longer a recharge will take if you're using the computer at the same time. Some smart batteries communicate with a similarly smart charging system to tailor the voltage to the battery's requirements. The result: more efficient charging and longer service life.
Additional savings are possible with savvy power management subsystems. For example, if the computer detects a power surge--perhaps because a disk drive is spinning up to speed--a smart battery can alert the system to reduce power elsewhere, perhaps by dimming the screen momentarily or slowing the processor clock. You also get more working time because of the lowered discharge rate, and the reduction in the average current drain pushes the battery to ope
rate at higher efficiency.
Wielding Power
Clever power management, with or without the benefit of smart batteries, is essential for extending battery life in portable computers. That's why power management functions are now integral to the CPUs, supporting chip sets, and BIOS firmware that are at the heart of the latest mobile computers. These subsystems monitor the computer's activity, throttle its system clock, and control power to the screen, disk drives, and other devices. Even on the desktop, the latest green PCs are adopting these techniques to meet the U.S. government's Energy Star guidelines.
In mobile computers, the power management subsystems take a more aggressive approach because they're balancing operating time against performance. The better ones let you set these priorities by adjusting sliders and other controls. You can select a power management strategy biased toward maximum operating time, peak performance, or anything in between. Smart-battery technol
ogy simply extends this flexible power management to the battery and its charging system.
Compaq introduced smart batteries in its LTE notebooks in 1993, and IBM began using them in its high-end ThinkPads at about the same time. Apple, which is another major laptop vendor, put smart batteries in its PowerBook 500 series last year. Canon recently announced that it would combine smart batteries with advanced power management in a pair of laptops scheduled for introduction this spring and summer. By the end of the year, at least a third of all new notebooks will include smart-battery technology, estimates David Heacock, marketing manager at Benchmarq Microelectronics, a company that makes chips for smart batteries.
High-end PowerBooks have two battery compartments; one of them accepts an optional PCMCIA-card cage. Each NiMH battery integrates a tiny processor card that communicates important variables to the power management subsystem, which is called EverWatch. Either the user or the subsystem can
cut power consumption by spinning down the hard disk, dimming the screen, or slowing the clock speed of the computer's 68LC040 CPU. As these changes are made, the on-screen fuel gauge indicates the precise effect on battery operating time.
Another bonus you get with smart batteries is that the computer can adapt itself to more advanced battery chemistries in the future. As new batteries are developed, their integrated controller chips will supply different information to the computer's power management and battery-charging subsystems, thereby adjusting the operating-time calculations and charging characteristics.
Canon's new laptops will offer similar features, but with one exception: Their smart batteries could become as widely available as the AA penlight batteries that are used in your Walkman. The reason for this is a proposed standard that could eclipse the proprietary designs that are typical of most batteries for today's mobile computers.
Custom Designs
Mo
bile-computer vendors generally prefer to use custom-designed batteries because it allows them more freedom to match the batteries and the power management subsystem to the particular requirements of the mobile system. These proprietary designs can prove to be expensive and risky, however.
Every time the battery is modified, the changes affect all the phases of product development, including systems design, manufacturing, inventory, and distribution. An unforeseen problem, such as the shortage of a key component, can bring manufacturing to a dead stop. And you may have a hard time finding the right battery to fit your computer, especially after the model is discontinued.
An industry standard for batteries could minimize these risks. Critical components would be readily available from suppliers who are eager to sell to a broad alliance of companies. You would have less trouble finding replacement batteries, and the additional competition should reduce prices.
However, some mobile-system ve
ndors are reluctant to adopt an industrywide standard that neutralizes their competitive edge in battery technology. A standard that's too rigid might also restrict their flexibility to experiment with advanced designs. For these reasons, proposed battery standards are often viewed with caution--if not outright suspicion.
Nevertheless, Intel and Duracell have proposed a pair of complementary standards that are known as SMBus (System Management Bus) and SBD (Smart Battery Data). Together, these two proposals map out a relatively low-cost and reliable plan that would allow mobile-computer makers to add smart-battery technology to their systems.
In the approach from Intel-Duracell, the two-wire SMBus carries clock signals, data, and instructions to a smart battery, an SMBus host, a smart-battery charger, and other devices. The SMBus specification allows for any type of battery, regardless of its chemistry, voltage, capacity, or physical package. And it's designed to work equally well in single- or
multiple-battery systems.
The SBD specification defines a wide variety of battery-related information that can be carried over the SMBus, including battery characteristics, manufacturer data, the current state of charge, low-power alarms, predicted and measured discharge rates, and control, status, and error messages. This information originates from logic and memory chips embedded within the battery.
The SMBus isn't meant just for smart batteries. Modeled after Philips Semiconductors' I2C communications bus, it's really a general-purpose communications channel that can share power management information with any number of devices. These devices can supply their model designations and part numbers, save their states immediately before a power-down event, indicate errors, accept control parameters, and answer requests about their status.
By carrying messages on a standard bus instead of on individual control lines, the SMBus can reduce the number of pin-outs on the battery and other power
management components, reducing costs. It's also easier to add devices to the bus.
SBD feedback improves battery charging, too. When connected to AC power, the charger adjusts its output in response to periodic messages from the battery about its charging requirements. The battery can notify the charger if it detects a problem, such as overcharging, high voltage, or high temperature. In this way, the battery--regardless of its chemistry, construction, or condition--controls its charging cycle.
Gradually, the Intel-Duracell proposal is gaining supporters. Chip makers such as Benchmarq and Microchip Technology have announced that they will supply system components, as have BIOS makers Phoenix Technologies and SystemSoft. Canon is the first major system vendor to announce support for the standards.
However, most of the vendors are for the moment watching and waiting. Still other companies actively oppose the idea. One chip maker, Opti Computer, objects to paying royalties for using the SMBus
. Apple says the SMBus would require a time-consuming and software-intensive redesign of its PowerBooks. The standards will probably appeal most to smaller vendors, who lack the resources to develop their own smart batteries and power management subsystems.
Duracell also wants the industry to adopt
five standard sizes
for smart batteries--similar in concept to the standard batteries (i.e., D, C, AA, AAA, and 9-V) that are used in other consumer devices. However, vendors may be reluctant to accept this idea, too. Mobile-computer designers tend to leave the battery for last, assigning a higher priority to the overall shape and size of the computer, along with styling and ergonomic considerations. As a result, it's the space that remains after the other components are designed that defines the battery's exact shape.
As every laptop owner knows, today's batteries are anything but standard. Users will welcome Duracell's proposal if it doesn't impair performance. But battery d
esign is an important decision that most system vendors would rather reserve for themselves.
At the Crossroads
To make it easier for system vendors to custom-design their own smart batteries, National Semiconductor and Energizer Power Systems, which is a division of Energizer Battery, have teamed up to offer an alternative. National Semiconductor supplies the electronics, and Energizer supplies the battery, which generally includes its own microcontroller. Thus, system vendors will save money because they don't have to start from scratch to make a custom design.
This partnership reflects a growing choice of discrete and chip-level circuits for power management and battery intelligence. For example, National Semiconductor also offers a smart-battery controller called the NeuFuz LMC6984. Using so-called neural-fuzzy logic, this controller is designed to optimally charge most types of rechargeable batteries without the need for more complex charger circuits.
It's bec
oming quite clear that battery technology for mobile computing is at a crossroads. If system vendors can be convinced that easy access to standard, low-cost batteries will help sell their laptops, they will be more amenable to the Intel-Duracell proposal. Otherwise, they will cling to the competitive advantages of proprietary batteries.
Dale Stolitzka, a senior applications manager at National Semiconductor, sees both camps thriving in the immediate future. "Companies that want a unique design will use custom batteries," he says. "Those driven by low cost and aiming for broadest market appeal will opt for standard sizes."
WHERE TO FIND
AER Energy Resources, Inc.
Smyrna, GA
(800) 769-3720
(404) 433-2127
fax: (404) 433-2286
Apple Computer, Inc.
Cupertino, CA
(800) 776-2333
(408) 996-1010
fax: (408) 996-0275
Benchmarq
Microelectronics, Inc.
Carrollton, TX
(8
00) 966-0011
(214) 407-0011
fax: (214) 407-9845
Canon Computer Systems
Costa Mesa, CA
(800) 848-4123
(714) 438-3000
fax: (714) 438-3099
Compaq Computer Corp.
Houston, TX
(800) 345-1518
(713) 370-0670
fax: (713) 378-1442
Duracell
Bethel, CT
(800) 431-2656
(203) 791-3274
fax: (203) 791-3273
Energizer Power Systems
Gainesville, FL
(904) 462-3911
fax: (904) 462-6210
IBM Personal Computer Co.
Somers, NY
(914) 766-1900
Intel Corp.
Santa Clara, CA
(800) 548-4725
(408) 765-8080
fax: (408) 765-1821
Microchip Technology
Chandler, AZ
(800) 437-2767
(602) 963-7373
fax: (602) 899-9210
National Semiconductor Corp.
Santa Clara, CA
(408) 721-5000
fax: (408) 739-9803
Opti Computer, Inc.
Santa Clara, CA
(800) 398-6784
(408) 980-8178
fax: (408) 980-8860
Panasonic Communications & Systems
Secaucus,
NJ
(800) 726-2797
(201) 348-7000
fax: (201) 392-4441
Phoenix Technologies, Ltd.
Norwood, MA
(800) 452-0120
(617) 551-4000
fax: (617) 551-3725
SystemSoft Corp.
Natick, MA
(508) 651-0088
fax: (508) 651-8188
BATTERY TECHNOLOGIES FOR TODAY'S MOBILE COMPUTERS
BATTERY TYPE:
Nickel-cadmium
GRAVIMETRIC ENERGY
(WATT-HOURS/KILOGRAM): 40 to 50
VOLUMETRIC ENERGY
(WATT-HOURS/LITER): 80 to 125
STRENGTHS:
Delivers high current output; relatively
tolerant of overcharging; withstands
up to 500 charging cycles.
WEAKNESSES:
Mature technology with little
tolerant of overcharging; withstands
room for improvement; cadmium is
environmentally troublesome;
noticeable memory effect.
BATTERY TYPE:
NiMH
GRAVIMETRIC ENERGY
(WATT-HOURS/KILOGRAM): 50 to 60
VOLUMETRIC ENERGY
(WATT-HOURS/LITER): 100 to 170
STRENGTHS:
Environmentally safer than nickel-
cadmium; somewhat less memory
effect; 300 to 500 charging cycles.
WEAKNESSES:
More easily damaged by high
charging currents or overcharging.
BATTERY TYPE:
Lithium-ion
GRAVIMETRIC ENERGY
(WATT-HOURS/KILOGRAM): 80 to 100
VOLUMETRIC ENERGY
(WATT-HOURS/LITER): 220 to 240
STRENGTHS:
Higher energy than nickel-cadmium
and NiMH; no memory effect; 500 to
800 charging cycles; low self-discharge rate.
WEAKNESSES:
Susceptible to damage from
overcharge and overdischarge.
Energy ratings may vary with cell size and application.
photo_li
nk (24 Kbytes)
Duracell is proposing five standard sizes for smart batteries. This would be more convenient for users, but some system vendors worry that it would restrict their design freedom.
screen_link (15 Kbytes)
A typical fuel gauge for a laptop with smart batteries. When coupled to the computer's power management subsystem, smart batteries give accurate information about their remaining operating time. The computer can offer more options for balancing operating time and performance.
Gil Bassak is a freelance technical writer and journalist in Ossining, New York. You can reach him on America Online at GLBassak, on CompuServe at 72230,3526, or on the Internet or BIX at
editors@bix.com
.
Brainy, Brawny Batteries
