Battery Talk
Portable debate turns to batteries
By Ron Wilson, EE Times
Santa Clara, Calif. -- Batteries and battery management pushed the envelope at Portable by Design, as vendors described
several new chemistries, each of which could make important contributions to portable equipment. Additional papers discussed
the latest in battery management technology, often based on the Intel/Duracell Smart Battery Data Specification.
The shift from NiCd and Ni-metal-hydride batteries to lithium-based cells was virtually a given for the conference. One
presenter claimed that nearly all the new notebook computers introduced in 1997 would use some sort of lithium battery
system.
But the new lithium cells, while offering advantages in operating life for the space and weight, have brought their own
problems, some papers warned. The cells require careful supervision by a microcontroller or dedicated battery management IC
to achieve their potential. In the extreme, intelligent battery management is necessary in lithium-ion batteries to prevent fire or
explosion.
In answer to this challenge, papers discussed a host of battery-management strategies based around the Intel System
Management bus (SMbus) and the SBDS. Dedicated ICs, such as those described by Benchmarq Microelectronics Inc., can be
integrated into a battery pack to provide a full range of management features, disconnecting the battery from an overcharge,
protecting it from exhaustion, controlling the charging process in conjunction with a smart charger and providing an accurate
capacity gauge.
Other papers, notably one from Upal Sengupta, principal engineer at Rayovac Corp., argued in favor of a
microcontroller-based rather than dedicated approach. Sengupta pointed out that microcontroller firmware could be altered to
take into account the needs and behaviors of multiple battery chemistries. Even within a particular group, such as lithium
batteries, the charging and discharge characteristics can vary widely.
Whatever approach was taken, there was general agreement that safety and accurate capacity measurement were vital. The
safety issue spoke for itself, particularly in light of Li-ion batteries' ability to grow tiny slivers of metallic lithium, which could
then short out a cell, resulting in fire.
But capacity measurement emerged as an equally important issue. Speakers from SystemSoft Corp. pointed out that with
typical battery measurement systems today, capacity is only measured within about 10 percent. By having a more accurate
knowledge of the remaining energy in the battery, and by carefully choosing a power-management scheme in the computer to
best use the remaining power, the paper estimated that operating life could be extended by as much as 20 percent with the same
hardware.
But the greatest changes in portable equipment life may come from the batteries themselves. Despite the current wave of
enthusiasm over Li-ion batteries, several competing technologies--some recent and some not yet ready for production--spoke
their piece. They focused not only on energy density but also on manageability and safe disposal.
Perhaps the most familiar of the competing technologies was RAM--a rechargeable alkaline battery developed by Battery
Technologies Inc. and marketed by several major companies. RAM cells, according to a paper, offer discharge rates in the
hundreds of milliamps, hundreds of useful recharge cycles under modest operating conditions and extremely low
self-discharge. They are used primarily to replace conventional alkaline cells in consumer applications, but they also have OEM
uses.
A more unusual approach came from AER Energy Resources Inc., which described a unique Zinc-air battery design: the cell
essentially respires, giving off oxygen during charging and absorbing it during discharge. A Zinc-air battery uses a
combination of air tubes and a fan to provide or remove air when the battery is in use and to isolate the battery from air when it
is not in use.
Zinc-air technology, according to AER, offers much lighter weight per Watt-hour than even Li-ion batteries, at about the same
volume. The batteries are also potentially much less expensive than lithium solutions, and can be disposed of as household
waste. This latter point is important because, unlike Li-ion batteries that provide hundreds of recharge cycles in even heavy
operation, Zinc-air batteries are reduced to half capacity by about the 50th deep discharge. AER marketing manager Tim Cutler
estimated that in practice, the Zinc-air battery in a notebook computer would have to be replaced every couple of months.
Other new and promising technologies used lithium ions, but in different ways. Li-ion polymer batteries, discussed by
Powerdex division of Gould Electronics Inc., offer essentially the same performance as liquid-electrolyte Li-ion batteries. But
because the electrolyte is a flexible polymer instead of a liquid, cells can be made in virtually any shape, from millimeter-thick
sheets the size of a notebook-computer LCD panel to complex curved surfaces.
Another emerging technology came from Battery Engineering Inc. The company has devised a Li-ion cell in which both anode
and cathode are carbon, and in which the electrolyte is an inorganic material containing sulfur dioxide. A sacrificial electrode of
metallic Lithium is placed in the cell, and over the first few operating cycles, the metal is transported into the carbon electrodes.
This gives the cell the remarkable characteristic of actually increasing its energy capacity during the first 100 or so cycles. The
cell shows an initial capacity of about 90 mA-hours, which degrades only slightly over 250 cycles.
More important, the cell exhibits one very valuable characteristic of previous sulfur-dioxide/lithium cells--it is chemically
resistant to overcharging. Cells overcharged to many times their capacity would show no abnormal behavior on discharge. But
previous sulfur-dioxide cells had the unfortunate habit of exploding violently during operation. Replacing the lithium anodes in
these cells with the carbon anode, and inserting the sacrificial lithium plate, eliminated the growth of Lithium dendrites and the
explosions.
The new battery technologies promise greater energy density, greater safety, more flexible packaging and less need for battery
management. But none offers an enormous increase in operating time for portable equipment. As one speaker commented, "We
have made big advances in energy density. But the systems vendors will find ways to use more current, so the operating hours
have stayed about the same."
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