Truth Seeking in Secret Life of Batteries
- WHAT'S NEXT
Researchers Seek Truth in the Secret Life of the Battery
By ANNE EISENBERG
December 23, 2004
DRIVERS can trust the fuel gauges in their cars to tell them whether the
tank is half full or down to its last gallon. But laptop owners don't have
the same security when they check the time left in a lithium-ion battery. A
click on the battery icon may suggest that 20 minutes are left when, in
reality, only a few minutes remain.
Now Texas Instruments researchers have come up with a monitor they say
laptop users can trust to measure remaining battery energy accurately. The
new gauges should help people avoid unexpected shutdowns that could result
in a loss of data.
''Whether the battery is new or old, you will know within 1 percent accuracy
what the remaining energy is,'' said Scott Eisenhart, director of battery
management at the company.
In the technology, each battery pack contains a two-chip system that can
constantly calculate the remaining energy. The chip sets have algorithms
that take into account impedance, the opposition to electrical current flow,
which changes with conditions like age, temperature and use. Based on
mathematical models of lithium-ion chemistry, the chips use these impedance
measurements to adjust predictions of the charge remaining in the battery.
The new chips have not yet proven themselves in consumer products. They will
not be available in laptops, Texas Instruments' first target for the new
technology, until the middle of 2005 or later.
''We're ready to go,'' said Dave Heacock, vice president for portable power
management, ''but our customers need some time to engineer the gauges into
their new products.''
If proven successful, the new gauges could be used in other devices as well.
In digital cameras, for example, they could provide an accurate estimate of
how many pictures could be taken before the battery was exhausted. In an MP3
player, a screen could indicate how many more songs could be played, Mr.
The devices currently used to measure remaining run time on laptops are
typically known as gas or fuel gauges. ''Right now, fuel gauges on computers
are basically worthless,'' said Robert M. Spotnitz, who heads Battery Design
in Pleasanton, Calif., a small company that makes software to simulate
battery performance. ''They will say an hour when you have 10 minutes.''
But the new devices actually monitor the battery and determine its state
throughout its lifetime, he said. ''These devices measure continually to see
you lost 8 percent or 10 percent, and are therefore more accurate.''
Newer laptops typically have chip systems within them that include a gauge
to monitor charge in and out of the battery, as well as a microprocessor
that runs software that estimates remaining battery energy, Mr. Heacock
said. But because the programs do not include continuing, real-time
monitoring, they can soon produce large errors.
The use of lithium-ion batteries is widespread in portable devices, and the
batteries must work in a range of environmental conditions. But run time and
performance of batteries vary depending on storage conditions and use, as
well as how fast the batteries are discharged, how old they are and other
''When the car gets hot, a laptop stored on the front seat gives you a
completely different usage than one stored at room temperature,'' Mr.
Users will know something is wrong, although they can't measure the
degradation of performance accurately. ''You start off with four hours' run
time on your laptop,'' Mr. Eisenhart said. ''But after a year, it's 1.5
hours, and the system starts shutting down on you.''
Part of this change in performance has to do with impedance, which builds as
the battery ages. ''Typically you are looking at 500 to 800 charge cycles as
the lifetime of a lithium-ion battery,'' Mr. Eisenhart said. ''But after as
few as 70 charge cycles, impedance of the battery can double.''
The Texas Instruments chip takes into account temperature, current and
voltage recovery time of the battery. ''It looks at how the computer is
pulling energy out of the battery over the 500 to 800 cycles,'' said Mr.
Heacock, who added that it also tracks impedance changes as the battery
This constant tracking of a battery's actual state of health is innovative,
said Jiang Fan, an engineering manager at Gold Peak Industries North
America, a manufacturer and distributor of batteries based in San Diego.
''In older systems, all the modeling factors for prediction have been
determined, and they can't be changed once the battery is installed,'' he
Texas Instruments may have gotten around this problem, he said, by tracking
actual impedance changes. ''Only battery energy can predict running time
accurately,'' Dr. Fan said, and impedance has a significant effect on this
Impedance builds up normally in batteries as electrolytes dry up, he said,
inevitably affecting the rate of the chemical reaction inside the battery.
''When electrolytes decompose,'' he said, ''there is no carrier for the
lithium ions to transfer from negative to positive during discharge.''
It's like a river with 200 boats to transfer 2,000 people, he said. ''When
100 of the boats burn, it takes longer to transfer the 2,000 people.''
But the new controls will lead to greater efficiency, he said. ''Battery
control can be set so that the user doesn't have to stop when the gauge says
there's still 20 percent energy left.''
Lance Chandler, a computer scientist and senior manager for design
engineering at Intersil in Milpitas, Calif., a competitor of Texas
Instruments, said that the new technology merited consideration.
''But it is still in its infancy as far as acceptance,'' he said. ''It will
take time for the technology to prove itself.''