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Truth Seeking in Secret Life of Batteries

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  • RemyC
    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
    Message 1 of 1 , Jan 15, 2005
      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.
      Eisenhart said.

      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
      factors.

      ''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.
      Heacock said.

      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
      ages.

      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
      said.

      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
      energy.

      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.''
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