powerZONE Products for the week of September 13, 2004
Texas Instruments Says . . .
Bq20z80: Battery Management System Calculates Remaining
Capacity over Life of Battery
"Impedance Track" Gas Gauge Technology Extends Battery Run-Time;
Makes Designing with Batteries Easier and Faster
Building on extensive research and development in battery management, Texas Instruments Incorporated (TI) announced a unique 'gas gauge' technology that calculates remaining capacity in lithium-based battery packs with up to 99 percent accuracy throughout the entire life of a battery. The new Impedance Track technology allows designers and users of portable medical, industrial appliances and notebooks to prolong battery use and always know the exact amount of potential energy left inside the battery.
"Portable devices continually rely on an accurate reading of remaining battery capacity, so the system won't lose data or worse, suddenly shut down during operation. Because current battery measurement solutions do not calculate impedance as a battery ages, the resulting error rate may be over 50 percent after a few months of use," said Dave Heacock, vice president of TI's portable power management. "Impedance Track lets end-users and designers realize the energy potential of their batteries, and effectively maintain the most accurate measurement possible over the life of the battery."
Taking Battery Gas Gauge Technology to the Next Level
TI's innovative Impedance Track technology precisely gauges changes in impedance,
or resistance caused by battery age, temperature and cycle patterns, to
accurately predict run-time of two-, three- and four-cell battery packs.
The technology, which sits inside TI's Flash-based bq20z8x gas gauge chipset,
analyzes precise state of charge when a battery pack is in a relaxed state
by correlating between a battery pack's open circuit voltage and the current
state of charge and temperature.
An exact "starting position" is determined for instant state of charge, and total capacity is calculated from the amount of capacity that exists - eliminating the need for a full charge and discharge cycle. For certain applications such as heart defibrillators or back-up battery packs used in telecom systems that never fully charge and discharge, Impedance Track will ensure instant and accurate capacity information on a continual basis.
Plug and Play Battery Design
Impedance Track relies on a dynamic modeling algorithm to learn how much
a battery has degraded through age, temperature or usage, and then correlates
typical chemical properties of the anode/cathode system in the battery's
cell - no matter what brand of battery cell used. In fact, Impedance Track
allows for the mixing of different manufacturers' cells in a single pack,
providing flexibility and continuity of supply. Many of today's gas gauge
integrated circuit technologies depend on static and unreliable modeling
techniques that require the creation of large databases in order to measure
each attribute of hundreds of available battery parameters. The patent-pending
Impedance Track technology significantly reduces development and implementation
set-up time required by original equipment designers and manufacturers to
ensure proper characterization because those databases are no longer necessary.
analogZONE Says...
Most users of equipment that are fitted with battery gas gauges can relate to how inaccurate they can be. Most laptops, for example, seem to have gauging that is rather akin to those of the Ford Motor Company: they slowly decline to half-full and then crash to empty -- always it seems when you are too far away from a wall socket to do anything about it.
TI has spent some time in looking at the existing technologies based either on voltage measurements or "counting" current. Both have problems although the open-circuit voltage measurement approach has been the most successful: but it still has battery loading issues and problems with the exact state of relaxation in the battery. Most systems are quite inaccurate and they get to be even more inaccurate as the cells age; and inaccurate systems allow batteries to go into deep discharge or not use the full range of the charge actually available. Instead, TI has come up with an algorithm of the discharge cycle that applies to the chemistry of Li-Ion and Li-Polymer cells. The shape of the curve is applicable to any battery, any combination of cells, from any manufacturer. The actual point on the curve depends on the internal resistance of the cells and conditions such as temperature and age. It boils down simply to a magnitude issue recognizing, for example, that I can have a lower termination voltage working in a bus shelter in Toronto in January compared to working on the beach in Malibu in August.
There is no magic formula for setting the end points of the algorithm and it requires a single full charge cycle for them to be set. Because, however, the same batch of batteries will have the same algorithm settings the process can be done off-line in a factory and does not have to be part of the production process.
In operation the system needs to monitor about 500 s (just over 8 minutes) of discharge cycle to be able to recognize the battery's position on the curve. TI claims an accuracy of better than 1% over the life of the battery and cell brands can be mixed together, but any change requires that the single charge cycle be performed.
The IC is based on a RISC CPU core with Flash memory on chip. It uses a 16-bit self-calibrating integrating converter to measure charge flow with an offset error of less than 1 µV and offering a resolution of better than 0.65 nVh. A 16-bit ?-S converter is used for temperature and open-circuit voltage measurements. Although an external crystal source can be used a timebase clock is provided on-chip. A pulse-charging mode can be deployed to speed charge times.
The IC provides an output to drive an LED display (3, 4 or 5 segments) and can be used with 7.2-V, 10.8-V and 14.4-V battery packs using the bq29312 as front-end battery protection. Capacity information is passed to the system host processor over an SMBus.
The accuracy and dependability on this system make it a must-have in products that are safety or mission critical, and that includes medical equipment and portable T&M equipment being used in time or event critical modes. The price premium required by the part will be less attractive to the generic laptop vendors of the world but the higher-end systems will probably pay the difference as an additional talking point to compare with the competition.
Having derived the algorithm for Lithium-based cells TI can now go and repeat the exercise for Nickel-based chemistries.
The bq20z80 is sampling in TSSOP-38 and is priced at $4.35 in 1000-piece lots. Production is slated for Q4 2004. The bq29312 protection IC has already been in production in TSSOP-24 priced at $1.30 in 1000-piece lots.
The data sheet for the bq20z80 had not been published
on TI's website at press time.