Originally Published MPMN November 2004

HOTLINE

Changes in Battery Capacity Are Accurately Gauged

Technology simplifies designing with batteries

Susan Wallace

A gas-gauge technology calculates the remaining capacity in lithium-based battery packs. Impedance Track from Texas Instruments (Dallas) claims 99% accuracy throughout the life of a 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,” says Dave Heacock, vice president of the company’s portable power management group. “Because current battery-measurement solutions do not calculate impedance as a battery ages, the resulting error rate may be over 50% after a few months of use. Impedance Track lets end-users and designers realize the potential of their batteries, and effectively maintain the most accurate measurement possible over the life of the battery.”

The 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. Impedance Track sits inside a Flash-based bq20z8x gas-gauge chipset and analyzes a precise state of change when a battery pack is in a relaxed state by correlating between the 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. This eliminates the need for a full charge-and-discharge cycle. For applications such as heart defibrillators, Impedance Track can provide instant and accurate capacity information on a continual basis.

The technology 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 and cathode system in the battery’s cell. The system significantly reduces development and implementation setup time required by OEM designers to ensure proper characterization because large databases of attributes of battery parameters are not necessary.

Texas Instruments
12500 TI Blvd.
Dallas TX 75243
800/336-5236
www.ti.com

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