Battery fuel-gauge chip provides 99% accuracy

By Margery Conner, Technical Editor -- 10/8/2007

Battery-gauge ICs for lithium-ion-battery packs have always involved complex algorithms. For example, battery gauges in laptop computers and cell phone measure current, voltage, and temperature and integrate the current over time to find the charge. They must model the cell’s reaction to discharge rate, temperature, age, and self-discharge rate, and they must relearn the full-charge capacity over time. They also must predict and accumulate the error for all of these numbers.

TI addresses this complexity and bets that user demand for more accurate gauging will make consumer-electronic vendors eager to incorporate its new gauges, which employ the company’s Impedance Track technology. The new system-side bq27500 incorporates Impedance Track to directly measure the effects of discharge rate, age, and temperature on battery charge. This sophisticated direct-measurement technique allows the gauge to calculate the effect on remaining and full-charge capacity with no modeling or learning. TI claims that the technology provides 99% accuracy in gauging remaining battery charge, and that figure is accurate, according to Robin Tichy, PhD, technical-marketing manager at battery-pack-design house Micro Power.

Tichy points to applications in medical electronics for life support as appropriate uses for Impedance Track. These applications require batteries that provide a precise countdown to 30 minutes before the battery power runs out. As a result, most life-support electronics rely on sealed lead-acid backup batteries rather than lithium-ion devices because lead-acid batteries’ predictably constant slope makes them easy to gauge. In contrast, lithium-ion batteries’ discharge profile is constant until just before they completely discharge, when it decreases sharply. However, TI’s Impedance Track provides an accurate measurement on the remaining battery power, allowing the use of Micro Power’s lithium-ion backup-battery packs in critical medical applications, according to Tichy.

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