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A Better Gauge on Battery Life

A new battery-gauge chip could make mobile phones more reliable and help them last longer on a single charge.
August 31, 2007

Texas Instruments (TI), based in Dallas, has developed a battery-gauge chip that can tell mobile-phone users down to the minute how much talking or standby time they have left–a degree of accuracy much greater than that provided by existing battery gauges. Such a precise gauge could allow smart-phone developers to squeeze more energy out of the battery, potentially increasing by half or more the amount of time that it lasts between charges.

The new TI gauge is more accurate than today’s gauges, which measure a battery’s voltage, because it measures a number of electrical properties. Voltage-only-based gauges are erratic and unreliable because voltage doesn’t fall steadily as the battery is discharged. What’s more, the voltage changes as the battery ages and experiences different temperatures. It also varies with different power demands on the battery.

The TI gauge is more accurate–to within 1 percent of the actual energy left in the battery–because it measures electrical properties besides voltage. Most important, it measures a feature of battery cells that is at the root of the voltage changes that make today’s gauges unreliable. This feature, called impedance, is a measure of the opposition to current flow, and it changes with temperature, battery age, and the power demands on a battery.

Knowing the changes in impedance allows the chip to reinterpret voltage changes, keeping it from being fooled by voltage changes caused by these factors. For example, when a person makes a call, the voltage drops as soon as the phone transmits the signal. A conventional gauge would interpret this as a sudden drop in the amount of energy left in the battery, which could engage battery-saving measures in power-management software. The new gauge would recognize that the cell still has plenty of energy. The approach also works with low voltages caused by cell age.

The new gauge chip, which is incorporated either into the circuit boards of a phone or directly into a battery pack, could be particularly useful in smart phones. Some phone users have to assume that after the gauge has reached the halfway point, the battery could die at any moment. What’s more, poor battery gauges make it difficult to employ power-management software on phones that could extend battery capacity. Power-management software slows down processors, turns off the camera’s flash, and dims the screen to save the battery once it’s low. It may also save data and shut down applications just before the battery dies. Such software, however, may engage power-saving measures too soon if it relies on an inaccurate battery gauge, resulting in sluggish device performance while there is still plenty of charge.

The problem gets worse as the battery ages and, as the battery is depleted, voltage drops more quickly. With conventional gauges, this could trigger the phone to shut down when there is still quite a bit of energy left. Indeed, much of the perceived loss in battery life in older phones is actually just a problem with the battery gauge. “You can lose 30 percent of the energy in a battery simply because the device shuts itself down too early,” says Richard DelRossi, an engineer at TI. He says that the new, more accurate battery gauge could increase the usable battery capacity by as much as 50 to 100 percent, depending on the power-management strategy.

Other phone and chip makers are also developing better battery gauges. Approaches taken by Motorola, based in Schaumburg, IL, and PowerPrecise, a chip maker based in Herndon, VA, that’s funded by Intel, combine voltage data with current measurements to determine how much energy has been used. Subtracting the amount of energy used can give a good idea of how much is left, as long as it’s known how much energy was there to start with. But the capacity of the battery, as with the voltage, depends on certain conditions, such as temperature, battery age, and power demand. To adjust for these factors, these systems can refer to models of battery behavior based on earlier tests to guess how these conditions will affect the battery’s overall capacity. Such a system gives a much more accurate gauge of battery charge than do voltage measurements alone, says Jerry Hallmark, who heads Motorola’s research on energy consumption in mobile devices.

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