Power Electronics

Compact Fuel-Gauge Increases Li+ Battery Runtime

Maxim Integrated Products introduces the MAX17047 fuel gauge for single-cell Li+ battery packs. Featuring the company's new ModelGauge m3 algorithm, the MAX17047 is the industry's only coulomb-counting fuel gauge that does not suffer from the abrupt corrections that occur with traditional coulomb-counter algorithms. Compared to other coulomb-counters, this ModelGauge m3 IC uses a smaller current-sense resistor and fewer external components. This saves both space and cost. The IC is ideal for fuel-gauging handheld single-cell and multicell Li+ batteries in a wide range of portable applications, including wireless handsets, smartphones, tablets, e-readers, portable game players, digital cameras, financial terminals, portable navigation equipment, and portable medical equipment.

Traditional Li+ battery fuel gauges are mounted inside the battery pack and require multiple supporting discrete components. To estimate of battery capacity, the fuel gauges rely on coulomb counters. These coulomb counters have a problem because of the small ADC offset errors that accumulate indefinitely. To correct for these offset accumulation drifts, a large and expensive sense resistor is needed. In addition, the battery must go into full, empty, or standby states on a regular basis.

Some recent design advances implement the fuel-gauging function on the system side, instead of inside the battery pack. While this approach reduces the cost of the application, it has little effect on the board area, as a large sense resistor may still be required.

Finally, the currently available algorithms that correct these drifts have an undesirable side-effect: they introduce sudden jumps in the state of charge (SOC). This happens because the algorithms rely on measuring the battery voltage in a standby state, and then make a correction based on the relationship of the battery's open-cell voltage (OCV) to the battery SOC.

ModelGauge m3 technology overcomes the limitations of the currently available fuel-gauging techniques. It combines the excellent short-term accuracy and linearity of a coulomb counter with the excellent long-term stability of a voltage-based fuel gauge. ModelGauge m3 cancels offset accumulation error in the coulomb counter, while providing better short-term accuracy than any only-voltage-based fuel gauge. This algorithm makes tiny corrections continually over time, so it does not suffer from the abrupt corrections that normally occur in coulomb-counter algorithms.

Since the ModelGauge m3 algorithm reduces the sensitivity to current-measurement ADC offset errors, the MAX17047 can use a smaller current-sense resistor without compromising the accuracy of the battery SOC estimate.

The MAX17047 also automatically compensates for aging, temperature, and discharge rate. It provides accurate remaining capacity in mAh or SOC % and time-to-empty over a wide range of operating conditions. It uses 75% less power than the competition, even adapts to changes in the battery over use and time, and warns of abnormal battery conditions. The device provides two methods for reporting the age/health of the battery: reduction in capacity and cycle odometer.

The IC provides precision measurements of current, voltage, and temperature. The temperature of the battery pack is measured using an external thermistor supported by ratiometric measurements on an auxiliary input. If the temperature information is available through some other means, then the system microcontroller can also write the temperature directly to the IC. That eliminates the thermistor, plus a resistor and capacitor associated with this external thermistor network. .

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