Keeping an Eye on Battery SOC in Wearables and Hearables

May 31, 2018

Christine Young By: Christine Young
Blogger, Maxim Integrated 


Do you have a wearable or a hearable? According to industry analysis cited by Forbes, global sales of wearables are projected to grow by an average of 20% each year over the next five years. At this rate, by 2022, wearables will be a $29 billion market. CCS Insight, the analyst firm, points to smartwatches as the engine behind this growth.

Smartwatches provide so much more than time-keeping. They are devices for monitoring health and fitness, for messaging, for hailing a ride, and for any number of transactions based on the apps supported. Above all, these small, but powerful devices are supposed to make our lives easier and more convenient. Long battery life is a key part of this equation.

A good smartwatch will last at least a day before it needs to be recharged. If you're designing smartwatches and other wearables as well as hearables, integrating the right fuel gauge IC into your design can make a difference between a customer who's delighted or annoyed by the battery life of her device.

SmartwatchThe right fuel gauge IC can help extend battery life in wearables like smartwatches.

Selecting the Right Fuel Gauge IC

So, what are some of the features you should look for in a fuel gauge IC for size- and power-constrained, connected electronic designs? Consider these factors:

  • Accurate prediction of battery run-time. As an example here, consider a smartwatch that runs for 5 hours in active state and 19 hours in passive state over the course of one day. If such a device consumed 40mA in active mode and 4mA in passive mode during this timeframe, it would consume a total of 276mAh, which is roughly the capacity of a typical smartwatch battery. This is why accurate prediction of battery run-time is important in avoiding unexpected or premature interruptions of the device as it is operating.
  • Good accuracy at low temperature, an important point since wearables will be operated in outdoor environments.
  • Low quiescent current. Since a fuel gauge is continuously monitoring the device’s battery, its energy consumption does reduce system runtime. So, the lower the quiescent current, the better to extend the device's battery life.
  • Any features that potentially minimize lengthy battery characterization work. Since the energy stored in the battery depends on load, temperature, and other parameters, developers typically need to characterize the battery under a variety of conditions in a lab. Characterization can contribute to more accurate battery performance, helping to minimize state-of-charge (SOC) errors and to correctly predict when the battery is almost empty. The extensive lab work for this process, however, can hinder time-to-market goals.
  • Small size is critical to achieve a comfortable and stylish form factor.

Accurate SOC Without Characterization

Maxim has developed an algorithm, the ModelGauge™ m5 EZ algorithm, that, for most common lithium batteries, generates accurate battery SOC estimates without requiring characterization. The algorithm uses a battery model that’s been tuned to a specific application and is embedded in the fuel gauge IC. It brings together the short-term accuracy and linearity of a coulomb counter with the long-term stability of a voltage-based fuel gauge. To further enhance accuracy, the algorithm also provides load and temperature compensation. A simple configuration wizard included in the evaluation kit software lets you generate battery models.

The MAX17055 fuel gauge IC includes the ModelGauge m5 EZ algorithm. This IC is suited for wearables, providing just 0.7µA shutdown current, 7µA hibernate current (IC still monitoring the battery), and 18µA active supply current in a 0.4mm pitch, 1.4mm x 1.5mm, 9-pin WLP. The device automatically compensates for cell aging, temperature, and discharge rate. Another simple fuel gauge IC suited for small, lithium-ion battery-powered electronics is the MAX17048. This IC has a 3µA hibernate current (IC still monitoring the battery) and 23µA active current, and requires just one external capacitor (compared to multiple external components with competitive devices). It’s available in an even tinier 0.9mm x 1.7mm, 8-bump WLP. The fuel gauge doesn’t require any sense resistor, which is typically required by a fuel gauge to measure current.

As you plan your next wearable, hearable, or similar design, choose a fuel gauge IC that allows you to meet customer expectations for long battery life and accurate battery health information.

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