Overcoming Battery Power Limitations in Wearable Devices

By Deborah Borfitz

February 11, 2019 | Explosive growth in wearable devices has put the battery conundrum front and center for companies in the business of making these pint-sized, often power-hungry technologies. “The number one problem is we want the battery to last longer and be smaller—or nonexistent,” says Walt Maclay, founder and president of electronic design company Voler Systems.

But the answer lies not with the batteries themselves but in applying creative workarounds to their limitations, including how data gets transmitted and alternatives to traditional electronic and software components. Batteries could get two times denser in the next few years but will never improve at the rate seen with semiconductors due to chemical energy storage constraints, says Maclay. Plus, there’s a well-demonstrated tradeoff between energy density and safety.

Sensible Sensor Selection

The power consumption differential among sensors is huge, says Maclay, from a tenth of a milliwatt for a 3-axis accelerometer to 300 milliwatts for a typical image sensor in a camera. Choosing the right one for the job is therefore crucial to extending battery life.

Voler Systems has a unique universal health sensor platform used for early, preproduction prototyping to quickly see if an intended sensor is a good fit for a device. Multiple sensors can be tried in a test environment with connection to the cloud to narrow the list of options, Maclay explains. “There is usually more than one way that you can sense something. Motion, for example, can be sensed with an accelerometer, by measuring distance or changing light, or by changes in sound as you walk around a room.”

How often data needs to be collected is one of the more common afterthoughts of companies looking to collect, store and transmit data digitally, Maclay says. Sensors can often be shut off much if not most of the time, dramatically lowering average power consumption. Wearables that measure blood oxygen, for example, “could take a measurement every five minutes or every hour versus every minute. Even for someone with sleep apnea, measuring every few seconds is probably not necessary.”

Similarly, a GPS sensor tracking users’ whereabouts doesn’t need to run continuously, says Maclay. Such devices might intermittently use a low-power dead reckoning chip to calculate their position based on the last GPS check-in. The old-fashioned navigation technique may only be accurate for a few minutes at a time but can significantly extend battery life—in some cases, from several hours up to several days.

For blood pressure (BP) monitoring, alternatives to energy-consuming automated cuffs include deriving BP estimates from electrocardiogram (ECG) signals or pulse transit time (PTT, the time it takes the pulse to reach an extremity), says Maclay. Measuring blood pressure from an ECG is a technique still under investigation. PTT is in use, although “It is questionable whether it can produce medically accurate results.” Not all applications may need that precision, he adds.

Power-Conserving Pointers

Wearables don’t necessarily need a display feature, which tends to guzzle a lot of power. Those that do usually display numbers only, and in black and white—and still the battery drains quickly if a user turns on the backlight to look too frequently or long at the readings, says Maclay. A “good compromise” is the e-paper display of a Kindle, which offers the same limited features but at a power grab nearly as low as the LCD display on a watch.

A 3-axis accelerometer can often be used to limit the power-on time of wearables. It might be an ideal solution for a sleep monitoring device that collects data only when people are moving, Maclay says. The accelerator could be programmed to turn the processor on when people hit a certain movement threshold and turn it off again when they’ve fallen below that baseline.

Even securing a device can be done in a power-conserving fashion, says Maclay. Advanced encryption generally requires a powerful processor that uses more battery life. But one company, Secure RF, is taking advantage of an encryption technique that runs on tiny and low-cost 8-bit microcontrollers, offering significant energy savings and “moderately good” encryption that is likely good enough for most healthcare wearable devices. “It depends on what happens if someone hacks into the device,” says Maclay. “Are they going to shut off your heart or interfere with collecting data when you’re sleeping?”

Battery power needs aren’t a big concern for wearables that spend most of their life cycle in sleep mode, notes Maclay. Emergency call buttons, which may not transmit data for days or weeks at a time, often rely on power-hungry cellular modems.

And a quick word on the topic of energy harvesting, which Maclay says frequently comes up in conversations with customers: don’t count on it. Energy can be scavenged from “temperature differences, motion, solar energy and RF signals in the air and pretty much all of these will generate microwatts … but it is rare that a customer comes to us with an application that needs less than a milliwatt. We have yet to put energy harvesting in a product we designed.”

Moving Data Wirelessly

Low-bandwidth, long-distance transmission protocols—including LoRa, NB-IOT, and Sigfox—are becoming more widely available, with many cities completely covered, says Maclay. That makes them an increasingly attractive means to send device data directly to the cloud. For now, their appeal is limited to geography-specific applications. LoRa, unlike all the others, allows organizations to put in their own base stations, he notes, making it an ideal option for devices that only need to be operational inside a hospital or factory. “The downside is that LoRa requires more complicated software development than other wireless technologies and doesn’t provide as much software support.”

Florida Power & Light is having NB-IOT infrastructure installed across its home state, says Maclay, suggesting uptake of low-power, wide-area network technologies may happen sooner rather than later. Entire countries, including the Netherlands and South Korea, are actively creating such networks devoted to serving IoT devices.

Which brings up wireless charging, whose time has not come for devices worn on rather than implanted in the body, Maclay says. It’s not that wireless charging lacks maturity—the de facto standard is Qi (pronounced “chi”)—but that the cost outweighs the benefit. People are accustomed to plugging in USB connectors to charge their phone and headset, and companies are generally differentiating themselves with bigger product features anyway.