By Allison Proffitt
September 4, 2020 | “Don’t always assume that smartphone [battery] requirements apply to wearables,” Lawrence Pan, Principle Battery Engineer with FitBit, warned last month at the 2020 International Battery Seminar and Exhibit. Pan outlined the state of the wearable battery market and highlighted the areas for growth and innovation—many unique to wearable applications.
The IDC Quarterly Wearable Device Tracker data showing an 89% growth in the number of wearable units shipped in 2019. Much of this growth, Pan said, is driven by growth in earwear or “hearables”, but he focused his discussion on wrist-worn smart watches and trackers.
In the wrist-worn segment of the market, functionality has continued to increase—from simple step counters a few years ago to voice assistants—and size and design has become more streamlined. The challenge for the battery designers, Pan explained, is that wearable batteries have unique design requirements, often quite different from batteries for smartphones and certainly much different from electric vehicle batteries. The volumetric energy density is inherently lower and the impedances are inherently lower; both must be addressed.
As wearable batteries—and the devices they power—have shrunk, the percentage of overhead goes up: pouch, foils, separators, tabs, packaging and taping. These overhead parts don’t store charge but are still needed, and are fixed, Pan explained. “Because of all these fixed overheads, the energy density decreases quickly as the capacity decreases.”
Because wearable batteries are so small, volume matters more than weight. These batteries usually weigh only a few grams, Pan pointed out—an opposite set of priorities than for electric vehicles. Thus efforts to improve wearable batteries have focused on both decreasing overheads as well as chemistry improvements to energy density. “We have to look at every detail of the battery to get the most out of the battery,” he said. But small size can also be an advantage. Some new chemistries and technologies—Pan mentions solid state batteries in particular—may not scale to larger sizes, but can easily handle the demands of wearable applications, some in the low 10s mAh.
Wearable batteries also have unique charging requirements—including some advantages. Many wearables have long run times lasting days or even weeks between charges, and consumers tend to tolerate longer charge times for devices with long run times—two hours to charge a device that runs for a week is more acceptable than two hours to charge a device that runs for a day. These longer runtimes mean fewer lifetime cycles, Pan explained, and fewer cycles leads to less swelling allowing a slightly thicker battery and higher initial capacity.
Relaxed cycle life requirements also open the field to new materials that may have limited cycle life, Pan pointed out. “Unlike smart phones where a cycle life requirement is 800 cycles or more, in wearables it’s often 300 cycles or even less. So this makes wearables somewhat of a low-hanging fruit for new materials that can’t quite meet high cycle life requirements yet,” he said, though he added an important caveat: new materials must still meet all safety and reliability requirements and compliance testing.
An added bonus for manufacturing: small batteries need less material, so production can be ramped up even with relatively low factory capacity.
Pan issued several challenges to the industry, highlighting all of the details that could be tweaked to improve wearable batteries. Battery makers should focus on decreasing overheads, improving impedance even at smaller sizes, improve accuracy through FET sensing, and further lower consumption currents.
There are also manufacturing and regulatory challenges that must be addressed to keep up with the demand for more and smaller wearable batteries. Can automation equipment handle smaller sizes? Can we lower the minimum size and thickness of batteries? Can regulatory and labeling requirements be updated to label very small batteries?
“I hope you understand that it’s not trivial to design batteries for wearables. There are plenty of challenges,” Pan concluded, but ended with outreach. “We want to work with battery manufactures and component suppliers to address these particular pain points.”
Editor’s Note: Did you miss the 2020 International Battery Seminar? Because the event was virtual, you can still access the event including all of the recorded sessions, presentations, and materials. Register for PREMIUM POST-EVENT ON-DEMAND.