Smartwatches, fitness bands, GPS trackers, earbuds and health monitors are just a few of the wearable electronics we have come to utilize and enjoy in our daily lives. These devices almost always use lithium-ion or lithium polymer batteries because of their higher energy density and superior battery charge capabilities. Wearables have inherent space constraints making newer, higher energy density batteries a popular choice for these designs.

Unfortunately, lithium-ion batteries are susceptible to short circuit and overcurrent events during the energy transfer process that can trigger the battery to overheat leading to a condition known as a thermal runaway. Not only is this a safety issue causing discomfort to the person wearing the device, but it can reflect poorly on the manufacturer resulting in large numbers of product returns. This issue was documented last year by the U.S. Consumer Product Safety Commission. In the Commission’s Status Report on High Energy Density Batteries Project*, it found that more than 25,000 overheating or fire incidents in some 400 types of lithium battery-powered consumer products had occurred over a five-year period.

A solution for this issue is Bourns® Multifuse® Model MF-ASML/X series. It acts as a secondary overcurrent and overtemperature protection device to prevent such a thermal event from occurring within the device’s specifications. This resettable PPTC fusing device was developed to keep parasitic resistances or impedances to a minimum to enable lower voltage and longer device operating times. Offered in a small 0402 form factor, designers are able to integrate the Bourns® Multifuse® device into both the connector head of the USB charging cable and the PCB in the wearable device itself. The beauty of this dual approach is that it protects the wearable device battery unit during a charging cycle and will also protect the circuitry during discharging, and powering of the device during usage in the event a short circuit or another overcurrent or overtemperature condition occurs.

A new application note from Bourns details the battery issues that can arise in wearable applications. It outlines how to design-in our Multifuse® device for maximum protection.

Don’t overlook adding circuit protection to a wearable design. Your company’s reputation for quality products is too important. The integration of ever-shrinking sub-micron semiconductor technologies can increase the harmful effects of electrostatic discharge (ESD) transients on a growing list of wearables. While a good quality lithium-ion battery is generally safe and unlikely to fail, who wants to take the chance that a faulty charging unit causes overheating or, even worse, a fire hazard?

You can mitigate the effects of an unspecified charging event, transient, or overtemperature condition of the battery to ensure your wearable design is safe against these threats. I invite you to learn more about the Bourns® Multifuse® Model MF-ASML/X series advantages.

*United States Consumer Product Safety Commission Status Report on High Energy Density Batteries Project, February 12, 2018: https://www.cpsc.gov/s3fs-public/High_Energy_Density_Batteries_Status_Report_2_12_18.pdf?UksG80UJqGY0q4pfVBkbCuUQ5sNHqtwO