Invisible Batteries for Wearables and the IoT



The wearable devices market and the Internet of Things (IoT) movement are both forecast to expand rapidly over the next decade. The primary technical demands placed by these application sectors on electronic components are ultra-low power, low cost, wireless connectivity, and in the majority of cases, ultra-small size.

Energy harvesting is already identified as an important technology for powering both wearable devices and wireless sensor nodes. Batteries for energy storage, meanwhile, are regarded as essential for the majority of applications. However, today’s conventional form-factors, including coin cells, are seen as a serious limitation. Thinness, flexibility and capacity are the critical factors. Hence, thin-film/solid-state and printed batteries offer the greatest potential.

This article will outline the challenges of providing continuous power for wearable devices with respect to battery technology, battery lifetime and rechargeability. It will then highlight the advantages of the latest generation of solid-state batteries, particularly their small size, design flexibility, and their ability to be recharged wirelessly via harvested ambient energy.

Reference will be made to the Cymbet Corporation range of EnerChip solid-state batteries with integrated real-time clock and power management features and their respective evaluation kits.


Figure 1: The Enerchip CBC34813-M5C from Cymbet combines a solid-state, rechargeable battery with real-time clock and integrated power management. Capacity is 5 μAh with an output voltage of 2.5 V. Device size is 5 x 5 mm. Charge time is 15 minutes at 2.5 V.

Market growth

Although thin-film and solid-state batteries have been around for a decade or more, it is the growth in emerging markets including wearable devices and the IoT that is generating significant new interest. According to market research company IDTechEx in its recent report, “Flexible, Printed and Thin-Film Batteries, 2015 to 2025”,¹ the market will be worth $300 million by 2024 at the device level.

The report highlights that different segments, including wearable devices, IoT, RFID, consumer electronics and medical devices, will require batteries in different form-factors, power densities, lifetimes and, of course, price points. Wearable applications will largely require high-energy sources, such as thin-film and flexible lithium batteries, which are expected to show the highest market potential, the report concludes.

According to Steve Grady of Cymbet Corporation, in a just-published White Paper, the limitation for many applications is power. “New power solutions will be required: power solutions that are small, thin, self-recharging and never need replacement. Conventional batteries simply won’t meet the requirements,” Grady says.²

The primary driving technologies behind the growth in wearable devices and the IoT sector, namely ultra-low power processors, sensors and RF/wireless networking circuitry, are all readily available, and integrated into extremely small packages – just 1 mm³, he adds.

Battery life

Most wearable products will be designed with a power source that has to last the lifetime of the product. Some devices can be powered solely through energy harvesting techniques. Heartbeat-powered implantable pacemakers and ultra-low-power sensor-based nodes powered by RF or electromagnetic waves are proving viable to allow battery-free designs.

Some applications require very little power and/or have a limited lifetime (such as some medical devices or RFID tags), and a single non-rechargeable battery may be sufficient to last the expected lifetime of the product.

However, in many cases, a battery is required, and typically it cannot be removed for charging, nor can a charging cord be connected. Wireless charging is becoming a desirable option. Battery life is already the most important aspect of many smart mobile devices, and can severely limit the number of increasingly power-hungry features and functions that can be incorporated, despite the focus on ultra-low-power circuitry. For wearable devices, form-factor is just as important as capacity, as ideally batteries need to be small and thin, and co-packaged with the electronic circuitry.

Cymbet’s White Paper outlines a number of drawbacks of conventional chemical batteries, including lithium ion and coin cell types. These include risk of fire or explosion, especially if exposed to high temperatures and repeated overcharging.

Meanwhile, research abounds worldwide, focused on increasing battery capacity, reducing battery size, making them flexible, and importantly, low cost. A variety of material combinations are under development and trial, including ceramic combined with lithium, lithium sulfur, carbon/graphene lithium, crumpled graphene paper, graphene-based paper, and many more.

Thin-film, solid-state and paper flexible batteries have been available for some years, but most tend to be low capacity and expensive. Solid-state lithium technology can deliver higher capacity, making it more useful for some applications, but it comes at the expense of size. See Figure 2 below for an example of a recent introduction.


Figure 2: The EFL700A39 paper-thin, solid-state lithium thin film rechargeable battery from STMicroelectronics has a capacity of 0.7 mAh with an operating voltage range of 3 to 4.2 V. It measures 25.7 x 25.7 mm. Charge time is 20 minutes at a constant voltage of 4.2 V.

Meanwhile, IDTechEx has observed an increasing effort into the development of a wider range of printed components, starting with RF antennas for tags, but now expanding to include sensors, memory and logic, as well as storage devices, such as batteries and supercapacitors. This leads the way to more highly integrated, miniaturized solutions, more suitable for IoT nodes and ultimately wearable applications.

Lithium free

A key advantage of silicon-based solid-state designs is that they can be manufactured cheaply and reliably on proven semiconductor processes. Further, they can be packaged as stand-alone devices, or integrated in die form with other circuitry. This is the route taken by Cymbet with its EnerChip ranges of lithium-free solid-state battery chips. The smallest, in die form, is the CBC005 with 5 µAh capacity, measures just 1.37 x 0.85 mm, and is 175 µm thick. It can be packaged with the company’s integrated power management circuitry and/or an ultra-low-power real-time clock. Battery capacities currently available are 5 μAh, 12 µAh and 50 µAh.

One of the company’s most recent introductions is the EnerChip RTC CBC34803-M5C, combining a real-time clock and a calendar optimized for low-power applications, with an integrated rechargeable solid-state back-up battery and all the power management functions. Although primarily designed to provide a low cost, small size (5 x 5 x 1.4 mm) back-up power solution, it illustrates the capabilities of the technology. Output voltage is 2.5 V and recharge time to 80% is just 15 minutes. Up to 100 hours back-up for the real-time clock is available per charge and more than 5,000 recharge cycles are possible.

An evaluation kit, CBC-EVAL-12-34803, is available, which features a USB Interface Board, CBC-TAB-34803, which plugs into a PC. The kit allows designers to explore the capabilities of the low-power real-time clock with I²C interface bus, power management features, such as power fail sensing, battery charging and discharge monitoring, and the solid-state battery itself that can provide up to 100 hours of back-up power to the real-time clock.


Figure 3: The Cymbet evaluation kit for the CBC-34803 integrated solid-state battery, real-time clock and power management circuitry plugs into the USB port of a PC.

A key advantage of Cymbet’s EnerChip battery technology is wire-free charging, normally achieved via energy harvesting, near field (NFC) induction or RF charging. Energy harvesting can be a useful technique for wearable devices, particularly using motion, piezoelectric or thermoelectric techniques. Fitness monitoring gadgets, for example, could be powered by body movement while in use, but then re-charged using solar, RF or magnetic induction techniques when not in use. Health monitoring devices in contact with the skin can exploit the difference in temperature between the body and the ambient air. The energy harvested can be used to power the sensors and circuitry directly, and/or to recharge the battery.

The critical design factors for wearable devices are to maximize the energy density of the energy storage device and minimize power consumption. Ultra-low power, tiny microprocessors are now readily available, including the Ambiq Micro used by Cymbet, together with extremely-low-power sensors and power management devices. An important design consideration for power efficiency is to ensure the design is only waking up to take sensor measurements at intervals appropriate for the application, and that the circuitry reverts to a low-power sleep mode in between times.

Cymbet’s White Paper provides a useful and interesting calculator for comparing the costs of incorporating a rechargeable or replaceable primary battery (providing the application can accommodate the bulk) and employing energy-harvesting technology. The devil is in the detail, which requires some analysis of manufacturing, operational and total product lifetime costs, including such factors as aging characteristics and end of life disposal procedures.

Finally, it outlines some techniques for optimizing energy-harvesting designs for wearable technology and wireless sensor nodes, and provides some design examples.

Conclusion

Battery size and form-factor are arguably the most significant challenges to overcome as wearable technology and the IoT movement expand rapidly. Solid-state rechargeable batteries that can be readily integrated with sensors and electronic circuitry can provide a solution. However, there will always be a trade-off in terms of battery capacity and size. Ultra-low power circuitry, smart power management and energy harvesting can make a major contribution to keeping power demands under control. Lithium-free solutions offer a more ecological approach, while wireless charging helps keep size to a minimum.

References:
  1. IDTechEx Market Report: Flexible, Printed and Thin Film Batteries, 2015 to 2025
  2. Cymbet Corporation: White Paper: Powering Wearable Technology and Internet of Everything Devices
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