Energy Harvesting for Wearable Devices

Having to remember to recharge a wearable device is still a major irritation for consumers. A wearable that fails to operate for as long as expected between charges is as good as useless.
Yet a previous generation of wearable devices, the wrist watch, lasted for months before their batteries needed replacement. Indeed, some watches never needed a battery: harvesting their energy requirements from their wearer’s body motion.

The idea
Self-winding watches, patented by John Harwood, have been available since 1928. The early mechanical self-winding watches were powered by kinetic motion; the swinging of the wearer’s arm would move a weight winding the main spring. In 1977, Seiko introduced the all-digital solar powered watch using light to power time keeping and displaying mechanisms.

Energy for nothing
Watches consume very little power (0.001 milliwatts) compared to a basic wearable which would require at least 500 times more (0.5 milliwatts).

Power games: demand V. supply
The graph shows the power available from four different sources, including a typical wearable-device battery with different periods between charging. Against these sources, the power consumption of a typical wearable with different data update rates is displayed. In this example, the data packet rate is used as a proxy for sensing and transmission frequency.

Batteries, small enough to fit in a wearable device, are capable of storing enough power to transmit around 45 Bluetooth Low Energy (BLE) data packets every minute to a phone and need recharging every 10-14 days. The data rate can be doubled at the cost of halving the battery life.

Thermal energy harvesting, using body heat, is impractical due to the limited surface area and temperature differential. Harvesting energy from kinetic motion allows 7 BLE updates a minute. This limits the frequency of data capture making kinetic energy harvesting viable for monitoring slowly changing conditions. For example: for monitoring body temperature, blood pressure or cholesterol over the course of a day.

Future
In the short term, battery technology is unlikely to advance sufficiently fast for wearable devices to have significantly longer periods between charging for high-demand use-cases, such as heart-rate monitoring. However, as BLE technology matures more quickly (Moore’s Law), the microprocessors become more efficient and the power requirements reduce. This could mean that in the near future energy harvesting will become an increasingly viable option for powering wearable devices. And as such, we can expect Energy Harvesting to appear on Technology Roadmaps for wearable devices.

Rob Karpinski - Project Engineer at Plextek Consulting

Rob Karpinski – Project Engineer at Plextek Consulting

About the author
Rob Karpinski is a Project Engineer at Plextek where he specialises in low-power systems design for Medical and Defence applications.
Rob holds a BEng MIET in Electronic Engineering from the University of Surrey and Westminster

About Plextek
Plextek is an electronic systems specialist: designing and implementing novel technology solutions that enable our clients to realise the full potential of their future goals. For more than 25 years our team of consultants, engineers and project managers has seen our clients’ opportunities through to fruition, giving their businesses the leading-edge in their sector.
Visit us at www.plextek.com/medical