Supercapacitor stores energy from a harvester
Some wearable devices such as health monitoring tags are designed to be very thin and low power, An energy harvester theoretically allows the device to run indefinitely but energy harvesters like micro generators and RF harvesters cannot supply the peak power required for data transmission and are an intermittent energy source as the device may not always be moving or near a RF source.
Supercapacitors store ~1000 x more energy than a traditional capacitor with capacitance in the Farad range. A supercapacitor can store enough energy for the wearable electronics to operate for a long period when the harvestable energy source isn’t available and can be charged at average current from either an energy harvester of battery.
The energy harvester can charge the supercapacitor directly or supply a power management IC which charges the supercapacitor at very low power. The supercapacitor provides the peak power burst for data transmission as well as maintaining a steady supply voltage.
The following are some examples of supercapacitor and energy harvester:
Solar cells to power sensors (Charging a Supercapacitor from a Solar Cell Energy Harvester) reporting data wirelessly.
RF for rapid wireless charging of a supercapacitor (Wireless Charging) that then enables the supercapacitor to power the wearable to for example monitor an exercise session.
Vibration transducers (micro-generator or piezo electric), eg in a shoe, to power sensors (Charging a Supercapacitor with Vibration energy harvester)
Size matters CAP-XX prismatic supercapacitor
CAP-XX produces some of the thinnest supercapacitors in the world which allows CAP-XX supercapacitors to be elegantly integrated in even the thinnest wearable device. See CAP-XX’s latest range of DMF low ESR high power, DMT long life high temp or DMH ultra thin supercapacitors. Some CAP-XX supercaps, e.g. DMH may be slightly bent to conform to the contours of the wearable device.
Sizing your supercapacitor
Supercapacitors, which can deliver high power due to their low ESR, have high C to supply sufficient energy to support the data capture and transmission for its duration, have “unlimited” cycle life, and can be charged at very low current are the perfect power buffer between the energy harvester and sensor (Powering Pulse Loads).
A range of ICs are available to charge supercapacitors from energy harvesters. To select your IC consider the characteristics of your energy harvesting transducer, the minimum voltage the IC requires to start, if the IC has peak power tracking and the method it uses, min and max power levels and efficiency at those levels.
Inrush Current Limiting
A CAP-XX very low ESR supercapacitor will try to draw very high inrush current when initially charging from 0V. In many cases the battery’s internal impedance will be a sufficient current limit, but if inrush current limiting is required, then see Current Limiting for Supercapacitors.