Hi again! I haven’t pulled the trigger on one specific model yet but I was initially looking at Li-Ion supercaps by Vinatech. Unfortunately they aren’t rated for more than 3.8V so not ideal for my application. I found some interesting alternatives though, the SCM series from Kyocera AVX or the KVR series from Eaton seem more appropriate. The goal would be to provide a sustained charge to power my Xiao MCU and my heart rate sensor, considering that there should pretty much always be energy coming from the harvesters (an array of TEG cells and - since I couldn’t get my hands on a Kinetron kinetic energy harvester - a small solar panel, both complete with the appropriate voltage boost circuitry).