As the solar cell (1”x1”) can deliver no more than 5mA/4V, I intend to do a very simple schematic: one Zener diode to clamp the voltage bellow 4V and one schotky diode to connect to the Vbat of the Xiao board.
Does anybody have tried such stuff, and do you have any comment or see any issue ?
SO it’s a great topic and becoming more mainstream , IMO.
For the Xiao, a couple things.
The Schottky idea is fine, but I would be careful about using only a Zener clamp as the “charger.” With a tiny solar panel, the panel is a high-impedance source, so a simple clamp tends to waste energy and lets the panel operate far from its best point.
The bigger concerns are startup current into the 55 F storage element, poor harvest efficiency in weak light, and brownout behavior around dawn/dusk.
For a quick experiment, diode + current limiting + clamp can work, but for a dependable node I would strongly consider a real energy-harvesting / PMIC solution or at least add current limiting and UVLO.
Also double-check which XIAO pin you feed, because XIAO boards have a charger/power-path IC on the battery input rather than a raw storage node.
Budget for darkness honestly.
Your instinct is right that 55 F is a lot of storage, but whether it covers “a few days” depends entirely on the usable voltage swing and regulator path, not just the farad number.
I have used AHCR’s (40F) in a project and they work really well…
They do require some “extra” energy to charge so provided your solar panel can deliver enough to charge your cap and power your device (averaged) your simple design may work (given 300uA average consumption, and 4V/5mA generation).
I added a low voltage cutoff (2.5V) to protect the cap and used the VBus (not VBat). Alternatively, you could go into Deep Sleep if the cap voltage got too low (say 2.6V).
concerns are startup current into the 55 F storage element, poor harvest efficiency in weak light, and brownout behavior around dawn/dusk.
PJ 