What are the best protective practices for solar cells and supercapacitors when they are used for small outdoor projects? Moreover, are there any way of widening the useable voltage output range of a small solar cell?

Consider the following scenario: two 2.7v supercapacitors in series for energy storage, powered by a small solar cell that outputs max 6v.

Firstly, there are a few protection features necessary for such circuit, chiefly, ensuring that the supercapacitors do not exceed their 2.7v rating. I found a fantastic video which tackles this exact problem here. Each solution outlined in the video (using either Zener diodes, voltage references, dedicated protection circuits) features a component placed parallel to each capacitor. As the capacitor's voltage approaches a set limit, the component's resistance essentially decreases, such that current bypasses the capacitor, and flows through the protection component instead. This solution works pretty great for the capacitors, as it naturally allows even unevenly charged capacitor pairs to eventually reach equilibrium when charged to the full voltage.

Unfortunately, as I understand, as the capacitors approach their fully charged state, a short circuit essentially begins to form across the charger, as the protection across both capacitors have a very low resistance in order to ensure no further current flows through capacitors. (please do let me know if that assumption is incorrect)

My question is, for the scenario I initially outlined, as the solar cell becomes limited to the 5.2v of the 2 capacitors in series, under sunny conditions where the cell may generate 6v, is this... damaging to the solar cell? Does this scenario also require protection for the solar cell?

Secondly, for a project that, for example, has a 3.3v device being powered from the capacitors, a voltage regulator may be used to step down the voltage. In this case, the regulator may have a dropout voltage of around 4.4v. This means, any power the solar cell generates under this voltage is essentially useless, as the voltage of the supercapacitors will never exceed this value, and the 3.3v device will not be powered.

Is this a necessary limitation of this small-scale design, or can some sort of boost converter circuit be used (either to boost the solar cell voltage, or to boost the supercapacitor voltage) to get a wider usable voltage range out of the solar cell? (Perhaps something simple like a joule thief?)

  • \$\begingroup\$ Apart from the supercaps, you’ve given us little to work with. First up - solar panel - how many Watts? The load - Volts/Amps. For a small (< 10W) solar panel, you could simply have your supercap circuit clamp the voltage and let the solar panel suck it up. But your clamp circuit would have to be able to dissipate the energy. Or more commonly you’d have a charge controller to disconnect the panel past a certain voltage. For a slightly better energy harvest you’d use a MPPT controller. Ultimately it comes down to a cost/benefit decision. Is the few % of MPPT worth it? Or use a larger panel? \$\endgroup\$
    – Kartman
    Nov 20, 2021 at 1:45
  • \$\begingroup\$ @Kartman The solar panel is quite small, maybe 100x70mm. The actual load for the project is a MCU dev board that will be under deep sleep mostly, waking perhaps every hour for a short, bursty load on the supercaps, then returning to sleep. Project does not need to be powered through the night, so the actual power requirements are quite low, hence the small solar panel, which should be able to slowly charge the caps over time, even in moderate sunlight. The only optimisation I would like is making voltages below 4.4v usable, which would allow the project to work under dimmer lighting conditions \$\endgroup\$
    – skillz21
    Nov 20, 2021 at 2:33
  • \$\begingroup\$ "Unfortunately, as I understand, as the capacitors approach their fully charged state, a short circuit essentially begins to form across the charger, " Why is that unfortunate? That's what a shunt regulator does. It doesn't harm the solar cells, nor the capacitors, nor could you do anything else with the excess power anyway. (OK you CAN use it as a heat source; for example by water cooling the shunt regulator; or you can simply rate the regulator to survive the full power of the panel, and let it get warm) \$\endgroup\$
    – user16324
    Nov 20, 2021 at 11:47
  • \$\begingroup\$ @user_1818839 Oh, I thought the solar panel's 6v output being clamped to 5.2v might potentially damage the cell (I don't have much experience with using solar cells). I'm happy to hear this shouldn't be an issue for the actual solar panel, but rather it's more a problem of dissipating the extra power. \$\endgroup\$
    – skillz21
    Nov 20, 2021 at 12:18
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    \$\begingroup\$ Just pick suitably rated zeners; and protect them from unwary fingers. They (the zeners!) don't mind getting a little toasty. \$\endgroup\$
    – user16324
    Nov 20, 2021 at 12:20


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