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Pitfalls in charging a supercapacitor from a small solar panel

I’m designing a circuit which occasionally requires small amounts of power (it’s switched off most of the time). I want to use small solar panels to charge a supercapacitor, and the cap then serves as an energy reservoir in the absence of full sunlight.

I have already set up a basic circuit with a EDLC supercap (VINAtech, 100F, 3V), a small solar panel (3V, 270mA) and a 1N4001 diode. It seems to work fine, the supercap voltage appears to stabilise at around 2.85V with the panel pointed at the sun, full sunshine and the panels clean. Such ideal conditions will be rare though, the panel may be shaded most of the time. I know that an MPPT charger would be more efficient but I want this to stay as cheap and simple as possible.

simulate this circuit – Schematic created using CircuitLab

In this schematic, V1 is the solar panel, C1 is the SuperCap and U1 is a generic boost converter based on a CE8301A50T. I’m not sure about the final circuit yet, but there will certainly be a microcontroller (Atmega16A) and an HD44780-based LCD with LED backlight. SW1 will only be closed for a few minutes at a time (no more than 15 minutes), very few times a day (probably not at all for several days in a row).

I just want to know if there might be any pitfalls I might have missed. The circuit will operate outdoors, i.e. it will have to survive temperatures of -25°C (-13F) with no problems. Unfortunately, manufacturers don’t seem to test the behaviour of their supercaps at low temperatures (as far as leakage current or change in capacitance goes) so I’m not sure if there might be issues. I don’t think temperatures will rise above 50°C (122F) in the summer.

Any thoughts?


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