These dark and cold winter days are not really effective for solar energy: the entire month of January will probably not generate more
power energy than two peak days in July!
Still, it’s a good baseline to try things with. And one of the experiments I haven’t given up on is making nodes run off solar power, using a supercap to hold the charge. Maybe they’ll not last through the night, but that’s fine – there are still various uses (especially outdoor) where being able to run during daytime with nodes that never need to have their batteries changed would be really nice.
In previous attempts, I’ve always immediately tried to power the actual node, but now I’d like to try something simpler: a solar cell, a supercap, and a resistor as load. Like this:
I’m using a tiny solar cell by Clare again, the CPC1824, with the following specs:
Not much, but then again, it’s a cell which is just the size of a fingernail. As SOIC-16 package, and with the specs of the available current next to it:
In fact, I’d like to try this out with 4 different resistive loads:
- one cell, 2x R = 20 MΩ = 0.2 µA current draw at full output
- one cell, 2x R = 2 MΩ = 2 µA current draw at full output
- one cell, 2x R = 200 KΩ = 20 µA current draw at full output
- two cells in paralel, 2x R = 200 KΩ = 20 µA current draw at full output
There may be a flaw in this approach, in that the leakage of the supercap could completely overshadow the current draw from the resistors. But my hope is that supercaps get better over time when kept charged. Hmmm… not sure it applies if they run down every night.
So the second part of the idea, is to alternate solar cell use and dumb charging – just to measure how that affects output voltage over time. One hour, DIO will be on, and put the supercap on about 2.7V, the other hour it’ll be off and the solar cell takes over. With a bit of luck, the output voltage changes might show a pattern, right?
I think it’s worth a try and have made a setup with 4x the above – more tomorrow…