WeatherShield + supercapacitor + tiny solar cell

There were some interesting discussions in the Low Power Techniques forum about solar power, running motes on super capacitors, and running motes without batteries. I had some tiny solar cells I got long ago from ebay, and I wondered if these could run a low power stock Moteino+WeatherShield node, without any assistance.

Update: the shop now has an offering for a 7.5F SuperCap and a 1W mini Solar Cell which can be used in a similar project as this shown here.

For the experiment I added this 7.5F low-ESR supercap to charge from the solar cell during the day, and keep the mote going at night. To avoid discharge I added a shottky diode from the cell to the cap. The solar cell is actually composed of three tiny cells wired in parallel, the combined capacity is around 0.7W. Here’s the “schematic”:

I initially charged the cap from a 5V source to get it going, and then I attached the cell to a basement blurred glass block window which hardly gets a ton of light:

Note that the shottky does drop 0.2V from the actual capacitor voltage. Even so this worked surprisingly well. Here is the new node in my Gateway UI:

The node transmits temp/hum/pressure/voltage data every minute. Below is a look at a few weeks of the voltage readings. The solar cell charges every day to about 4.25v (actual voltage is ~4.45V b/c of the diode) and discharges to just below 4V.

Quite encouraging, I was pretty sure this would work since the WeatherShield and Moteino sleep at under 7uA, I just wasn’t sure how these old small cells would behave with the supercap. I recon if this node would be placed outside and facing direct sunlight, the charge voltage and overnight dips would be even higher values.

This would also work with a LiPo battery instead of the super cap. Of course, if this was placed outside in the freezing cold, those cheap LiPos from china can die in the cold. But at just $6.50, the supercap is a cheap alternative, much safer and resilient to the cold, and shows how low power is not that complicated.