After reading Jean-Claude's initial information about supercap charging, I became interested in exploring the concept as well. I had done previous tests in which I'd charged a 1F supercapacitor for a few seconds then used it to power a jeenode for up to a minute or two. My conclusion was that the self-discharge rate was incredibly high, as I didn't realize they could falsely appear fully charged. JCW's insight prompted me to undertake a longer set of experiments.
- Node CHARLIE: A 16MHz JeeNode with AA power board powered by a ~4.5V (open circuit) 100mA (short circuit) solar panel.
- Node DELTA: An 8MHz custom design powered by a NCP1400 3.3V boost converter powered by a 2.3V (OC) 70mA (SC) solar panel.
- Both nodes use a 1F 5.5V supercapactor as their energy storage
- Temperature is provided as a reference as to when the panels are in the sun
- The sketch transmits a 13 byte payload packet every 5 seconds
For some reason CHARLIE, the AA board jeenode, always drops off when the supercap voltage hits around 2.0V. When I measure the +3.3V out, there is 3.3V. I can't figure out why it stops transmitting but it drains power pretty quickly at that point. I keep thinking there's something hooked up wrong, but really how complicated is it? Solar panel, diode, AA board's +/-.
Based on the curve in the graphs though, it doesn't look like it would make it through the night anyway. You can definitely see when it gets out of the sun the supercap voltage drops quickly to its "actual" voltage level, so they're not getting fully charged. I'm going to try upping the transmit delay to a minute to see where I end up for comparison with jcw's numbers.