I hacked together a simple node with 3.3V Arduino and RF24 sender, just to measure the power consumption and see how long the node will be actually able to run on the energy stored in the three supecaps 0.047F. To my disappointment, it runs for about a minute, despite the fact the node is sleepping most of the time. There's something drawing much more current than I anticipated.
I need to do some further measurement tomorrow, and I need to think it over. There's an TPS77033 LDO that has relatively high quiescent current, but not THAT high. My previous measurements of Arduino (the Pro Mini with LDO and LEDs removed) showed I am somewhere around 20uA when the thing is sleeping, but the current being drawn with this setup looks more like miliamps, not just tens of microamps... strange.
I even disconnected the RF24, just to be sure it's not this thing, but that didn't help.
If I understand this right: the capacity is 0.15F, I am charging it to 4.2V and I draw current from it until it drops to 3.3V, that gives me 0.9V * 0.15F = 0.135 Coulombs. With current draw of let's say 1mA, I am able to operate for 0.135C / 0.001A = 135 seconds... hmmm...
That tells me two things:
- these supercaps won't be enough to power the sensor for the whole night, even when the average power consumption will be below 50uA as I expect. It will be enough for about one hour of operation, and I'll need something like 3F at least.
- The efficiency of this setup is terrible :) I really need to get back to the idea of using boost regulator to drain the supercaps till they drop.