That’s a whopping 0.47 Farad, the size of a little coin cell, and as you can see, this unit is rated 5.5V (most supercaps are 2.7V, I suspect that this is actually made of two 1F 2.7V units placed in series).
The beauty of a supercap is that it’s like a little battery, but with fewer limitations – you can’t really overcharge it, for one, because it doesn’t turn electric energy into chemical energy. There is no conversion: put 5V on it, and it’ll draw current and gobble up electrons until it reaches 5V, then it’ll stop.
So for example for solar-powered ultra-low power nodes, this could be a pretty nice solution. Solar cell -> diode -> supercap -> JeeNode. Max charge rate while the sun shines, and then it simply stops once the supercap is full. Only thing is to not exceed that 5.5V maximum, for which supercaps are very sensitive.
But there’s a problem. Supercaps can have a substantial self-discharge rate. When I connected 5.3V to it, the voltage immediately jumped to 5.3V, but when I disconnected that cable, it also dropped back to around 4.7V in just a few seconds – a normal capacitor sure isn’t supposed to work that way!
As it turns out, supercaps tend to “learn” to keep charge better over time. The longer you expose them to a voltage, the lower their self-discharge rate becomes. The isolation barrier needs time to build up, apparently (I’ve had this supercap on the shelf for over a year). Which is great, because presumably these cells would be kept charged most of the time, with the node only depleting them slightly when sending out a packet. So ideally, all we really need is for the supercap to retain enough energy overnight.
It’s time to put these unique components to the test!
The first encouraging fact is that indeed, when fed 5.1V for a couple of minutes, the discharge no longer jumps as radically when disconnected. It now drops to 5.03V in a few seconds, but tends to hold its value after that. So it does indeed look like these supercaps can be “taught” to better retain their charge.
This test is going to take some time. First thing I’m going to do is to just keep the supercap charged to 5.1V (note that the power supply voltage calibration is pretty good – slightly less so for the low mA’s):
Let’s just leave it there to stabilize for about 24 hours. Stay tuned…