Now that I have this super-high-impedance multimeter, it’s time to revisit the venerable supercap:
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…
They do have the dielectric absorption effect, but I wonder if what you describe isn’t just high internal resistance (connecting for a few seconds didn’t allow enough time for charging to happen through the effective internal resistor.) Years ago I played with a 5V supercap with an internal resistance around 1k ohms.
Hm, good point. A few seconds might be insufficient for a full charge, but I did notice that the voltage starts off all right – it just drops really quickly. The mental image I now have of this process is that the cap is made up of lots of little caps, and some haven’t been “reached” yet when only charging very briefly. No idea whether that corresponds to reality. More curious behavior coming up, BTW.
I’m curious to see where these experiments will go. It would be great to power a node through a small solar panel during daytime and from a cap during night time. Because right now I’m using a solar phone charger to do the same job.
I second the point that these coin-cell super caps are actually designed with a high input impedance. This prevents the super cap from drawing too much current while it’s charging, since it’s meant to be used as a trickle-charged backup for a RTC, etc. Most people aren’t used to dealing with a discrete capacitor with a time constant in the range of minutes. It’d be a little tricky, but you could come up with a test arrangement to deduce this input impedance.
Just to add another layer of difficulty in the analysis, there are physically two caps in series here which cannot match exactly; usually leakage-balancing resistors are integrated in parallel with each cell to swamp out the variation of true leakage. So what is observed is the highly variable cap leakage in parallel with a fixed external leakage.
Why waste charge? Without some mechanism to fine tune this, over time, the higher leakage cell becomes ‘undercharged’ and the lower leakage cell becomes ‘overcharged’ – eventually exceeding its individual Vmax.
The better charge management circuits use individual cells/packaged with node taps so this effect is minimised – often by periodically generating an extra discharge path for the “best” cell to bring it into line with the others. Still losing charge, but less than the method above.
In micro-energy apps – seems like Nature conspires against you saving the electrons!
I really recommend the Panasonic guide to Electric Double Layer Capacitors which you can download here: http://industrial.panasonic.com/www-ctlg/tech/tABC0000_WW.html
The effect you are observing is not related to self discharge. As you already noticed it is instant and it reacts to current flow so this has to be the internal resistance of the capacitor. It is not there by design but rather as a technological necessity. The ultra-high capacitance is achieved in a porous material and getting all the charges from the electrolyte inside these pores takes quite a lot of energy (and time).