I have procured some supercaps.
No datasheet was provided. The only data I have is what is printed on the caps themselves: "
4.0F 5.5V cda®"
Since my DMMs don't measure capacitance in Farads, I setup the following circuit shown below. Two "
10Ω 1% 2W" (
20.2Ω and 20.3Ω measured) caps in series with the cap.
Note that I did not actually use a switch, I simply connected a banana plug. I did that to avoid extra variables (switch resistance and power rating, as well as power supply startup time).
That being said, with a large expected time period of 80+ seconds, I figured that using a stopwatch and monitoring my DMM would suffice.
The RC time constant is:
R*C = (20.3Ω)*(4) = 80.2 seconds
Which I take to mean that if I apply
5V as shown in the circuit and close SW1, the cap should reach
5V * 0.632 = 3.16V at
80.2 seconds. The current limit on the supply was set to 2A (more than enough).
The cap went from
3.16V in approximately
38 seconds, at which point I removed the cap from the circuit.
Solving for capacity:
C = (38 seconds)/(20.3Ω) = 1.87 F, only
47% of the
About a minute after being removed from the circuit, the voltage on the cap had stabilized at about
1.28V. Should I be using this value instead? That would suggest
6.43F, so I'm guessing "No".
I then tried the same test with another cap of the same specs... same result.
I next tried a discharge test, going from
1.64V. That should have taken
87 seconds, but instead took only
28 seconds, hinting at a capacity of only
However, by leaving the cap to discharge for
55 seconds showed
1.1V, suggesting a capacity of
1.82F. That's odd to me because it means that it's not following the predicted curve. And that would mean that I will end up calculating a different capacity depending on the time I record at. But that shouldn't be.
The following image is from hyperphysics.phy-astr.gsu.edu:
I'm wondering what margin of error I should expect from a test like this. Assuming my multimeter is calibrated beyond practically needed, and that recorded times are valid, is it possible that the capacitance is closer to