Using the formulae for Coulombs to measure the capacitance of a SuperCapacitor.
https://en.wikipedia.org/wiki/Coulomb
https://en.wikipedia.org/wiki/Coulomb
By Julian
Youtuber, shed dweller, solar charge controller aficionado
Good morning all…
Youtuber, shed dweller, solar charge controller aficionado
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I am not good in electronics but I think it has some capacitance between 0 and 1V and 2.5 and 2.7V as well. This should be added to the calculated 60F.
i ADVISE YOU TO CHECK THE THEME NON LINEARITY OF ULTRA-CAPACITORS, YOUR EXPERIMENT WILL WORK, IF YOU ARE USING NORMAL CAPACITOR. HOWEVER WITH UC, THIS WONT WORK…
My "500F" caps were measured to be around 330F using a similar method. That's terrible even assuming a 20% tolerance. Do you recommend any sources for good quality caps?
Assuming this formula would work the same while charging the capacitor at 1 amp, then measure the time in seconds till voltage raised 1.5 volts.
You don't need to go from Vmax to zero to measure capacitance. even a capacitor tester can check a capacitor quickly by looking at this same voltage and current change over time. it does not max out the capacitor and drain to zero to do a test. it would take forever and drain the on board battery to do that. Julian did it correctly.
For supercap, the value C is not a constant versus V and versus Q. If they were, they would charge almost instantaneously under R = 0 in serial. Still, if C was constant, you could easily compute it from an oscilloscope since their charge/discharge in time will follow the exponential law with an RC circuit: 0.5 V = V (exp(-t/RC)), so RC = -t_1/2 * ln(0.5); knowing R will give you C by measuring t_1/2 which is the time required to get (to lose) half the constant voltage, Vcc, at the end of the group resistor-capacitor, time that you measure on the scope (quite easy on digital scope allowing to "track" the curve with a cursor: start anywhere on the curve and end when your end point is at 0.5 Vcc more than your starting point, the t_1/2 is displayed as the Delta t between these two points).
I think that for supercap, the announced C value is for Q=0, or a discharged cap. You can try RC = – t_1/10 * ln( 1/10 ), with t_1/10 the time required to reach Vcc/ 10, if it takes too much time for t_1/2.
Thanks Julian, I bought a bunch of 500F Green-Caps from the Great Ebay and I'll test their real capacity with the same technique
Discharge from 2.5v to 1v you get 60F, there is 1v to go to totally discharge it, if you continue to discharge the rest of the 1v, you might get Farad calculation closer to 100F.
1 Columb is 1 Amp sec at 1 Volt, or 1 Wattsecond.
So when discharging 2,5 volts at 1 amp You are draining 2,5 Wattseconds. That is why you observe a discrepancy. You have to count wattseconds in order to count columbs.
One of the things we've found at Loughborough University is that the capacity depends upon how long you charge for. I.e. the state of charge isn't purely based upon the measured voltage. Similar to old lead acid batteries. The reverse of this effect is if you charge the cap, short it for a second to take it to 0V. Then use a high impedance multimeter to measure the OCV, it will start increasing (recharging itself!)
100F is from 2,7v to 0v
I love these analog stopwatches. I once measured the length of a fart with one.
After constant charging of 2.7 and 2.5 volt type DL caps 5 or more in parallel I found leakage rate of under .001 amp.. the more in parallel the more the leak current. I don't think it's too bad compared to a super low impedance OSCAP alum.electrolytic @.001F
Julian brother, I think when above one volt 4 times the power each time you double the cap voltage. 2v times 100 farads power used down to 1v gives you more power compared to1v to 0, see?
I'm just an amateur here, but wouldn't it make more sens to mesure from completely full to completely empty ?
I don't know if this analogy is correct, but if I want to know how much water a bottle can contain, I'll mesure all the water I can put in it or all the water I take out of it, if it was full.
So many people commenting are confusing capacitance with capacity, and concluding that Julian's method was incorrect. They're different things, and Julian was correctly measuring capacitance (measured in Farads), NOT capacity. To better understand the concepts, a useful analogy for a capacitor is a cylindrical container holding water, with a tube attached to a hole in the base. In the analogy, the water is the equivalent of charge, the water level is the equivalent of voltage across the capacitor, the height of the cylinder the equivalent of the maximum rated voltage. The water flow rate through the tube is the equivalent of the current into/out of the capacitor. Exactly like a capacitor, if you have a constant flow of water through the tube into the cylinder (equivalent to a constant current charging the capacitor), then the water level (voltage) increases linearly. The equivalent of capacitance in this analogy is the cross-sectional area of the cylinder, NOT the total water capacity. As you can see, the cross-sectional area determines the rate at which the water height increases for a given input water flow rate, completely independent of the maximum water capacity. This is analogous with the capacitance of a capacitor determining the rate at which the voltage across the capacitor increases, for a given current, again unrelated to the maximum capacity. A battery on the other hand is not analogous to a capacitor, and there's nothing equivalent to capacitance in a battery.