I have a big 500 F 2.7 V capacitor, and a module of six 120 F 2.7 V capacitors. If I calculated the capacitance correctly, the module has 20F total capacity.
If I calculate the energy stored in them by E = 0.5 * C * V^2, I get:
- For the module: E = 0.5 * 20 * 16.2^2 = 2624 J
- For the big cap: E = 0.5 * 500 * 2.7^2 = 1822 J
So far, so good.
I bought a battery tester device and charged the capacitors to 2.7 V and 15 V, respectively (my DC source only goes to 15 V), and I've connected them like a battery to the tester device. I set 0.5 V as the lower cutoff voltage. The results I got are:
- The module yielded about 58 mAh.
- The big cap yielded about 220 mAh.
These results are in the ballbark of what an "Ah equivalence" equation, Ah = Farads * DeltaVolts / 3600 gives, but both the measurements and this equation are in conflict with the capacitor energy equation, which shows the energy stored in the module should be greater than in the big cap. The battery tester puts the current through a (constant) resistor, so it should be fairly simple as far as calculations go.
So my question is: how to reconcile the two?