(Continued from previous page that discussed ultracapacitors)
Many capacitors have an usual characteristic known as dielectric absorption, soakage, or memory effect. If you charge a capacitor for a while, then discharge it quickly to 0 V, and then leave it disconnected, the capacitor’s voltage magically increases.
Here are two newly-manufactured, different chemistry capacitors that have been charged to 2.5 V for three minutes. A toggle switch disconnects the capacitors from the voltage regulator and shorts the leads for about one second.
Voltage rebound due to soakage in capacitors.
This seems utterly impossible. Absent a power source, how can a capacitor magically regain voltage after being discharged to 0 V?
A clue is that the longer you charge the capacitor, the more energy reappears. Another clue is that the longer you short the capacitor, the less energy reappears.
It turns out that capacitors store energy in multiple forms. There’s the classic model of charges accumulating on the plates, but there are other storage forms, such as a chemical change. The plates can charge and discharge quickly. Chemical changes happen more slowly. It’s almost as though you have two capacitors connected together.
Capacitor schematic symbol showing leakage and absorption capacitance.
Think of the internal second capacitor as being a much smaller capacitance and having a high-value resistor in series. Thus, it can’t restore the full voltage of capacitor and it takes longer to charge and discharge.
After the main capacitance is discharged, the secondary capacitance slowly charges the main as much as it can. Eventually, due to leakage, both capacitors will reach 0 V.
So, the power is not magical nor is it coming out of thin air. The energy is supplied during charging and is returned after the capacitor plates are discharged.
Why do you care about soakage? If you make a circuit with a slow timer based on a capacitor, the secondary capacitance may affect your results. Professional electrical engineers that create sample-and-hold analog-to-digital converters need to compensate for this affect by choosing proper capacitor chemistries. Lastly, it would make a good bar bet, if there were drinking establishments dedicated to electrical engineers where fresh-out EE grads could be found.
I put a lot of work into measuring a wide variety of capacitors and to refining the test circuit to reduce the effect on the component being measured. Each time I discovered and corrected sources of electrical drain in my measurement circuit, I had to throw away my prior results and start again. In the end, it was very frustrating and unfulfilling to not be able to create an accurate capacitor self-discharge test apparatus.
However, I learned that the most significant source of power drain for small capacitances is unavoidably everything it is attached to, including the board, connectors, and flux residue. For ultracapacitors, a high-impendence source can successfully measure self-discharge -- and that discharge, along with a low maximum voltage, greatly reduces the viability to store a useful voltage level overnight.