When an ultracapacitor is charged to its voltage it will lose its voltage over time. This is known as a "leakage current". In a capacitor there are two electrodes (positive and negative) that are separated by an insulator or a dielectric. The purpose of the separator is to isolate the positive and negative from one another so there are no short circuits. The separator is not a perfect insulator though, which means that small currents will move from one electrode to another, or "leak".
When charging an ultracapacitor ions, or charged particles, are transferred from one electrode to another creating a strong magnetic field between the two. This magnetic field is how energy is stored (voltage) and when an ultracap is charge or discharged it is the transfer of ions between the two electrodes. The leakage current is caused by the influence of the electric field built up on the electrodes.
Industry standard to measure the leakage current is to charge the capacitor to its voltage, and then monitor the current supplied to keep the capacitor charged. Measuring the current needed after 72 hours is the capacitors leakage current value.
GRAPH OF 72 HOURS OF CELL AT MAX VOLTAGE
You can also measure the leakage current, or self-discharge, by charging the capacitor to maximum voltage, and then putting the cell into an open circuit stage, so there is no source or load on the capacitor. Then by monitoring how the voltage changes over time you can figure out the self-discharge rate of the capacitor.
GRAPH OF CELL GOING OPEN CIRCUIT WITH SELF DISCHARGE