Capacitance of an ultracapacitor is a measure of how much electrical charge is required for a given change in voltage. The amount of energy a capacitor can hold is proportional to its capacitance thus it's an important parameter when evaluating a capacitor.
Capacitance is related to charge and change and voltage by the following equation.
By setting a fixed charge/discharge current, I, and measuring the change in voltage over time, we can accurately measure capacitance, C. It should be noted that capacitance is slightly dependent on voltage. Capacitance values are typically reported through measuring over the full operating voltage range. However, a restricted voltage range may be suitable for some applications.
The following test instructions are used to gather necessary measurement.
The plot below shows the corresponding voltage waveform.
Finally, the below equations are used to calculate the capacitance in the charge and discharge cycles. Capacitance is reported as the average of these two values.