Two main factors affect the voltage drop of all capacitors, including EDLC's. Ohmic voltage drop and capacitance related voltage drop.

Omhic voltage drop - the capacitor has an internal resistance, known as ESR (Equivalent Series Resistance). As current flows through the capacitor, there is an instantaneous voltage drop, in conformity with Ohm’s law. It diminishes when current is no longer being drawn.

Capacitance related voltage drop - the voltage of the capacitor is directly proportional to the charge accumulated in the capacitor. During current discharge, the consumption of capacitance (current emitting from the capacitor) causes a linear voltage decrease in the capacitor. When the current is stopped, the voltage of the capacitor indicates the charge left in the capacitor.

The actual working voltage window of an EDLC is the combination of the Ohmic related voltage drop and the capacitance related voltage drop under drain conditions as described in the drawing.

Like an EDLC, a battery undergoes a voltage drop under drain conditions .Because of many physical and chemical constraints, the battery with its limited power often cannot supply the power required while still retaining its open circuit voltage. The larger the voltage drops of the battery, the larger the load on the battery. If the battery must supply high power at short pulse widths, the voltage drop may be too great to supply the power and voltage required by the end product (cutoff voltage), and the large load may decrease the useful energy stored in the battery, harming it and shortening its work life. This problem may be resolved by connecting the battery in parallel to an EDLC, to achieve a voltage damping effect. The decreased voltage drop of the battery will lead to better energy management and superior energy density in the battery. Power will be produced by both the EDLC and the battery, each supplying power inversely to its own ESR.

Batteries become inefficient at lower temperatures, as their capacitance decreases from the slowed kinetics of the chemical reaction in the battery, which increases its internal resistance. The voltage drop of the battery can be reduced greatly by coupling of the battery and the EDLC.