AC impedance measurements have been taken for five different double-layer capacitors (DLCs) operating at different DC bias levels. Using these measurements, circuit parameters for different DLC equivalent circuits have been determined. The effect of DC bias on the circuit parameters has been investigated. Variations in manufacturing were also examined. Equivalent circuit parameters were calculated for a small group of 50F DLCs and then compared. The equivalent circuit parameters varied about 10% over the sample set.