Aluminum Electrolytic Capacitors

®3 Calculation Method of Discharge Time3-1 Approximating the Discharge Time of BasicConstant Current DischargeThe discharge time at the constant current of acapacitor can be calculated by the followingequation.t = (CΔV)/IWhere,t : Discharge time (sec.)C : Capacitor capacitance (F)ΔV : Working voltage range (V)I : Discharge current (A)ELECTRIC DOUBLELAYER CAPACITORSAs an example, we calculate the discharge time whena capacitor of the DB series 5.5V 1F is charged with5V and discharged to 3V at a constant current of 1mA. Since the working voltage range ΔV is 2Vfrom 5 3V, t = (1F2V)/0.001A from the aboveequation, and the discharge time can be calculatedas 2,000 seconds (about 33 minutes). Note thatthe actual discharge time may be different becausethis equation does not cover the effect of theself-discharge and the IR drop by internal resistancedescribed below.3-2 Effect of Self-discharge at MicrocurrentsWhen backup is made by discharge with a microcurrentbelow some µA especially for the memorybackup application and the like, the discharge timemust be determined while taking into account the selfdischargeas shown in Fig.4.The value closer to the actual discharge curve isobtained by adding the voltage drop through the selfdischargedetermined from the voltage retentioncharacteristic test to the discharge curve given bycalculation.Note that the value of self-discharge varies bythe charge time, charging current and an ambienttemperature.TECHNICAL NOTE3-3 Effect of IR Drop at Large CurrentsWhen a large Current discharge and a capacitor witha high internal resistance are used, the effect of IRdrop by the product of the internal resistance and thecurrent must be considered as shown in Fig.5.When a large current is required in a very short time,or a large instantaneous current flows at the start ofdischarge, the voltage drop indicated with ΔV1 counts.However, when the discharge continues as it is, thedischarge curve indicates in a manner showing a slowdiffusion and then keeps a constant straight line.We also make calculation including ΔV2 of theintersection extending from the initial discharge andthe discharge straight line section including thediffusion curve when indicating the DC internalresistance.VoltageEV2ChargeDischargeFig.5 Example of Discharge Curve involving IR DropDue to IR drop, the shape of the discharge curvevaries by the internal resistance and ambienttemperature for each series.V1Discharge curve given by calculationVoltageActual discharge curveTimeFig.4 Example of Discharge Curve involving Self-DischargeElectric Double Layer CapacitorsTECHNICAL NOTECAT.No.2010/2011E