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Super capacitor discharge time calculator
This calculator determines timekeeping operation using a super capacitor (supercap) based upon starting and ending capacitor voltages, discharge current, and capacitor size.

Formulas used:
Bt(seconds) = [C(Vcapmax - Vcapmin)/Imax]  This formula is valid for constant current only.
Bt(seconds) = -log(Vcapmin/Vcapmax)(RC) = t  This formula is valid for linear current only (simple resistive load).
Where:
Imax is the discharge current in amperes (A)
Vcapmin is the ending voltage in volts (V)
Vcapmax is the initial voltage in volts (V)
R is the equivalent resistive load in ohms(Ω) based on : R=Vcapmax/Imax
C is the capacitor value in farads 1F=1000 000μF=1000 000 000nF=1000 000 000 000pF
t is the time in seconds(s)
    Operating time with SuperCap
Vcapmax: V  
Vcapmin: V  
Capacitor Size: Farads  
Capacitor ESR: Ohms  
Imax uA seconds (constant current discharge)
R (calculated) Ohms seconds (resistor load discharge)
Graph: Capacitor voltage vs time
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More about this calculation

  • Vcapmax is the VCC maximum value that the capacitor is charged to.
  • Vcapmin is the minimum operating voltage you can tolerate before your circuit or component, which is backed up by the capacitor, stops working.
  • Imax is the maximum current that your circuit will discharge the capacitor. This can be a constant current or the initial linear current at Vcapmax.The Imax and Vcap values are used to calculate the equivalent resistance of the circuit, which is used in the equation to calculate the backup time.

RC Capacitor discgarge circuit
Figure 1.

From basic electronics, the formula to determine the voltage across a capacitor at any given time (for the discharge circuit in Figure 1) is: V(t) = E(e-t/RC)

Rearranging this formula for time gives us: t = - log(V/E)(RC)
Where:
V is the ending voltage in volts (V)
E is the initial voltage in volts (V)
R is the resistive load in ohms(Ω)
C is the capacitor value in farads 1F=1000 000μF=1000 000 000nF=1000 000 000 000pF
t is the time in seconds
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More about super capacitors
A supercapacitor, supercondenser, pseudocapacitor, electrochemical double layer capacitor (EDLC), or ultracapacitor, is an electrochemical capacitor with relatively high energy density, typically on the order of thousands of times greater than an electrolytic capacitor. For instance, a typical D-cell sized electrolytic capacitor may have capacitance in the range of tens of millifarads. The same size electric double-layer capacitor might reach several farads, an improvement of two orders of magnitude. Supercapacitors usually yield a lower working voltage in the range 2,5 - 20V.
As of 2010 larger double-layer capacitors have capacities up to 5,000 farads.[1] Also in 2010, the highest available supercapacitor energy density is 30 Wh/kg,[2] lower than rapid-charging lithium-titanate batteries.
EDLCs have a variety of commercial applications, notably in "energy smoothing" and momentary-load devices. They have applications as energy-storage devices used in vehicles, and for smaller applications like home solar energy systems where extremely fast charging is a valuable feature. Super capacitors are widely used as a backup power source for realtime clock circuits and memory in microcontroller applications for years. More information in Wikipedia here.

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