# Cable voltage drop - capacitor reservoir

#### Cirkit

Oct 28, 2015
153
If you have a scenario where there is a cable (which cannot be shortened) between a battery and a circuit, is it better to have electrolytic capacitors or a super capacitor on the circuit input to help with transient demands from the circuit?

#### Delta Prime

Jul 29, 2020
1,807
If you have a scenario where there is a cable (which cannot be shortened) between a battery and a circuit, is it better to have electrolytic capacitors or a super capacitor
instantaneous current through a capacitor is ( I=C*du/dt).
If you have a scenario where there is a cable (which cannot be shortened) between a battery and a circuit
(Strike one).The capacitor acts as a short circuit when it is discharged .
You say" transient" that implies time. How long is this transient (du/dt)?
Your capacitor needed is based on energy.
It has to store enough energy so that during this mysterious transient you can provide the needed energy.

#### Bluejets

Oct 5, 2014
6,863
electrolytic capacitors or a super capacitor
Same same..... also depends on the power requirements and the circuit in question.
Will a wheel roll up hill...yes, in certain circumstances...... no in others.

#### crutschow

May 7, 2021
834
It depends upon the amount of capacitance needed to handle the current transient.

A super capacitor is just an electrolytic capacitor designed to have a very high capacitance.

#### kellys_eye

Jun 25, 2010
6,497
Rule of thumb is 1,000uF per amp of peak current draw.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,651
This is nonsense:
1. using a capacitor to supply peak current for transients is good standard practice.
2. please watch your language. The scenario in no way s*** and even if it would, there are gentler ways to express that.

A super capacitor is just an electrolytic capacitor designed to have a very high capacitance.
No, it s not. From Wikipedia (link):
Unlike ordinary capacitors, supercapacitors do not use the conventional solid dielectric, but rather, they use electrostatic double-layer capacitance and electrochemical pseudocapacitance,[3] both of which contribute to the total capacitance of the capacitor.

#### ramussons

Jun 10, 2014
462
No, it s not. From Wikipedia (link):
Wikipedia only mentions about the making of a super capacitor. It does not say that it is not a Electrolytic capacitor.

#### Delta Prime

Jul 29, 2020
1,807
This is nonsense
Capacitors store electrostatic energy...E=CV squared÷2.
current-voltage relationship is given by i(t) C×dv÷dt.
For a capacitor a instantaneous changing voltage dt=0 requires an infinite current; "Good Practice" in electrical engineering states, this cannot be achieved.
The change in voltage in a capacitor requires a finite time!
Recognition that the rate of energy produced must be equal to the sum of the rate of energy
dissipated and the rate of energy stored at all times...
(Principle of Energy Conservation).
The equation provided in my post describes the response of a capacitor formulated as linear
time-invariant differential equations with constant coefficients.
The classical method of treating transients, is the use of capacitors
and analytical solutions of linear differential equations.
Transient analysis based upon the laplace transform method is what
I use when analysing transients in a power system.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,651
For a capacitor a instantaneous changing voltage dt=0 requires an infinite current
Right.
But: in a real world circuit the transients are not Dirac pulses. A real world transient is a pulse with a limited rise or fall time in the nanosecond range and therefore dt > 0.
This is what decoupling capacitors are used for. ANd in the case of a long wire, a bigger decoupling capacitor can help to supply power for bigger transients.
"Good Practice" in electrical engineering states, this cannot be achieved.
So yes, using decoupling capacitors explicitly for the purpose stated is good practice.

Wikipedia only mentions about the making of a super capacitor. It does not say that it is not a Electrolytic capacitor.
One may have to search and read a bit further. An electrolytic capacitor stores energy in the form of opposing charges across a dielectric, typically aluminum oxide (tantalum or niobium based electrolytic capacitors are also well known and work by the same principle).
Supercapacitors store energy "via the physical adhesion of ions along the surface of active carbons" (from Nippon Chemicon who as a manufacturer should know).

@crutschow : Admittedly the same site also states that " the basic electricity storage mechanism and structural principles of both types are basically the same. " So in a very wide sense a supercapacitor may be regarded as a kind of electrolytic capacitor. Nevetheless I stand by my assessment that a supercapacitor is not "just an electrolytic capacitor designed to have a very high capacitance".There's considerably more to it than just winding more aluminum foil.

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