What's all this supercapacitor stuff anyhow?

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Discussion Overview

The discussion revolves around supercapacitors, focusing on their types, charging and discharging characteristics, and potential applications in low power DC storage systems. Participants explore the nuances of Equivalent Series Resistance (ESR), the feasibility of custom designs, and the search for advanced materials or technologies that could enhance performance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks expert guidance on selecting the best supercapacitor for a specific application, expressing interest in exotic varieties and custom designs.
  • Another participant questions the initial poster about their prior research and suggests narrowing down their questions based on that information.
  • Concerns are raised about how ESR affects charging and discharging capabilities, with one participant noting the importance of understanding leakage and charge retention.
  • There is speculation about whether materials exist that allow for faster charging than discharging, with uncertainty expressed about the feasibility of such technologies.
  • One participant provides a practical example of charging and discharging times for a specific supercapacitor, illustrating the challenges faced in achieving faster charge rates.
  • Another participant suggests that a lower ESR typically leads to faster charging and discharging, but emphasizes the need for more specific application details.
  • Discussion includes inquiries about the relationship between ESR and DC applications, with confusion expressed regarding the specification of ESR for components not intended for AC use.
  • One participant expresses a desire for information on cutting-edge supercapacitor technologies that may offer lower ESR and better performance.
  • Another participant raises the possibility of using higher current pulses for faster charging, questioning if this could be a viable approach.

Areas of Agreement / Disagreement

Participants express various viewpoints on the characteristics and applications of supercapacitors, with no consensus reached on the best approaches or technologies. There is ongoing debate about the implications of ESR and the potential for advanced materials.

Contextual Notes

Participants mention limitations in their understanding of supercapacitor technologies and the need for more detailed specifications to make informed decisions. There are unresolved questions about the performance of different materials and the practical implications of ESR in specific applications.

  • #31
I calculated that your source should charge a 3.3F cap from 0V to 3V in about 160 seconds ignoring losses. In the charged state it would hold about 14 Joules of energy.

If your load works from 3V down to 0.9 V the capacitor could deliver 60mA for about 115 seconds.

Useful theory...
Q=CV
dQ/dt=CdV/dt
dQ/DT=I
So...
I=CdV/dt

You can rearrange that last equation to calculate a lot of things such as the charge or discharge time given the current, capacitance and allowed change in voltage.
 
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  • #32
dnyberg2 said:
That's the rub. I can generate up to about six volts at 1 to 4 mA. Takes a long time to charge a supercap like that. I can charge a very large STD cap much quicker but it drains out of juice in seconds at 60 mA and that won't work for this remote sensor application.

Ah I used your earlier figures or perhaps missunderstand post #14.
 
  • #33
Then somethings wrong somewhere. With a load of 60mA I get about 4 minutes of power down to about 1.5 volts. With a charge current of 10mA, it takes 24 minutes to charge the cap back to 3 volts...
 
  • #34
I find it interesting that the low current charger I have can be six volts @ no load but the that supercap pulls that down until it gets up near full voltage. Then it seems that the "load" of the cap itself can hold the low current CHG CKT down around 3 volts. I was surprised to see that behavior but I've never really worked so closely with supercaps before.
 
  • #35
I'm going to to have to do a side by side experiment. CW DC at as much CHG current as I can get versus storing small amounts of it faster in a CKT that bursts a higher current pulse into the supercap...
 
  • #36
dnyberg2 said:
Then somethings wrong somewhere. With a load of 60mA I get about 4 minutes of power down to about 1.5 volts. With a charge current of 10mA, it takes 24 minutes to charge the cap back to 3 volts...
Could be the capacitance of the actual capacitor is much greater than specified. Some have a wide tolerance of -0% to +80%.
 
  • #37
Oh yea, I see that now. Crazy wide tolerance. I wonder if that moves around very much over the life of the part?
 
  • #38
dnyberg2 said:
Then somethings wrong somewhere. With a load of 60mA I get about 4 minutes of power down to about 1.5 volts. With a charge current of 10mA, it takes 24 minutes to charge the cap back to 3 volts...
Both charge and discharge figures suggest the actual capacitance is 9.6F.
 
  • #39
Thats a MUCH bigger # than the 3.3F on the wrapper!
 
  • #40
I wonder why the tolerance band is so crappy for caps unless you spend $100 a piece for them?
 
  • #41
That brings up another important question. Is there such a thing as an accurate capacitance meter that can measure tens of farads? If I were to market this thing, I'd need a way to quality control the actual capacitor value.
 
  • #42
dnyberg2 said:
I find it interesting that the low current charger I have can be six volts @ no load but the that supercap pulls that down until it gets up near full voltage. Then it seems that the "load" of the cap itself can hold the low current CHG CKT down around 3 volts. I was surprised to see that behavior but I've never really worked so closely with supercaps before.
C=eA/d where d is the plate separation. So for max C you want min d but small d reduces the breakdown voltage. I think it's more complicated with supercaps but my guess is that over their rated voltage they are either breaking down or the leakage is going way up dragging down the source voltage.
 

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