Super Capacitor Charging/Discharging

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Discussion Overview

The discussion revolves around the design and performance of a phone charger utilizing super capacitors, specifically addressing issues related to charging speed, current output, and circuit configuration. Participants explore theoretical and practical aspects of capacitor behavior in charging applications.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant notes that while capacitors can output high current, this depends on the circuit's impedance, with low impedance allowing for higher current output.
  • Another participant emphasizes that the characteristics of capacitors, including equivalent series resistance (ESR), play a role in their performance, though there is uncertainty about whether all super capacitors have low ESR.
  • Concerns are raised about the charging circuit potentially exceeding the maximum voltage rating of the capacitors, which could lead to damage.
  • There is a discussion about the configuration of capacitors, with one participant initially misstating their connection type, leading to questions about the implications of series versus parallel connections on voltage and capacity.
  • Participants express uncertainty about how to ensure equal charging of capacitors in series and the effects of differing capacitance values on voltage distribution.
  • One participant points out that the voltage drop during discharge could affect the charging capability of the phone, questioning whether a voltage of 4.5V would be sufficient.
  • Another participant mentions that charging current varies and that a current above 600mA is needed for effective phone charging.

Areas of Agreement / Disagreement

Participants express differing views on the characteristics of super capacitors, particularly regarding ESR and their ability to deliver current. There is no consensus on the best approach to configure the capacitors for optimal performance, and several technical concerns remain unresolved.

Contextual Notes

Participants highlight limitations related to the assumptions about capacitor behavior in different circuit configurations, the potential for voltage exceeding safe levels, and the implications of capacitor tolerance on performance.

Who May Find This Useful

This discussion may be of interest to individuals designing power supply circuits, particularly those involving super capacitors, as well as those exploring the practical applications and limitations of capacitors in electronic devices.

Akmalidin
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Hello,
cap.jpg


I am designing a phone charger with super capacitor with capacitance C=100F. The max voltage of cap is 2.7 V. When I charge the capacitance and then charge my phone, it is charging really slowly with low current (around 700mA). However, as I read, capacitance should output high current. The other thing is, when I charge my capacitor while phone is connected to it, it is charging itself then charging my phone slowly. Please let me know how I can make my cap output more current to charge my phone. The schematics are below. I used both . Please share your experience and give advice.
 
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Akmalidin said:
... capacitance should output high current ...
No, not at all. A capacitor in an open circuit output no current at all. A cap in a high impedance circuit outputs very little current. A cap in a very low impedance circuit outputs high current (briefly).
 
Yes you are right. Supercap has a low ESR .
 
Akmalidin said:
Yes you are right. Supercap has a low ESR .
It has nothing to do with whether it is a "super" cap, it just has to do with the fundamental characteristics of caps.
 
Akmalidin said:
Yes you are right. Supercap has a low ESR .
I'm not sure all supercaps have low ESR. Check the datasheet to be sure...

https://en.wikipedia.org/wiki/Supercapacitor
The first supercapacitor with low internal resistance was developed in 1982 for military applications through the Pinnacle Research Institute (PRI), and were marketed under the brand name "PRI Ultracapacitor". In 1992, Maxwell Laboratories (later Maxwell Technologies) took over this development. Maxwell adopted the term Ultracapacitor from PRI and called them "Boost Caps"[11] to underline their use for power applications.
 
berkeman said:
I'm not sure all supercaps have low ESR.
Sure, but what I'm trying to get him to realize is that the ESR might well be a red herring in this case, because he doesn't seem to understand Ohm's Law and expects a high Farad cap to produce lots of current regardless of what it's hooked to.
 
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I believe most super caps have a relatively high ESR. The trend has been that the ESR is dropping, but from one cap to the next super caps do not typically have a very low ESR.
 
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When I said low ESR, i compared them to batteries so caps outputs higher current. Please give a feedback the circuit I am using to charge my supercap.
 
Ok, the circuit you are using to charge the cap will destroy it. There is nothing to prevent the voltage from exceeding 2.7 volts when being charged from a 5 volt source.
 
  • #10
Oh sorry, I forgot to mention that I am using two of caps connected in parallel. SO they form 5.4V together.
 
  • #11
Akmalidin said:
Oh sorry, I forgot to mention that I am using two of caps connected in parallel. SO they form 5.4V together.
No they don't.
 
  • #12
I meant in series connection.
 
  • #13
Probably best to draw it that way then. And do you have anything to guarantee that they will charge equally? What is the spec on the caps for tolerance? If their capacitances differ then one will charge to a higher voltage than the other.
 
  • #14
Akmalidin said:
I meant in series connection.
That's probably not a good idea. As ASN points out, there are symmetry issues with it, and the way you usually handle that is with resistors in parallel around the caps, which robs charge and power in the overall circuit. Do they not make supercaps with a rating > 5V?
 
  • #15
There are caps with 5 v but with lower capacity.
 
  • #16
Akmalidin said:
There are caps with 5 v but with lower capacity.
And what is the straightforward way to increase that capacity and keep the 5V (plus margin) rating?... :smile:
 
  • #17
I gues adding the caps in paralel.
 
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  • #18
Akmalidin said:
When I charge the capacitance and then charge my phone, it is charging really slowly
A separate issue to consider: when you power anything from a capacitor that was initially charged to 5V, after the capacitor has given up just 10% of its charge then its voltage will have dropped by 10%. So, I'm wondering is 4.5V still going to be any good for charging a phone?

Or maybe you have this solved ... ?
 
  • #19
4.5V is giving out about 800ma to charge but it is varying (going down). But I think when the current is over 600ma ,the phone can be charged
 
  • #20
An interesting thing to note is that you were planning on putting higher value caps in series. And you avoided the higher voltage caps because they had smaller capacitances. Do you know what happens to total capacitance when capacitors are placed in series?
 

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