Is My Super Capacitor Power Bank Circuit Safe?

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Discussion Overview

The discussion revolves around the safety and functionality of a power bank circuit designed using supercapacitors. Participants explore concerns regarding voltage ratings, charging mechanisms, and the use of balancing techniques in circuits with large capacitors. The scope includes theoretical considerations, practical applications, and troubleshooting of the circuit.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related
  • Experimental/applied

Main Points Raised

  • One participant expresses concern about the safety of using a 2.7V, 600F capacitor in a power bank circuit, questioning whether the circuit could apply a voltage greater than the capacitor's rating.
  • Another participant suggests that the low voltage and current capabilities of the circuit may not pose significant safety risks, but questions remain about the implications of exceeding the voltage rating.
  • There is a discussion about the necessity of balancing resistors when using large capacitors in series, with some participants noting uncertainty about the effectiveness of this technique for supercapacitors.
  • Participants mention the concept of passive and active balancing for supercapacitors, referencing external resources for further information.
  • One participant shares their experience with a charging board that did not function as expected, indicating potential issues with the circuit design or components used.
  • Concerns are raised about the need for initial charge supply to supercapacitors before a charger can operate effectively.
  • Another participant suggests using a resistor in series with the power supply output to limit charging current, emphasizing the importance of resistor power ratings.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement regarding the safety and functionality of the circuit. While some believe the circuit could work without balancing resistors, others highlight the need for caution and further investigation into the design and components used.

Contextual Notes

Participants note limitations in understanding due to language barriers with external resources and the potential for miscommunication regarding circuit specifications and safety measures.

Who May Find This Useful

This discussion may be useful for individuals interested in designing power bank circuits, particularly those using supercapacitors, as well as those troubleshooting similar projects.

emtae55
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TL;DR
I'll make a power bank with capacitors and I made circuit of it. But I'm worrying about whether the circuit is safe, because it's dangerous to use capacitor.
I'll make a power bank with capacitors and I made circuit of it. But I'm worrying about whether the circuit is safe, because it's dangerous to use capacitor.
So, can you check the circuit i made??

pic2.png

My capacitor is 2.7V, 600F and the power bank circuit has "Charging current : 1A maximum, output current : 2.1A(5V) maximum, ouput voltage : 5V DC","http://www.11st.co.kr/product/Selle...%DB_PC_S_%BC%EE%C7%CE&utm_medium=%B0%CB%BB%F6"
I wonder if you understand this site because it's korean :(
Thanks. :)
 
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Web site in Korean is useless here and I have no idea what it is that you think is unsafe. You seem to be working with very low voltage and very low current capability. What do you think is unsafe?
 
phinds said:
Web site in Korean is useless here and I have no idea what it is that you think is unsafe. You seem to be working with very low voltage and very low current capability. What do you think is unsafe?
I thought whether the power bank circuit gives voltage to the capacitor that greater than 2.7V. Isn't it?
 
emtae55 said:
I thought whether the power bank circuit gives voltage to the capacitor that greater than 2.7V. Isn't it?
Well, yes, if you are using a capacitor that is rated too low for the voltage you are using that could make the capacitor have dialectic breakdown. Why would you do that?
 
emtae55 said:
I'll make a power bank with capacitors and I made circuit of it. But I'm worrying about whether the circuit is safe, because it's dangerous to use capacitor.
I'm not familiar with placing large value supercaps in series, but for more normal size capacitors in series, we generally would put large resistance value resistors in parallel with each capacitor, to help to equalize the voltage on each. But with such large value capacitors (600F !), I'm not sure that technique works...
 
DOH_small.jpg


I clearly was not paying attention. Thought the caps were in parallel. @emtae55 you should be OK since each cap only gets 1/4th of the voltage, so about 1.25V per cap. Still, you should check equalization, as @berkeman pointed out.
 
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Although I have not worked with super caps we do need to keep in mind what a capacitor is able to do. And that is accumulate a large quantity of charge from a harmless low current source. It may take many hours to charge a super cap or bank of super caps but by definition a capacitor can dump it's charge very quickly. This will depend on equivalent series resistance (ESR) but when it comes to safety I would assume the capacitor bank can dump its charge with NO ESR.
 
emtae55 said:
I wonder if you understand this site because it's korean :(

If the translate got it right then this thing is for Li-Ion batteries. Current seems to be OK, but I'm not sure if it's for single or for double cell?
Quite wasteful to use that suparcap bank this way, but it's expected to be safe.

Though I would put a small fuse in series somewhere.

Ps.: wait, your circuit is actually four supercap in series :woot: Are you sure about that? It's safe up to 10.8V or so, but ... erm...
 
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  • #10
We're getting a number of similar questions recently. There may be some articles driving it. I imagine the application is mobile recharging of batteries for drones, scooters, hoverboards and the like. The advantage would be that the mobile charger could itself be recharged in a very short time from AC mains.

If any of our EE members know of a good tutorial article on mobile power banks, that would be welcome. As always, if you would like to write an Insights article on the topic that too would be welcome. What I would like to see most is a schematic for a commercial mobile power bank using supercaps.
 
  • #11
berkeman said:
I'm not familiar with placing large value supercaps in series, but for more normal size capacitors in series, we generally would put large resistance value resistors in parallel with each capacitor, to help to equalize the voltage on each. But with such large value capacitors (600F !), I'm not sure that technique works...
Passive balancing technique with resistors or ZDs is widely used and protection circuits are similar to circuits for series connection of Li-ion batteries . A colleague has written a paper with simuls included:
https://bib.irb.hr/datoteka/942041....balancing_using_resistors_T.Bari_pp_15-22.pdf
 
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  • #12
Um, guys, I tried to make a batteries, but it doesn't work. I used DMM but it displayed 0V when i measured the charging part.(The charging board's 'B+' and 'B-') I think it is because of the board, so i want to find another one.
The board's name is '864-KC v1.0'. Google it! and the circuit was like this. Thanks
2.png
 
  • #13
Any half-decent Li-charger board would need a bunch of different kind of protection circuits: likely against charging non-healthy (under-depleted, 0V) cells too.

I think you have to supply the initial charge of supercaps yourself, before the charger can kick in.
 
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  • #14
Rive said:
Any half-decent Li-charger board would need a bunch of different kind of protection circuits: likely against charging non-healthy (under-depleted, 0V) cells too.

I think you have to supply the initial charge of supercaps yourself, before the charger can kick in.
I tried it, but it didn't work. I charged the caps and tried to charge with the board, but i couldn't charge them
 
  • #15
emtae55 said:
I tried it, but it didn't work. I charged the caps and tried to charge with the board, but i couldn't charge them
83566975_165308411466635_161139996055568384_n.jpg
83617086_576837356200714_6872001261906952192_n.jpg
83094600_3441058965965106_5784204767316148224_n.jpg
 
  • #16
I'm in a project that make a powerbank with supercapacitor. I made it but it didn't work. Can you confirm it?? My circuit is here. The green one is a board (charging board, 864-kc v1.0). It gives 4.2V DC to capacitor.
2.png
83094600_3441058965965106_5784204767316148224_n.jpg
 
  • #17
emtae55 said:
Summary:: I'll make a power bank with capacitors and I made circuit of it. But I'm worrying about whether the circuit is safe, because it's dangerous to use capacitor.

I'll make a power bank with capacitors and I made circuit of it. But I'm worrying about whether the circuit is safe, because it's dangerous to use capacitor.
So, can you check the circuit i made??

View attachment 255083
My capacitor is 2.7V, 600F and the power bank circuit has "Charging current : 1A maximum, output current : 2.1A(5V) maximum, ouput voltage : 5V DC","http://www.11st.co.kr/product/Selle...%DB_PC_S_%BC%EE%C7%CE&utm_medium=%B0%CB%BB%F6"
I wonder if you understand this site because it's korean :(
Thanks. :)
So far, seems the circuit would work without balancing resistors. Super-capacitors typically have quite uniform Rseries and capacitance to connect them in series without balancing. Take care to equalize temperatures though (make a solid common heat sink) .
 
  • #18
The link you posted is in a language many of us can not read. Can you post a link for the English version?

The photo you show is for a USB power supply that will turn off if more than 1A output is needed (some will work to 2A).

Put a resistor in series with the power supply output for charging. A resistor value of 5 Ohms or more will limit the current to less than 1A. The power rating of the resistor must be 7 Watts or higher, otherwise it will get too hot and burn up.

If the power supply will really supply 2A, then the resistor can be 2.5 Ohms and 15 Watts or higher.

With a 5 Ohm resistor, it will take about 1 Hour to charge the capacitors to 2/3 of full. A full charge will take about 5 Hours. If you want a faster charge you will have to use a higher current supply and a lower value (or no) resistor. To charge in 10 minutes needs a 30A supply. These numbers are not exact but are probably within a factor of 2. (that means it might take twice as long or half as long)

Cheers,
Tom

(p.s. for those that are wondering about the above estimates, the charge time was estimated using the RC time constant. A Constant Current supply would speed things up but those cost more and are harder to find.)