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Using capacitors instead of batteries


Kerrowman
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Hi there,

I would like to know if I can use a large capacitor to store energy on the way that I currently do with an 18Ah lithium battery?

 
My understanding is that with the minimal internal resistance of a capacitor, they can discharge their energy very fast even though the energy density is not as big as that of a battery.
 
With this natural tendency for a fast discharge then how could one regulate the output current to a steady dependable rate without inserting some resistance to increase the RC value very significantly. Also there will surely be energy wasted as heat through any such resistance.
 
So to be more specific to my application, I am currently storing energy from HV transients in an 18Ah LiFePO4 battery and then later using that energy to run a small load.
 
From past experience I know that my HV transients can quickly charge up a bank of capacitors (50mF) but would it be feasible to use that energy, with additional energy from a PSU for example, to run the circuit that is producing the transients in the first place? Is that an option or are there too many technical hurdles, due to the nature of capacitors, to make that efficient and workable?
 
I also expect such large value capacitors (1-2F) would be very expensive.
 
Thanks
 
 
 
 

 

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Perhaps one could use a dc to dc converter without loading the capacitor, thus not discharging it quickly? They made various types for various voltage combinations. See for example:  https://www.amazon.com/Aceirmc-Current-Converter-Adjustable-Regulator/dp/B082XQC2DS among many.

This subject comes up often on the internet; I see a Youtube video for replacing the battery in an automobile with capacitors; I  need to look at that one.

If that interest you I have a 12v to 5v converter I could try.

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Hi Harry,

Thanks for your thoughts. I think it’s only fair that I give you some context for my query.
 
For the last few years I have been researching and testing a hypothesis regarding energy harvesting using inductively generated HV transients. Here pulses are delivered to a battery and certain energy gain effects have been observed.
 
If you are interested to read an internal report about this work then I have sent you a link via a pm. I will be writing a full scientific paper in the new year after I have completed the power tests. The capacitor issue is a way to add further validity to my findings.
 
In order to observe the process independently of battery chemistry, it is proposed to replace the batteries with capacitors. I know that capacitors charge up very well with the pulses as they formed part of a ‘cap dump circuit’ that was integrated into the system as part of the testing to compare dV/dt effects with dI/dt ones.
 
The question is, can a capacitor be used in the same way as a battery, which is in effect a big capacitor, and controlled in a similar way to a one?
 
Your suggestion of the DC-DC converter (either Buck or Boost?) sounds an interesting option and worth exploring but what would be a suitable capacitance to replace an 18Ah Lithium Phosphate battery and a what sort of cost?
 
As an aside I note you enjoy woodwork. Besides photography I also do wood turning that keeps me very grounded.
 
I look forward to your thoughts.
 
Regards
 
Julian

 

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Hi Harry,

I hadn't realised that super capacitors were now so cheap and so I have come up with a plan to test the energy in and out as shown in the attached graphic.

Am I right in thinking that after charging the capacitors, if I shorted the output terminals of the DC-DC converter that there wouldn't be a massive discharge?

Julian

 

Using capacitors for energy measurements.png

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I would think shorting the output would kill the converter thus acting as a smokey fuse. 

The solar power folks use converters that supply a constant output for varying input  voltages. One would like a converter that supplies say 5v as the capacitor discharges from say 12v to 7v. Perhaps this type:

https://www.amazon.com/DEVMO-Adjustable-Converter-XL6009-Voltage/dp/B07SZQMKLW

Also searching on the internet for "solar dc up down converter" will give you some useful information.

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Ok but if I’m using an electronic load to discharge the capacitor (so I can measure the energy it can deliver) once it’s been charged, then do I need the buffer of a DC-DC converter. Won’t the electronic load (https://www.westmountainradio.com/product_info.php?products_id=cba5) serve that function?

 

 

I would think you would not need a converter given: "Discharge current is software and electronically regulated using a solid state switch and fan cooled electronic load"

Why can't you  get the energy/power by simply charging a capacitor over some time interval?  Given work W = 1/2*C*V*V  in joules.  You may need to make allowance for the capacitor's ESR either charging or discharging.

For others: Equivalent series resistance (ESR)  describes losses associated with moving a charge through a capacitor. See:

https://www.digikey.com/en/articles/simple-explanation-of-capacitor-esr

 

 

 

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Hi Harry,

Yes, 1/2 CV squared is a quick way to calculate the energy in a capacitor but there are three reasons why using a CBA is preferable and helpful, bedsides the fact that I already have one for the extensive Coefficient of Performance tests done over the past six months, as detailed in the report I sent via the link in the pm.
 
Firstly, knowing the exact capacitance of the 6 supercapacitors I have in series is not easy to determine. 6 x 500F caps in series should be 83F but they are often only about 300F each so it’s more likely to be around 50F. Sticking my meter across them doesn’t give any reading even after 30mins due to their large value.
 
Secondly, the CBA provides me with the detailed graphical information for the total energy dissipated through the electronic load as well as the process and rate of charging etc.
 
Lastly the CBA gives me a safe way to dissipate and release the energy they contain to bring them safely down to zero.
 
I calculated that a 50F bank of caps at 15V ‘contains’ 5.63kJ which, if there was a short across the terminals could generate over 50kW of power in 0.1s so having a way to discharge them and see the process is also desirable.
 
I acknowledge that ESR is a factor but that mostly applies to AC circuits and this is DC all the way.
 
It’s good to know that I shouldn’t need any additional buffering between the caps and the CBA.
 
Julian
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