RC Charge/Discharge for Electrochemical Capacitor

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I am a student working on designing an electrochemical capacitor (an area which i have limited experience). When I charge this capcitor through an RC circuit I notice two non ideal effects. I need to adress the cause of these effects if I can ever hope to correct them.

The first problem is that the voltage immediatly jumpts up to about a quarter of the applied voltage before following the exponential function expected from RC circuit theory. I have discovered that internal resistance of the capacitor is the cause of this. However, I dont understand why the internal resistance does not simply add to the resistor in series with the capcitor.

My second problem is that the capacitor never reaches the applied voltage but approaches a lower voltage. If i continue to let it charge it will eventually start discharging on its own. This observation holds true even for low applied votlages. I have a feeling this is caused by leakage but can anyone confirm this?
 

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  • #2
berkeman
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I am a student working on designing an electrochemical capacitor (an area which i have limited experience). When I charge this capcitor through an RC circuit I notice two non ideal effects. I need to adress the cause of these effects if I can ever hope to correct them.

The first problem is that the voltage immediatly jumpts up to about a quarter of the applied voltage before following the exponential function expected from RC circuit theory. I have discovered that internal resistance of the capacitor is the cause of this. However, I dont understand why the internal resistance does not simply add to the resistor in series with the capcitor.

My second problem is that the capacitor never reaches the applied voltage but approaches a lower voltage. If i continue to let it charge it will eventually start discharging on its own. This observation holds true even for low applied votlages. I have a feeling this is caused by leakage but can anyone confirm this?
If it jumps up initially, that means it does not have much capacitance. Recall the relationship between current and a changing voltage for a capacitor. Probably it has to do with the way your electrochemical electrolyte is working with no initial bias.

And the end result sounds like a high leakage current, again probably related to your electrochemistry.
 
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  • #4
berkeman
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Low capacitance causes an inital jump in voltage? My conclusion of high ESR was based on figure 7 of this pdf.

http://www.sencore.com/uploads/files/UnderstandESR.pdf
From your description initially, it sounds like something changes after the initial jump. Whether it's ESR or capacitance, you will have to determine on your own based on your cap's electrolyte chemistry.
 

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