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I am trying to discharge my 1F capacitor over a 47 ohm resistance which i connected in parallel with the capacitor . İ have calculated the time it would take to discharge from 5V to 4V as 10.55 seconds with the following formula : V(t) = V0 * exp(-t/RC). Where V(t) is 4 Volts and V0 is 5 Volts. But whenever i connect the oscilloscope to observe the change in voltage i see this

İt takes exactly 0.2 second from 5v to 4v, any idea that explains why there is a big gap between my anticipated value and the one i measure is appreciated . Have a nice day people :)

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    \$\begingroup\$ That is a big deviation. From the scope capture it looks like the initial drop is much steeper than it is supposed to be. Could it be that super capacitors are not very "good" capacitors? Their V/I characteristics ony roughly approximate an ideal capacitor? Is the remainder of the curve somewhat as expected? \$\endgroup\$ – mkeith Apr 22 '16 at 8:15
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    \$\begingroup\$ What is the Internal ESR of that cap. give the part number/datasheet of the cap. \$\endgroup\$ – user19579 Apr 22 '16 at 8:20
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    \$\begingroup\$ Also 0.22 is not exactly 0.2 seconds. And yes I'm being pedantic. \$\endgroup\$ – Dmitry Grigoryev Apr 22 '16 at 8:41
  • \$\begingroup\$ Brand: KAMCAP Model:5.5V 1F \$\endgroup\$ – Dogus Ural Apr 22 '16 at 8:45
  • \$\begingroup\$ Rest of the curve is as it is supposed to be , fully decharged around 5RC seconds. \$\endgroup\$ – Dogus Ural Apr 22 '16 at 8:46
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You should check the datasheet to be sure, but the usual cause would be that ultracaps are often quite limited in terms of current they can provide. You're trying to draw about 100 mA, while most ultracaps are rated at 1-10 mA discharge current.

BTW, I suggest you stop your experiments if you need that cap later on. You can easily damage an ultracap by an out-of-spec charge/discharge current. The cap may fail open or lose some of its nominal capacity (maybe that has already happened, contributing to the short discharge time you're observing).

Update:

According to the datasheet your cap has a maximum Equivalent Series Resistance of 22 Ohm. When you connect a 47 Ohm load to it, it is allowed for the cap to drop up to 1/3 of its voltage internally. That's what you observe: the voltage quicly drops from 5V to about 3.3V and then keeps falling exponentially as you expected. In the long run, the discharge time constant should be around (R+ESR)*C.

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  • \$\begingroup\$ But it says 22 ohm resistance at 1 kHz frequency, and what can i do to prevent the sudden drop , since my priority is to observe 5V-4V range \$\endgroup\$ – Dogus Ural Apr 22 '16 at 11:15
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    \$\begingroup\$ You can't prevent it. The fact that ESR was measured at 1 kHz doesn't mean it becomes zero at lower frequencies. The cap is made from porous microstructures to maximize the electrode surface, and those microstructures have significant resistance. \$\endgroup\$ – Dmitry Grigoryev Apr 22 '16 at 12:02

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