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I have a super capacitor (1000F) laying around and I want to measure ESR and it's capacitance. I have access to a 8V/500A power supply and various meters.

If I wanted to charge/discharge the capacitor at constant current (75A) to it's rated voltage (2.7V) and half (1.35V) then I would just need a series resistor that pulled more than 75A at 1.35V?

1.35V/75A = .0173 ohms. So anything less than that with a power rating of over 205 should suffice? Am I missing anything?

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  • \$\begingroup\$ What is included in 'various meters'? Just a bunch of multimeters, none with a capacitance test? You aren't going to get a useful ESR measurement like this. >100W resistors aren't cheap, unless you have some spare heaters lying around. I'd also be rather careful at the stated currents. It can readily become more of a welding experiment than a measurement experiment. \$\endgroup\$ – replete Aug 9 '17 at 4:44
  • \$\begingroup\$ Multimeters. One does have a capactiance test but 1000F isn't in it's range. I've located such a resistor and it's not cheap but affordable. I would be using method from maxwell.com/images/documents/… \$\endgroup\$ – Frank Aug 9 '17 at 5:04
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That's not how to test ESR according to the OEM. Since there is a double charge layer in effect, two caps with two ESR's mainly and many more in parallel that you can neglect. THe Cap Voltage drop at DC under high current will have a memory effect when the load is removed. This is the other main charge layer which is used to compute the total C value for the part but a higher ESR so it restores the cap voltage as it discharges slower.

It is similar to almost any battery where you load short the battery for a second and it restores its charge voltage no load.

The pulse duration could be varied from 0.1s to 1s or as per directed in OEM specs. Another standard test ESR test is @ 1kHz , , but this is less important for measuring capacity and high current ESR and more for step transient slew rate in current.

1000 F is ballpark , about the same capacitance of a LiPo cell which can have ESR of 5~25mOhms typ. for fresh 18650 cell. ( with a wide tolerance due to supplier Ah and quality) An AA Alkaline is around 100F.

If you want to drop 75 Amps use AWG tables and expect the wire Resistance to change with temperature, so this is why one uses a 50mV current sense to dissipate low heat. Then capture the results with a scope.

The FoM or figure of merit in batteries and all electrolytic caps is the ESR*C product. where lower is better. Ultralow ESR caps in the 1~100uF range are 1us and std types > 100us and as e-caps increase in value so does the T constant to ms to seconds in poor quality supercaps. Recall the previous generations were only good for non-vol RAM backup and had >30 second time constants and high ESR.

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