I had bought a Maxwell ultracapacitor (this one) rated for 1200F at 2.7V. Its ESR according to the datasheet should in ballpark of 0.4 milliohms.
It got delivered today and is undamaged and new by appearence. However, I don't understand something.
To test it, I charged the capacitor briefly (with a 1.8V supply with 1A peak current) to 50mV and shorted it across a wire (it charged very slowly as expected from a supercapacitor). Nothing.
I tried again with 500mV. Still nothing. Then with 750mV. Still doesn't do anything.
To, investigate it further, I charged it to 750mV and shorted it across a wire with the multimeter in parallel set in voltimeter mode. When I shorted it for 2 seconds,the voltage dropped close to zero when the short was active. However, upon removing the short, the voltage climbed immediately back to 750mV.
This is behaving as if the ESR of the cap is really high and doesn't even discharge easily with a dead short.
Why is it doing this and how do I stop it from happening?

  • \$\begingroup\$ 400 uOhms is implausibly low. Link to a real datasheet. \$\endgroup\$ Apr 11, 2015 at 21:48
  • 2
    \$\begingroup\$ @OlinLathrop: those have indeed ~500-600 µOhms ESR but of course measured in the best possible way and circumstances, which are far from the situation described here. maxwell.com/images/documents/K2Series_DS_1015370_5_20141104.pdf If you are intrested in the "datasheet" \$\endgroup\$
    – PlasmaHH
    Apr 11, 2015 at 22:00
  • 1
    \$\begingroup\$ @OlinLathrop - The capacitor alone is the size of a soda can. You can arc weld with these things. ESR values in the uOhms is very reasonable. \$\endgroup\$ Apr 12, 2015 at 3:43
  • \$\begingroup\$ What are you shorting it with? Can you measure the current when shorting it? Perhaps short via a low resistor and see what voltage is across that \$\endgroup\$
    – Jodes
    Apr 12, 2015 at 14:03
  • \$\begingroup\$ @Jodes Shorting it with plain 20AWG 3cm copper wire. The voltage drops to nearly 10-20mV \$\endgroup\$
    – AvZ
    Apr 12, 2015 at 14:18


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