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Broadening my knowledge and experience about ultracapacitors I've made a test bench that was described in here. I've used an IEC methodology to calculate the capacitance: $$ C = (I_d*t_d) / (\Delta U) $$ where delta U is difference of voltages equal to: $$ \Delta U = 0.9 *U_r -0.7 * Ur $$ where Ur is a rated voltage equal to 2,75 V Other values that I've measured are:

  • td = 202 s
  • Id = 10 A

My calculation predicts C to be ~3600 F. Whereas rated capacitance of ultracapacitor is 3000 F. Are my calculations wrong or the calculated capacitance is a maximal capacitance available to achieve for this ultracap, but manufacturer limits it to 3000 F? Or assures only the 3000 F?

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    \$\begingroup\$ What's the tolerance on the capacitor? Electrolytics and double-layer capacitors tend to have really wide tolerances. \$\endgroup\$
    – Hearth
    Commented Jun 27, 2019 at 18:17
  • \$\begingroup\$ Product matrix doesn't show the capacitance tolerance. It is BCAP3000F with Ur = 2,75 Volts. I've searched in the Internet and found it to be -10/+20 % This site provides the info about ultracapacitor tolerance: picclick.com/… \$\endgroup\$
    – QST
    Commented Jun 27, 2019 at 18:19
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    \$\begingroup\$ maxwell.com/images/documents/K2Series_DS_1015370_5_20141104.pdf indicates the minimum value is 3000, and the maximum is 3600, so the tolerance is all to the positive side. \$\endgroup\$
    – Phil G
    Commented Jun 27, 2019 at 18:26
  • \$\begingroup\$ I think this answers my question. Topic to close. \$\endgroup\$
    – QST
    Commented Jun 27, 2019 at 18:30
  • \$\begingroup\$ I'll put this as an answer then so you can close it. \$\endgroup\$
    – Phil G
    Commented Jun 27, 2019 at 19:13

2 Answers 2

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Energy storage devices usually are toleranced to ensure that a minimum value is met.

For this cap, the datasheet indicates the minimum value is 3000F, and the maximum is 3600F, so the tolerance is all to the positive side.

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3600F is the start capacitance and 0.29 mΩ is the initial ESR.

3000F is the rated C after certain conditions.

a. -20 % after 1500 hours held continuously at Rated Voltage and Maximum Operating Temperature 65'C
or ...

b. -20 % after 10 years held continuously at Rated Voltage but at 25'C or ...

c. -20 % after 1,000,000 cycles at 25'C

These reductions are cumulative.

Also same is given for ESR , which can double (2x) or rise 100%.

Always Read The (OEM's) Fine Print

Don't assume the Distributor has valid specs.
e.g. Your link Ultracapacitor Cell Capacitance: 3000.0 F -10/+20%

the .0 should give it away to you, that Caps do not have 5 significant Figures.

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