# Ultracapacitor, capacitance higher than rated?

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?

• What's the tolerance on the capacitor? Electrolytics and double-layer capacitors tend to have really wide tolerances. Commented Jun 27, 2019 at 18:17
• 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/…
– QST
Commented Jun 27, 2019 at 18:19
• 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. Commented Jun 27, 2019 at 18:26
• I think this answers my question. Topic to close.
– QST
Commented Jun 27, 2019 at 18:30
• I'll put this as an answer then so you can close it. Commented Jun 27, 2019 at 19:13

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.

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%.

Don't assume the Distributor has valid specs.