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I have 2 identical PowerStor Series XB supercaps connected in series. Voltage rating of each is 2.5V. In series their capacitance is cut in half but the voltage drop over both of them should be double. So that's why I connected it to a 5V pin on a computer PSU for charging.

After about a minute I disconnected everything and found that the top cap had a voltage of 2.6V while the bottom cap had 0.8V. How could this happen? and why?

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  • \$\begingroup\$ Generally the "value" of a cap is it's "guaranteed minimum value", and it may have an actual value 2-3 times higher. Electrolytics are probably worse in this regard than non-polarized ("paper") caps. \$\endgroup\$ – Hot Licks Jan 14 '15 at 3:10
  • \$\begingroup\$ Did you try to swap? And when you charge connecting in parallel, are they charging at the level of 2,5V? If yes, have you try to measure the discharge rate of both? \$\endgroup\$ – GR Tech Jan 14 '15 at 3:54
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    \$\begingroup\$ what's missing in the question is the "what to do about it" part ... you can add circuitry to balance the charge in each. It can be as simple as a resistor in parallel with each C; with DC input, the voltage across each C should equalise. \$\endgroup\$ – Brian Drummond Jan 14 '15 at 13:16
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"How" is because of the differences in capacitance, - including parametric changes - ESR, and leakage between the caps, and "why" is because the manufacturing process isn't that good yet.

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