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I read that the underlying technology (the dual layers) of ELDC super capacitors is limited to about 2.5 V. Yet I see 5 V (and 3.3 V and other) supercap packages on the market.

Also, it is well known to put such capacitors in series with balancing resistors, to prevent one of the two capacitors from receiving voltages beyond their 2.5 V limit. And there are IC's dedicated to charging series supercaps without balancing resistors, e.g. the LTC3225.

For example, the Cooper Bussman brand, part number PB-5R0H104-R. Rated 5 V max with max ESR of 4 Ohms. That supercap appears to be two cylindrical capacitors bound together.

Does that particular model of supercap have built in balancing resistors? In general, is that the case for supercaps rated over 2.5 V?

Are there any tradeoffs to using such a supercap? As opposed to say a Cap-XX brand, a model from the "GW 2" family (having two cells in series). That model has a "balancing" pin and the mechanical drawings at the bottom of the datasheet show its use with an external balancing resistor network. (And a note "Please contact Cap-XX for further information.") When would you want to design your own balancing resistor network, or use a dedicated IC to charge, a series supercap?

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  • \$\begingroup\$ If there are resistors accross the caps, then you can find their minimum value by looking at the leakage current spec. However, I thought the technology leaked enough on its own. \$\endgroup\$
    – Olin Lathrop
    Commented Aug 31, 2012 at 12:58
  • \$\begingroup\$ I haven't torn a 5V one apart to determine whether there are internal resistors. As to leakage, I thought they were low leakage, after a day or so. Also, the leakage would not change the need for balance when charging? \$\endgroup\$
    – bootchk
    Commented Sep 1, 2012 at 18:19
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    \$\begingroup\$ If I were designing such systems, I would just go for parallel zener diodes + series polyfuse (potentially with a reverse current diode for fast discharge). \$\endgroup\$
    – qdot
    Commented Oct 7, 2012 at 17:07

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My company uses Maxwell ultracap modules all the time, which are series combinations of low-voltage high-capacitance cells. All balancing is taken care of internally, with no external connections or indicators, and we've never seen any problems. So unless the data sheet for a particular cap says to worry about it, I wouldn't.

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