Potential problems with series capacitors?

[Pirate88179]

Farmhand:

I have read that you can only put 2 supercaps in series/parallel.  Maxwell boostcaps on the other hand, claim that you can place as many of them in your circuit as you want.  I do not know why this might be but, I read this a few years ago...for what it is worth.

Bill

[MarkE]

I discovered the same issue with my supercapacitors, I wanted to put two in series to get a 5.4 volt capacitor but soon realized
that I would need to limit the voltage on each in order to avoid over volting one due to unbalanced charge.
(isn't volting a word ?) Seems not but i'll use it.

I could not think of any way to correct the issue without consuming power except to use a voltage source (battery) across each one.

Which of course is just silly unless the battery is a low output battery (home made cells) and I allow the supercaps to charge from
them until maximum voltage is achieved then use the caps until the voltage is too low then repeat. A circuit could do that
automatically for me and harness the low output of the home made cells via the capacitors intermittently.

Do 5.5 v double layer super caps have some kind of internal balancing ?

..

The series situation is one of  charge distribution.  A voltage limiter as shown above will with the right value for the zener protect the capacitor from over voltage.  If the application is  a series stack of capacitors with source/load only connected across the  whole stack then the two diode per capacitor solution's only limitation is the series impedance of the zener.  That will be a factor if you apply a steep current ramp such as turning on a power supply that does not have a reasonable current limit  Otherwise the diodes will conduct only during boundary conditions and will otherwise be invisible.  This technique has been used very successfully with many capacitors in series.

Capacitors of any kind should not have any problems with parallel connections per se.  Because of their very low impedance, one can get large current transients with super caps.  Those current transients can develop voltages across wiring resistance and inductance that could become a problem.

[d3x0r]

They actually sell charge balancing IC modules...


this one is op-amp based with a mosfet... http://www.discovercircuits.com/DJ-Circuits/supcapvoltlim.htm


http://www.instructables.com/id/Lets-learn-about-Super-Capacitors-A-Practical-G/step7/Balancing-Your-Series-Banks/


comparison of some balancing methods (zener, resistance, boost-converter (mosfet)) http://hal.archives-ouvertes.fr/docs/00/41/14/82/PDF/ESSCAP2006_Venet_2.pdf


looks like getting a series balancing chip might be a better way to go....
ALD810025 4 cap series thing $2.91 last 2 digits is voltage so a 27 is 2.7....

[MarkE]

The ALD8100 data sheet looks very nice.  If nothing else, it will save you space.

[d3x0r]

The ALD8100 data sheet looks very nice.  If nothing else, it will save you space.

ya... space...


It is unfortunately a unique part, and only package is 16-SOIC; There is a breakout board for another couple dollars.... *sigh* I'm no good with tiny things


2.7V zeners are only $0.10; but are really low current (500mA)
Although these chips are only 80mA...


---
Edit: Regarding your diode thing... won't the 0.7V drop of the PN affect it?  like then should I use a 2.0V zener?