# Balancing series stacks of supercapacitors during charge /discharge

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#### Flyback

##### Well-Known Member
Our contractor thinks that the balancing of series stacked supercapacitors during charging is needed because of differences in leakage current. However, we keep telling him that charging of series stacks needs balancing due to the tolerance differences in capacitance from cell to cell. Do you agree? (When the charger goes onto "float charge", thenn the leakage current effects take over)

Also, the problems inherent to charging batteries in parallel with a single charging current source do not exist for paralleled supercapacitors. This is because if one paralleled supercapacitor hogs all the charging current then it will not overheat like a battery could. Do you agree?

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#### ronsimpson

##### Well-Known Member
balancing due to the tolerance differences in capacitance
What is the tolerances? If 10% then the voltage difference is known. Is 10% in voltage a problem.
I can see if one cap is 2X the other then the "voltage divider" will be off and one cap might over voltage.

#### Flyback

##### Well-Known Member
Thanks, the point is, as you know, that capacitors that are (nominal -20%) Farads are in danger of overvoltaging if the two-stack is charged to (nomial x 2) volts.

The great advantage of supercapacitors in series stacks, that is never mentioned anywhere, is that if series stacked supercapacitors are discharged, then even if they have massively different capacitance values, no single capacitor gets a damaging reverse bias whilst the others have a positive voltage......this makes supercapacitors better than batteries...because a series stack of cells can see one cell go negative whilst the others are positive...destroying that cell.
Why is this great superb advantage of supercapacitors never mentioned..anywhere?

#### ronsimpson

##### Well-Known Member
a series stack of cells can see one cell go negative whilst the others are positive...destroying that cell.
Happens to batteries to.

#### Flyback

##### Well-Known Member
Thanks, thats actually what i meant...it happens to batteries in series stacks, but not to supercapacitors in series stacks

#### GromTag

##### Active Member
I seem to lack the means to properly convey what I comprehend of "Extended capacitors" supercaps.

Leakage current vs. temperature and series resistance temperature. Both charging and discharge need attention, charging more than.

Cell imbalance with temp increase should not be too uneven on the cell bank itself based on environment. Tolerance change. And no two cells are alike in reality. Parabolic shift per cell differs at temperature, one may go 3% at such and such temp, other 5% at both the same temperature. Needs attention again during charging.

Only examples can I offer in terms of IC datasheets for an reference that I've encountered with cell balancing extended capacitors. Hope it's of some use.

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#### Flyback

##### Well-Known Member
Thanks, but as you know, if a series stack of supercapacitors is discharged into a load, then it is not possible for one of the supercapacitors to go negative in voltage when the other capacitors in the stack have positive voltage. This is a massive advantage of supercapacitors over batteries, because in a series stack of cells, when discharged, one cell could go negative in voltage and be damaged when the others are still positive in voltage.

#### GromTag

##### Active Member
When the pack of supercapacitors are charged with proper polarity then not likely for a single supercapacitor to go inverted at that point. Somewhere I've noted reading that a supercapacitor can go inverted during charging IF have sat long dormant and goes completely discharged, can't find that info, was listed in fault conditions of a balancer inability to recover that cell and possibly the whole batch if that kind of fault occurs.

Caps have more tolerance over battery cells when facing inverted polarity under discharge, a hard ramp of current surge on any inverted cap (not possibly by standard supercapacitor series design) would take it out, usually the reason for the add on diodes recommended by the IC balancer over the internal diodes which would typically should not allow inverted operation of a supercapacitor period.

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