Multiple Capacitors : Series or Parallel ?

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Frédéric DruppelFrédéric Druppel 07/17/2017 at 18:264 Comments

As I will be using 2 capacitors, I have the choice to put them in series or parallel, with benefits on each choice. The capacitors I have are 2.5V 310F Ultracapacitors from Maxwell.

First of all some formulas :

Where C = capacity (Farads), Q = charge (Coulombs), U = Voltage(Volts), W = Work/Energy (Joules).

If I put the 2 capacitors in parallel, the overall voltage will stay 2.5V, but I'll have 620F in total. Which means I'll have a charge of 1550 Coulombs (Q=C*U), and a stored energy of 1937.5 Joules. As it is "only" 2.5V, it can have some trouble pumping the required current through the circuit (as I=U/R), but it's easier to charge as I won't have to "balance" the two capacitors as in the "series" option.

If I put the 2 capacitors in series, the overall voltage will become 5V, and the total capacity will become 155F. At the end that means I'll have 755 Coulombs of charge, but still 1937.5 Joules of stored energy. Because it's 5V, it could pump more current through the circuit (as I is still equal to U/R), but the overall charge won't be as high as the "parallel" option.

TL;DR ? Here's a quick table :

Option :ParallelSeries
Voltage :2.5 Volts5 Volts
Capacity :620 Farads155 Farads
Stored charge :1525 Coulombs755 Coulombs
Stored energy :1937.5 Joules1937.5 Joules

So yeah, I don't really know which one I'll be using yet, so I'll do some testing ^^


K.C. Lee wrote 07/18/2017 at 12:46 point

Probably should wire them in parallel.  
Your table has wrong heading.  First column should be parallel.

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Frédéric Druppel wrote 07/18/2017 at 13:00 point

Thanks for the correction ^^

Indeed in parallel the stored charge would be greater, but if I have power losses in the cables and switching mosfet of around0.25V, the end voltage at the contact would be 2.25V (assuming the capacitor bank was fully charged). The only way to find out what the best option is, is to test them out ^^

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K.C. Lee wrote 07/18/2017 at 13:31 point

It is easier to get thicker wires and a contactor than trying to balance the voltages for the caps in series and dealing with high current MOSFET.

BTW you want to pay close attention to the maximum SOA graph in the MOSFET datasheet.  Also use a proper sized gate driver.

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Frédéric Druppel wrote 07/18/2017 at 14:23 point

Indeed, thats' true ^^ I nearly forgot to look at that curve :3
Sir, You just saved me 10$ on a supercapacitor balancing IC and circuitry :P

I'll still test the two options, but my first choice is now the "parallel" option

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