What happens when charged capacitors are swapped from parallel to series connected?

Thread Starter

Mark Flint

Joined Jun 11, 2017
80
I am reading about capacitors and have a few blocks in my understanding...

If I have 3 supercaps, 2.7v 400F, and I charge them in parallel I will have 1200F at 2.7v, which is 4,374 joules.
If I have 3 supercaps, 2.7v 400F, and I charge them in series I will have 133.33F at 8.1v, which is 4,374 joules.

1) does it take more input power to charge caps in series than the same caps in parallel?
2) If I now swap the 3 caps that were charged in parallel TO a series configuration, do I loose joules?
3) If I loose joules WHERE do these joules go?
 

AlbertHall

Joined Jun 4, 2014
10,226
As you have calculated the stored energy is the same whether they are in series or parallel.
Therefore:
1) No
2) No
3) See 2)
 

Thread Starter

Mark Flint

Joined Jun 11, 2017
80
So therefore I can charge the caps at 2.7v in parallel, swap them to series and use them at 16.2v with no loss of energy. Makes sense to me, but I've been advised that capacitors "don't work like that".

Cheers,
Mark
 

Papabravo

Joined Feb 24, 2006
14,206
So therefore I can charge the caps at 2.7v in parallel, swap them to series and use them at 16.2v with no loss of energy. Makes sense to me, but I've been advised that capacitors "don't work like that".

Cheers,
Mark
How did you plan to do this swapping? What do you think you are going to use them for?
The reason I ask is that you may have some surprises in store for you.
If you are not familiar with RC circuits, I recommend the following background material.

https://en.wikipedia.org/wiki/RC_circuit
 

Thread Starter

Mark Flint

Joined Jun 11, 2017
80
I plan to run a DC motor from the capacitor bank. The caps would be charged in parallel, disconnected from the circuit, swapped to series, reconnected to the circuit.
 

wayneh

Joined Sep 9, 2010
16,399
...Makes sense to me, but I've been advised that capacitors "don't work like that".
There's the theoretical aspect and then there's the practical aspect. You can stack charged capacitors in series and the resulting voltage is the sum of all of them, in theory. But there are practical problems in doing this, one of which is that there is no simple way to insure that the voltage across each cap will stay below its rating. And while you get more voltage, you lose capacitance compared to the parallel arrangement. So energy is conserved but all you gain is voltage. There are other, more practical ways to get a DC-DC voltage boost.

The advice may have been related to these practical issues, rather than the "on paper" analysis.
 

AlbertHall

Joined Jun 4, 2014
10,226
Incidentally, it is important that the capacitors all have the same value or bad things will happen when they discharged in series.
 

Papabravo

Joined Feb 24, 2006
14,206
I plan to run a DC motor from the capacitor bank. The caps would be charged in parallel, disconnected from the circuit, swapped to series, reconnected to the circuit.
Running a motor from capacitors is a bad business. To start a motor requires a large initial current. The capacitor bank will provide this, but the cost is that the voltage will decline exponentially. What you have left may or may not keep the motor running and there will be less and less current as the voltage declines further. Where did you get the notion that this was a good idea?
 

Thread Starter

Mark Flint

Joined Jun 11, 2017
80
A conservative designer wouldn't operate components at their maximum ratings.
True.

Incidentally, it is important that the capacitors all have the same value or bad things will happen when they discharged in series.
Noted.

The advice may have been related to these practical issues, rather than the "on paper" analysis.
Yes, I think so.

Where did you get the notion that this was a good idea?
I just like finding different solutions - and don't let that stop me asking the next dumb question :)
When someone says "bad idea" - I mostly hear "design challenge".
 

Papabravo

Joined Feb 24, 2006
14,206
True.
...
I just like finding different solutions - and don't let that stop me asking the next dumb question :)
When someone says "bad idea" - I mostly hear "design challenge".
Sometimes bad idea means just that. By all means -- run the experiment and find out for yourself. Just don't be surprised, and don't say nobody told you so.
 

Papabravo

Joined Feb 24, 2006
14,206
You said it was a bad idea without any knowledge of my circuit.
That is because I know what will happen when you connect any kind of load to a capacitor bank. It's not rocket science, but it is freshman calculus and physics. If you missed them, then you get a pass.
 

Thread Starter

Mark Flint

Joined Jun 11, 2017
80
Here's an example of a similar motor, but smaller than mine, hence I am looking at bigger caps than this guy is using.


I'm currently running my motor on batteries.
 
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