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

#### Mark Flint

Joined Jun 11, 2017
145
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
12,292
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)

#### Mark Flint

Joined Jun 11, 2017
145
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
20,584
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

#### Mark Flint

Joined Jun 11, 2017
145
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.

#### dl324

Joined Mar 30, 2015
16,110
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.
A conservative designer wouldn't operate components at their maximum ratings.

#### wayneh

Joined Sep 9, 2010
17,463
...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
12,292
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
20,584
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?

#### Mark Flint

Joined Jun 11, 2017
145
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
20,584
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.

#### Mark Flint

Joined Jun 11, 2017
145
You said it was a bad idea without any knowledge of my circuit.

#### dl324

Joined Mar 30, 2015
16,110
You said it was a bad idea without any knowledge of my circuit.
You said you want to run a motor from some capacitors.

Do the experiment and see for yourself what's wrong with your idea.

#### Papabravo

Joined Feb 24, 2006
20,584
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.

#### Mark Flint

Joined Jun 11, 2017
145
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.

#### Papabravo

Joined Feb 24, 2006
20,584
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.
What is the load on the motors: his and yours? If there is no load then they're just toys.