Batteries and Capacitors in Series

[Drakkith]

I was looking up some information on series and parallel circuits, and a question popped in my head. I know that hooking up batteries and capacitors in series will give you a voltage equal to the sum of all the voltages in each component, but I was wondering why it does that?

 

[tiny-tim]

Hi Drakkith!

But that's how voltage is defined …

it's the difference in electric potential between two points,

ie the difference in potential energy per charge,

which in a conservative field (such as an electric field) is additive …

the difference between two points will always be the sum of the differences between intermediate points.

(and of course the difference between the same two points along different routes, ie in parallel, will always be the same).​

 

[Drakkith]

Hrmm. I see what you mean.

Well then, when you connect a couple of batteries in series, like in a flashlight, where is the current flowing when it passes from one battery into the other one? Into the electrodes and then the electrolyte and such? I think I'm actually wondering how the physical construction of batteries and capacitors allow more and more voltage to build up in series. Or something like that... I'm REALLY bad at wording my questions, so I apologize. I know what I want to say in my head, but I cannot get it out properly.

 

[tiny-tim]

… where is the current flowing when it passes from one battery into the other one? Into the electrodes and then the electrolyte and such?

Nothing really flows … each electron only moves a very short distance.

The current is the same all the way round the circuit.

I think I'm actually wondering how the physical construction of batteries and capacitors allow more and more voltage to build up in series. Or something like that... I'm REALLY bad at wording my questions, so I apologize. I know what I want to say in my head, but I cannot get it out properly.

They're like two pumps, their effect adds up.

 

[Drakkith]

I understand all of that Tim.
Does the extra voltage have an effect on the batteries? IE will a battery eventually fail because you put too many in series?

 

[tiny-tim]

No, a battery will only fail because it uses up its own stored energy (the electrolyte, or whatever).

(I suppose you could "blow" a battery by putting too much current through it, just as you can blow a light bulb filament with too much current, but that's current not voltage)

 

[Evil Bunny]

I'm struggling with these concepts as well... with the batteries in particular. When you take two batteries and hook the + of one to the - of the other and vice versa... you've just created something very close to a short circuit scenario and there is obviously current flowing (because of a very low internal resistance evidently)

Yet... when you connect them in series, this problem (of current flowing through them) does not occur... you just add the voltages. Likewise, if you connect them in parallel, (identical voltages), everything is fine... you've only increased the available current. Nothing is flowing through them like in the original scenario..

I've been asking similar questions around this stuff and I've still not been able to completely wrap my head around it...

 

[sophiecentaur]

I understand all of that Tim.
Does the extra voltage have an effect on the batteries? IE will a battery eventually fail because you put too many in series?

You could imagine a circuit with a high voltage, high capacity battery in series with a little battery and a low resistance load. This circuit could cause a very high current to flow forward through the small battery, which would cause it to overheat and knacker it. But a battery used under normal conditions will just add energy to the charges flowing through it so series connection will just produce an overall potential difference equal to the sum of all the PDs of the batteries.

 

[Evil Bunny]

You're saying that two 10V batteries in series would give you a 20V source to work with (which I would agree with).

But are you also saying that if we had (let's say) a 100V battery in series with a (let's say) 1V battery, we might not have a 101V source? At what point does the current flow through the little battery and overheat it? Is there a formula or something? This is something new that I've never heard of.

It's not hard to imagine it happening, and I wouldn't be surprised to see it documented, but I've never heard of it.

 

[sophiecentaur]

If the current flowing through it was within its spec, there would be no difference in that situation in which a single battery is powering a low voltage circuit than if the same number of Amps were flowing in a 'high voltage' circuit or . Each Volt, wherever it is, corresponds to one Joule for every Coulomb flowing.

There's no "formula" to describe what happens when you force too much current through a battery but the power that is dissipated inside it will be about Isquared R, where R is the internal cell resistance.

You haven't put your finger on something that hasn't been thought of before - just look at it logically and it will become clear.

 

[Evil Bunny]

Oh, so all you're saying is that you can't exceed the current rating on a battery...

 

[sophiecentaur]

I'm saying that the internal resistance of the small battery needs to be taken into account if you try this sort of thing. In a stack of similar batteries this wouldn't be a problem - as long as the load was appropriate.
The battery doesn't 'know' where it is or in what circuit. All it can do is behave like a battery and provide its volts up to a specified current.