Does current flow through a Battery?

[kevin original]

I recall Dave having a discussion about whether or not current flows through a capacitor because the electrons can't make it through the dielectric(unless you let the smoke escape). I've recently been looking into batteries and I've read that electrons do not pass through electrolyte solutions, which boggles my mind. We all know that adding salt of some form to water makes it conductive to electricity and "current flow" but if it is true that electrons do not journey across the electrolytic fluid and it is only the ions that travel between the electrodes then does current really flow through a battery? Perhaps electrons do travel through a battery just hitching a ride on the ion but when all of the ions "run out" the current flow stops. So it sounds to me like batteries are really just chemical capacitors.

[IanB]

So it sounds to me like batteries are really just chemical capacitors.

There is a sense in which this is absolutely true 

[bdunham7]

Perhaps electrons do travel through a battery just hitching a ride on the ion but when all of the ions "run out" the current flow stops.

That's pretty good simplified version of how an Li-ion battery works.

So it sounds to me like batteries are really just chemical capacitors.

No, absolutely not.

[bob91343]

Batteries have a lot in common with capacitors.  Ions and/or electrons are the means of charge transport in batteries.  In capacitors, it's the charge and discharge that 'imitates' current flow.

[eti]

One wouldn't get very far if it didn't - put 4 "D" cells in a Maglite and the current HAS to flow through ALL of the cells.

[bob91343]

The idea here is not whether current goes from cell to cell, but whether it flows internally within the cells.  Of course it flows externally but it's not clear what the mechanism is, especially within a capacitor.

[eti]

Well of course it does.

[ejeffrey]

I recall Dave having a discussion about whether or not current flows through a capacitor because the electrons can't make it through the dielectric(unless you let the smoke escape). I've recently been looking into batteries and I've read that electrons do not pass through electrolyte solutions, which boggles my mind. We all know that adding salt of some form to water makes it conductive to electricity and "current flow" but if it is true that electrons do not journey across the electrolytic fluid and it is only the ions that travel between the electrodes then does current really flow through a battery? Perhaps electrons do travel through a battery just hitching a ride on the ion but when all of the ions "run out" the current flow stops. So it sounds to me like batteries are really just chemical capacitors.

Current definitely flows through a battery in the form of ionic drift through the electrolyte which can be positive or negative.  The electrolyte is not a metal so it doesn't support "free" conduction electrons in the same way as a wire but there is still current flowing.

For what it's worth, even if you are so mentally inflexible that you can't consider displacement current a real current (which you absolutely should), current flows through most capacitors.  Dave's argument only holds for vacuum capacitors, but most capacitors have a deliberately chosen high dielectric constant material between the plates.  Microscopically, the electrons in the dielectric move in response to the applied field and generate a current.  Normally we don't think about this and we just lump that into the displacement current along with the displacement due to the vacuum permittivity, but if you are going to be pedantic about it you should at least do so correctly.

[TimFox]

The important difference between a cell or battery and a capacitor is that the voltage across a capacitor is proportional to the charge therein, while the voltage across a cell is close to independent of the charge therein.  Therefore, the energy stored in a capacitor is (1/2)QV, but the energy stored in a cell is approximately QV, where Q is the charge.
Note that a battery is a series connection of cells:  the common AA alkaline is a cell, but the common 9 V alkaline is a battery.

[kevin original]

Thinking a bit more on it. I don't believe the ions are actually accepting or releasing electrons at the electrode. Instead the ions are displacing the electrons at the electrode by their movement towards them and thus attracting/repelling them. So electrons are not actually being moved through the battery(except for the ones bound to the ion). "Current flow" however is the time rate of electric charge. Ions by definition have electric charge even though they are not free electrons. By definition then, electric current does flow through a battery albeit through mechanical action ie. protons are moving with the current.

[IanB]

Thinking a bit more on it. I don't believe the ions are actually accepting or releasing electrons at the electrode.

Of course they are.

The cell has to remain electrically neutral. Whenever an electron leaves one electrode an electron enters the other electrode to balance it. You can look at this from the electrical circuit's point of view, where the electrons are entering and leaving the electrodes through wires, or you can look at this from the electrolyte's point of view inside the cell, where the electrons are entering and leaving the electrolyte solution. Whichever way you look at it, everything balances, and overall electrons are flowing round the circuit.

[GlennSprigg]

Well of course it does.

Hmmm... I know what you say sounds logical, but just sticking to Capacitors for a sec, there is no actual 'current' flowing internally between the plates.
It just holds a 'Charge', where one side holds an excess of electrons compared to the other side. It's the 'Circuit' connected to such a Capacitor that
'transfers' the excess of electrons from one side, and balances it to the other side!!  Even with Batteries, this is so, but done chemically though.

Say there 'are' 4 batteries in series... you have to think about 'Charge-Relativity' to grasp it all. Again, it's the relative charge difference between the 'cells'
that causes a flow from the -ve end of one battery, to the +ve end of the next battery etc, and only when an external circuit is completed! So the END of
last one, flows into the beginning of the first one, via the external circuit. It is not flowing 'internally' within one cell !!   

[ejeffrey]

Well of course it does.

Hmmm... I know what you say sounds logical, but just sticking to Capacitors for a sec, there is no actual 'current' flowing internally between the plates.

Batteries are not capacitors. 

last one, flows into the beginning of the first one, via the external circuit. It is not flowing 'internally' within one cell !!   

It most certainly is.

[shakalnokturn]

I had to re-read the definition of electric current before having a word to say, to me it fits batteries and capacitors.
Considering self discharge, current even flows more through batteries than elsewhere... 
(To attempt an answer to the topic question.)

[GlennSprigg]

Regarding the last two answers here... you are mis-interpreting what I was saying.

Firstly, I'm NOT talking about imperfections/breakdowns in the real-world, where a Capacitor or Battery may 'leak' internally,
and lose some/all of it's charge internally. That certainly happens 'within' the cell, but is unrelated to what I am saying!   

If the above doesn't occur, then they hold their 'charge' until such time as the External Circuit connected to such devices performs
the 'actual' completion of the path of the Electrons, back to the other side!  I don't understand the problem of grasping that ??   
And the way a 'Battery' discharges, (through an external circuit), is the same as a Capacitor so discharges, but using 'obviously' different
technology, as the charge is Chemically Stored instead, not with a 'charge' either side of a non-conducting Dielectric. So I stand by my
statement the Electrons flow 'externally' from one side of such devices, and 'back' to the other side. I'm not trying to be silly here ?   

[tszaboo]

Current is defined as movement of charged particles.
Batteries have a separator. Positive ions go through the separator.
Their negative particles, the electrons go through the wire, because they cannot go through the separator.

There is a movement of positive particles, so there is current in a battery. Its different than the current we usually see in wires and resistors and whatnot, but it is still current.

[GlennSprigg]

Current is defined as movement of charged particles.
Batteries have a separator. Positive ions go through the separator.
Their negative particles, the electrons go through the wire, because they cannot go through the separator.

There is a movement of positive particles, so there is current in a battery. Its different than the current we usually see in wires and resistors and whatnot, but it is still current.

Again, I know what you are saying too, but it's all a matter of 'nomenclature' and ideas used to understand.
Your statement of "Movement of Positive Particles", smacks to me of the 'old' idea, (hence direction arrows etc in Diodes, Transistors, Valves/Tubes),
before they realized that it is actually 'Electron' flow, from -ve to +ve, and not the other way around!!  My old Instructors (about 50 years ago!), used
to try and explain it away by talking about... "A movement of Positive Holes", in the reverse direction of actual Electrons!  Here's how that worked!!...

"Put a line of blocks, (electrons!), on the table. Now starting from the right, move the last block to the right a bit. Then move the second block from the
right to join the 1st block moved. And then keep doing that with the remaining blocks in the same way".  Then they would say... "Notice how the GAP
between the blocks appears to be moving to the Left! That is the movement of Positive Holes!!".  Ggrrrr... Even then at 16, I would pull them up, and
tell them to start talking about 'Electrons', and that drawing 'standards' even 'today', should be changed !!!   

[shakalnokturn]

All I was saying is that IMHO what happens in a battery fits the definition of electrical current flow...
Although the smiley didn't make it obvious enough the internal leakage comment was a form of humour.

In what way would you say that batteries do not fit the definition of "electric current"?

[tszaboo]

Current is defined as movement of charged particles.
Batteries have a separator. Positive ions go through the separator.
Their negative particles, the electrons go through the wire, because they cannot go through the separator.

There is a movement of positive particles, so there is current in a battery. Its different than the current we usually see in wires and resistors and whatnot, but it is still current.

Again, I know what you are saying too, but it's all a matter of 'nomenclature' and ideas used to understand.
Your statement of "Movement of Positive Particles", smacks to me of the 'old' idea, (hence direction arrows etc in Diodes, Transistors, Valves/Tubes),
before they realized that it is actually 'Electron' flow, from -ve to +ve, and not the other way around!!  My old Instructors (about 50 years ago!), used
to try and explain it away by talking about... "A movement of Positive Holes", in the reverse direction of actual Electrons!  Here's how that worked!!...

"Put a line of blocks, (electrons!), on the table. Now starting from the right, move the last block to the right a bit. Then move the second block from the
right to join the 1st block moved. And then keep doing that with the remaining blocks in the same way".  Then they would say... "Notice how the GAP
between the blocks appears to be moving to the Left! That is the movement of Positive Holes!!".  Ggrrrr... Even then at 16, I would pull them up, and
tell them to start talking about 'Electrons', and that drawing 'standards' even 'today', should be changed !!!   

It's the definition of current in "The Art of electronics" page 2. There is no "old" or "new" definition of current. You can also look up the definition of Ampere, (and Coulomb) which is defined as X amount of charge in a second, not X amount of electrons.

[TimFox]

Once again, I suggest you go to any freshman physics textbook that discusses electrical current.
Examples I remember from freshman physics are:
1.  Flow of electrons through a vacuum from cathode to anode in a vacuum tube.
2.  Net motion of electrons or holes through a solid.
3.  Mechanical motion of charged patches on a moving insulating substrate, such as the belt of a Van de Graaff generator.
4.  "Displacement current" through a capacitor dielectric.  This is not a new phenomenon, it was discovered in the 19th Century.
5.  Net motion of charged ions (both polarities) in a liquid solution.
6.  Protons moving in a particle accelerator, accelerated by alternating electrical fields in synchrotron cavities.
etc.
Electrical current was discovered long before the discovery of the electron in the 1897 by J J Cavendish.
There is no need at this late date to redefine positive and negative charges and currents, obsoleting centuries of science (except for WW II-era electronics training in the vacuum-tube era).

[ejeffrey]

"Notice how the GAP
between the blocks appears to be moving to the Left! That is the movement of Positive Holes!!".  Ggrrrr... Even then at 16, I would pull them up, and
tell them to start talking about 'Electrons', and that drawing 'standards' even 'today', should be changed !!!   

Holes are completely OT to the question at hand which is electrochemical cells, but even if we redefined positive charge to mean electrons (which there is absolutely no reason to do), we would still have the concept of holes -- they would just be negatively charged.  Holes are not an annoying wart that we have to deal with because of the "wrong" charge convention, but an essential concept in semiconductor physics.

But in a electrochemical battery we have physical motion of charged ions through the electrolyte.  Depending on the battery chemistry these can be positively or negatively charged.

You don't have to try to reason about this from first principles or resort to analogies, you can just read about what exactly happens in a battery.

I really, really, really wish high school physics teachers would stop saying that "Ben Franklin got the conventional current definition wrong but we are stuck with it" -- this places way to much focus on electrons rather than the idea of fields and net charges which is really what matters. The fact that in metallic conductors the electrons are the mobile charge carriers is irrelevant unless you are studying solid state physics.

[TimFox]

Exactly.  Electricity was discovered and quantified long before a particle that is very important in electricity was discovered and named the "electron".

[GlennSprigg]

Aaarrgghh....  I 'get' what people are saying, but still think there is a miss-communication of ideas/concepts here...   

Old Scottish song... "I'll tak the high road and you tak the low road"... Oh well, what ever people feel comfortable with.

So far, no-one here has dared/tried to counteract what I said about a 'Capacitor', (which IS basically behaving like a battery,
but not chemically),  where there is NO actual flow of 'charge' within it, through the Dielectric, and that any 'flow' is EXTERNAL
through an attached circuit, and 'then' back to the other side of the Capacitor???  I do not grasp the problem of this thinking???

If I'm wrong, then I'll probably go to my not far off grave believing what I believe, so what does it really matter! hahaha... 

[TimFox]

"Displacement current" was discovered in the 19th Century and is included in Maxwell's Equations to explain the current through a capacitor.
Ionic current in batteries is similar to ionic current in salt-water solutions.
Current is a more general term than the motion of electrons in a vacuum tube.

[TimFox]

Here is a thought-experiment to show the importance of displacement current through a capacitor in the real world.
1.  Set up two parallel plates, each 10 cm by 10 cm, spaced by 1 cm, inside a transparent jar that can contain the sand discussed below.
2.  With air between the plates, k = 1 (dielectric constant), the capacitance will be approximately 0.9 pF.  (Even without a material dielectric, there exists a polarization field between the plates.  When we add material below, the polarization increases for a given applied voltage.)
3.  Connect an AC generator (output impedance 50 ohms) through this capacitor to a 50 ohm load and AC voltmeter.  The total circuit impedance is 100 ohms (resistive) plus the reactance of the capacitor.
4.  At 10 MHz, the reactance of the capacitor is about 18 k and dominates the total impedance of the circuit.
5.  Now, start adding SiO₂ powder (sand) to the region between the plates.  It is a good insulator, but solid SiO₂ has k between 3.7 and 3.9.  Since the powder has a lower density than the solid, assume k = 2.5.  As the sand level increases from zero to fill the gap, the capacitance increases from 0.9 pF to 2.2 pF, and the current in the circuit (measured by the voltage across the 50 ohm load) will increase by almost the same factor as the reactance decreases. 
With these parameter values, the voltage across the capacitor itself is almost constant (since its reactance is much larger than the total resistance of 100 ohms), but the displacement current increases with the increased polarization in the dielectric.  Current continuity includes the conduction current through the two resistors and the displacement current through the capacitor.
Moral:  just as there are many flavors of ice cream, displacement current is one type of the general phenomenon known as "current", along with ionic current, conduction current, beam current, etc.