BATTERIES
How do they REALLY work?
[NOTES FOR UNWRITTEN ARTICLE]
©2015 W. Beaty

How diodes REALLY work
How resistors REALLY work
How inductors REALLY work
How capacitors REALLY work

Batteries are like pumps

If an electric circuit is like a drive-belt, or like a loop of water-filled hose, then a battery is just a pump. It's a constant-pressure charge-pump. Like all pumps, the battery does not supply the liquid being pumped. Batteries take in electric charge through one terminal, pump it through themselves, then force it back out through the other terminal. Note that that's the concept called "complete circuit." A battery is a good conductor, is part of a complete circuit, and no electric charge is building up inside.

The pumps are at the surfaces of the plates!

If batteries are charge-pumps, then exactly what is doing the pumping? Corrosion! Natural corrosion provides the pumping action. Here's the one-sentence explanation: Metals in water dissolve as easily as sugar or salt, but when this metal corrosion starts up, a weird electrical effect halts it, and we can harness this dissolving-metal effect to produce a current-loop in a closed circuit. That's batteries.

Here's the longer version. Dunk copper into water and it dissolves rapidly. But then it instantly creates a bunch of volts between the copper and the water, since water is pulling positive-charged atoms away from the metal, leaving behind a sea of un-cancelled electrons, which leaves the copper strongly negative while the water becomes strongly positive. This water-copper static-electric force layer will push any further copper atoms back into the copper, and totally halts the corrosion process. (Those who are interested could google "half-cell potential.")

The water is now a "positive terminal" and the metal a "negative terminal," and if we could somehow short these together, the voltage which prevents the corrosion would reduce, the corrosion would kick into high gear, and a huge electric current would appear as positive copper atoms flooded into the water. The copper would rapidly dissolve away, its chemical energy becoming electric heating. But we can't. I mean, we can't connect water to copper and short out the voltage, since they're already touching.

So what we DO do, is to dunk some zinc (or any non-copper metal) into the same water. Let it momentarily dissolve-and-halt, which produces another voltage between water and zinc. But the zinc-water voltage is higher than the copper-water voltage. If we now touch the copper to the zinc, the zinc-water voltage gets smaller, and the copper-water voltage gets higher. A huge current does appear, since we've reduced the electrical repulsion force which had been stopping the zinc corrosion. (We've also *raised* the copper-water voltage, which *reverses* the copper corrosion and drives dissolved copper ions back onto the metal surface, electroplating it.)

That's it. That's the basic battery. Ah, we also have to add lots of dissolved copper ions to the water near the copper surface. Just dump copper chloride or copper sulfate into the water, both make the water very conductive (full of movable charged atoms,) and to guarantee that only copper is being plated back onto the copper surface, and not some other metal contaminant which would alter the usual copper-water voltage and screw things up. We don't want our copper plate to get covered with zinc!

If not charge, then what is stored?

What do batteries actually store? Chemical fuel, of course. A "battery" is just a fuel-cell, but where the fuel is stored inside. (Imagine a hydrogen fuel cell, but where the hydrogen was kept inside the cell.) A discharged battery contains waste products, and whenever we "charge" a battery, we just push the electric charges through a battery and back out again. Doing this injects chemical energy, and turns the waste back into fuel again. Or instead of using an automotive battery-charger, we can take out the car-battery plates and directly smear them with fuel: lead oxide and lead powder. (That's how manufacturers do it.)

Heh, batteries, they work like the gas tank of a car, but only if we could force the exhaust gases into cars' tailpipes while pushing the whole car backwards, in order to un-burn the waste products and re-create the gasoline. Not possible with engines, but perfectly possible with batteries. We can turn water into H2 O2 and back again. We can also turn metallic zinc into zinc chloride and back again. Or dissolve and corroed the lead, then uncorrode it again to make lead metal.

THEY DON'T STORE ANY CHARGE

Batteries don't store charge, inductors don't store charge, capacitors don't store charge, resistors don't store charge. In ALL of these components, the net electric charge remains constant, so for every bit of charge pushed into one terminal, an equal bit of charge pops out the other one. In this, batteries are no different than resistors. Note that saying "batteries store charge" is just as wrong as saying "inductors store charge." Yeah, there's electric charge involved, but it's never added or removed from these components, on average.

A lot of introductory textbooks get this wrong. They try to convince us that the plates store charge, and tell us that the current in the electrolyte is always zero, when actually the amperes in the battery acid is exactly the same as the amps in the battery cables. Batteries are very good conductors. Any accumulation of charges on the plates is immediately shorted out.

A wire is like a water pipe, but so is an inductor, capacitor, or battery. Or say it this way: inductors store magnetic energy, capacitors store electrostatic energy, batteries store chemical fuel(energy,) and none of them emit or accumulate any electric charge at all. Neither does a resistor, yet a resistor is able to take in a one-way flow of electrical energy.

Or, the only simple way to "charge" a battery is also the only way to charge a resistor or inductor: put them on top of a VandeGraaff generator! Or, inductors spin their internal charges like a massive flywheel, while capacitors stretch their charges like a spring. (Note that flywheels and springs store energy, not metal; their amount of metal is constant!) Or: capacitors store charge in exactly the same way that inductors store charge: they don't.

THE VOLTAGE-GENERATORS INSIDE BATTERIES

1. The potential-difference, the voltage in batteries is created in two 
separate spots:  the surfaces where the conductive liquid touches each 
battery plate.

The "charge pumps" which create the voltages; they are located in two 
microscopic layers, with one layer on the positive battery plate, and the 
other layer on the negative plate.  In other words, every electrochemical 
cell is actually two separate cells in series, with the electro-
lyte liquid acting as a conductive connection between them.  They're 
called "half-cells," search the www for lots of info on these.


2. Ask yourself what happens when a hunk of metal is placed in water.  It 
corrodes rapidly, right?  The water molecules behave as an extremely 
aggressive solvent, and they attack the metal surface quite vigorously.  
At the surface of the metal, the water molecules surround individual metal 
atoms and pull them loose.  But atoms in metals are in an odd situation:  
they're not neutral atoms.  Long ago they've lost their outer-shell 
electrons.  Their outer electrons became part of the "electron sea" of the 
metal.  So, whenever water grabs atoms from the metal surface, it's 
actually grabbing positive ions: metal atoms with an excess of positive 
charge.  These ions, each surrounded by a "shell" of water molecules, then 
diffuses away into the liquid.  The metal dissolves.  In theory, it should 
dissolve rapidly, almost as fast as sugar or salt crystals.

But as water molecules pull away these positive-charged atoms, the 
neutral-charged battery plate turns overall negative.  At the same time, 
the neutral-charged water turns equally positive.  It's almost like a 
VandeGraaff electrostatic generator, where the dissolving positive ions 
are the rubber belt.  As the metal ions are pulled away, they create a 
charge-separation, like a charged-up Leyden jar.  A voltage grows between 
the water and the metal.  This voltage becomes larger and larger as 
corrosion proceeds along.  Soon the voltage is big enough that it forces 
the dissolving positive metal atoms to stop leaving the metal.  The 
negative-charged metal surface is now attracting the positive-charged 
atoms in the water.  They're pulled back, and the water fails to yank them 
away from the metal surface.  And so, the corrosion process halts.  It's 
halted because the entire metal object has become charged overall 
negative, and the water charged equally positive.  (And scientists back in 
the 1700s discovered this using electrostatic force detectors, 
"Electrometers.")

Cool, eh?

In truth, metal should always dissolve instantly in water; disappearing 
almost as fast as sugar or salt.  But the "electrostatic generator effect" 
stops it.  In the everyday world, metal in water doesn't corrode like 
this.  In a tiny fraction of a second, the electrical attraction forces 
become enormous, and the corrosion halts almost instantly.  The metal is 
protected by an invisible layer of intense voltage-field.

Now the slightly-corroded battery plate has become charged to a few volts 
negative, relative to the water which is charged a few volts positive.  
Hey, what if we could use this potential-difference as a power supply?! 
After all, the dissolving of metal is a genuine electric generator.  If we 
could somehow connect it to an external circuit, then we could connect a 
light bulb.  This would slightly lower the potential-difference between 
the water and the metal.  The metal would again corrode.  The chemical 
energy would drive an electric current through the outside circuit.  The 
bulb should light up!

But try it.  It doesn't work.  

We can easily connect a wire to the piece of metal in the water.  But if 
we try to make a second connection to the water itself, we just create a 
seonc water/metal pair!  Doh!  Another "electric generator" or "halfcell" 
layer appears whenever we try to connect to the water.  But this time the 
voltage is pointing backwards, and the two voltages cancel out.  No 
potential difference on the two wires immersed in water.  The water is 
still positive.  But both metals are equally negative.

So we cannot connect wires to water?

Fortunately there's a way around this.  Suppose we first placed copper 
metal into the water. The corrosion-powered "charge pump" creates roughly 
4V between the copper and the water (with the copper being negative.)  
What if we try another metal?  Not copper?  Say, a plate of Zinc?  The 
voltage between Zinc and water is smaller than copper by about 0.9V.  The 
two half-cells still oppose each other.  But their voltages wrt the water 
are not the same.  They still subtract.  They leave behind a remainder of 
just under a volt (which turns out to be the "cell potential" of a 
copper/zinc pair in water.)  Connect the two different metal plates to 
wires, and the 0.9V should be able to run a small incandescent bulb, or a 
tiny sensitive DC motor.

But it still doesn't work!  The pure water is an insulator!  Our magical 
device is an open circuit, a turned-off switch.  To make water conductive, 
it needs to be full of movable charges.  Easily remedied: just fill the 
water with zinc salts.  Oops, but that drives zinc atoms into the copper, 
quickly covering it with zinc, so the two surfaces become the same metal.  
The battery works for a moment, then stops again.

Well, what if we put one electrode above the other, then fill the jar with 
two pools of electrolyte which have different densities?  One liquid layer 
full of zinc salts, the other full of copper?  Yes, works great!  We put 
our copper on the bottom, our zinc above it, fill the jar half way with 
dense copper salt solution, then the rest of the way with lighter zinc 
solution.  Both liquids are good conductors.  Connect it up, and we've got 
an electric energy supply, a charge-pump which is fueled by water's 
spontaneous corrosion of metal.  The zinc dissolves, and it mostly powers 
the copper plate, forcing the dissolved copper to un-corrode. A small 
remainder of energy will come out through the wires, and power our light 
bulb.  (Or our miles of Morse-code signaling wires.) This is the "gravity 
cell" or "crow-foot battery" used to power all the telegraphs of the late 
1800s.

Ah, one last thing.  The two separate corrosions are fighting, right?  
One wins.  And while one battery plate loses positive atoms and corrodes, 
the other metal plate must gain metal atoms.  It runs backwards and 
"un-corrodes."  The zinc plate gives out energy as it dissolves, but the 
copper takes it back again.  In our completed battery, the zinc electrode 
is corroding away; being dissolved to form more zinc salt solution.  This 
creates a voltage, and also an electric current (made of positive zinc 
ions) flows out of the zinc and into the water.  But at the same time, the 
copper electrode is getting new copper atoms driven into its surface.  
Positive copper atoms are going backwards; flowing out of the water and 
into the solid copper to build up new metal.  And so there's an electric 
current in the electrolyte.  The amperes of current are exactly the same 
as the electric current in the metal plates.  It's exactly the same as the 
amps in the two wires leading out to our light bulb.  The battery is like 
a short circuit, forming a complete loop including the tungsten filament 
of the bulb.  If there's a one-ampere flow of electrons in the metal 
connections, then there's always a one-amp flow of metal ions in the 
electrolyte solution.  Batteries don't store any electrons. 
Batteries don't accumulate any electric charge.  The path for electric 
current is **through** the battery, and back out again.  The electrolyte 
serves as a conductor.  It connects the two micro-thin "charge pumps" on 
the plate surfaces. The battery's voltage originates on those thin 
surfaces.

REALLY one final last bit.   Remember that thing about metals dissolving 
as fast as sugar?  We can watch that happen. Just connect the two battery 
terminals directly together.  This reduces the voltage inside the 
microscopic layer where the water touches the zinc.  Suddenly the natural 
"static electricity barrier" is lower, and the positive-charged metal 
atoms aren't driven back as they ordinarily would be.  Again the water can 
attack the zinc, and spontaneously dissolve the metal, dragging the atoms 
far away from the surface.  Our large zinc plate will corrode away in a 
matter of minutes, as if it were a big piece of salt.  If metals aren't 
allowed to charge up strongly negative, while the water charges up 
strongly positive, then any metal placed in water would fast-corrode like 
this. 

Batteries?  That's where we've figured out how to remove the natural 
electric barrier from metals immersed in water ...andthen use metals as a 
"chemical fuel" which runs a microscopically-thin charge-pump, which 
produces a voltage, and thus powers all kinds of electrical devices.