How does a battery determine its
output voltage?WJ Beaty, 12/2017
Where does battery voltage actually come from?
When I finally learned the answer, I was stunned, because none of my
engineering or chemistry college textbooks explained it. (They only
presented equations describing it. They obviously didn't understand it
themselves, since they couldn't explain it to Einstein's
Battery voltage is determined by micro-thin layers on the two battery
plates. All batteries always contain two separate independent batteries
inside them (well, they're actually called half-cells, not batteries.) One
of these appears at the surface of the positive plate, while the other is
surface of the negative. The "terminals" of each half-cell are a metal
wire on one side, and an electrolyte-filled volume on the other (perhaps
use a water-hose terminal instead of a water tank.) For example we can
zinc half-cell with zinc chloride solution, a lead
half-cell with acid, lithium
half-cell, carbon, etc. Each metal creates its own special voltage, and
possible to mix and match the half-cells. Normally we connect the two
half-cells back-to-back in series, with the water-conductors
So, actually your question is, inside a battery, how does each half-cell
decide on its own voltage?
Their voltage is based on water dissolving a metal. Water turns out to be
an extremely corrosive solvent. Usually we only notice this when we put
some sugar or salt into water. The water molecules attack the solid
surface, pulling out atoms/molecules and carrying them off into the water.
When we put metals into water, the same thing happens. Water will dissolve
any metal object within seconds, because the water molecules aggressively
pull out all the metal atoms and carry them away. Metals dissolve about
as easily as sugar or salt.
Hey, wait a minute. When I dip a screwdriver into water, sure, maybe it
corrodes after days or weeks, but it doesn’t vanish in minutes as if it
were made out of rock-salt. Yes, you're right, and the reason it doesn't
dissolve is weird and fascinating ...and electrical.
When you stick a screwdriver into water, the water starts vigorously
dissolving it. Iron atoms are yanked away, almost as fast as dissolving
rock-candy. But metals are weird. All of their atoms had originally lost
an electron when the metal first formed. The screwdriver is actually made
of positive-charged iron atoms immersed in a "sea of charge" composed of
movable electrons. So, whenever water dissolves the iron, it yanks away
the positive-charged atoms. When this happens, the water becomes
positive-charged, and the screwdriver becomes negative. A voltage appears
between the water and the metal, and it grows quickly larger.
Then, when the voltage has risen to a certain value, the dissolving
Think about it. If the dissolving atoms are always
positive-charged, then water cannot dissolve a strongly
negative-charged metal! The unlike charges attract each other. As
positive-charged iron atoms are dragged from
the screwdriver, a very strong electrostatic attraction will slowly
grow. The screwdriver progressively charges negative, and attraction with
positive particles grows
larger and larger as the water takes away more and more plus-charged
atoms. Quickly the attraction between negative metal and positive ions
wins out. The aggressive water is temporarily defeated, and the
Next, if water should take too
many iron atoms, the increased attraction pulls them back in. Or, if some
iron atoms wander back to the screwdriver, then the overall charge of the
water and metal get less, and this lets the water steal some more iron
atoms. Finally things even out, the voltage between metal and water
becomes constant, and the metal surface neither corrodes nor re-deposits.
(Or actually it grows and shrinks simultaneously, with a bit of dissolving
matched with some un-dissolving.)
As this voltage-attraction is preventing the metal from dissolving, a
particular value of voltage always exists between the water and the metal.
This voltage was created by the removal of positive atoms from the metal
surface. This voltage entirely exists across a micro-thin layer; right at
surface where water touches metal. After all, the ion-filled water is a
good conductor, and so is the metal. The water and the metal serve as
terminals connected to the micro-thin layer. And except for the
these two conductors would be touching! The water forms one "half-cell
plate," and the metal is the other plate. It's much like a charged
capacitor (it's a self-charged capacitor, powered by dissolving metal.
A constant-voltage self-charged capacitor.)
Notice that the actual "battery" is just a few molecules thick. But
also it's very, very wide, since it's composed of the entire surface of
metal plate inside the battery. Inside the battery-layer, the few volts is
concentrated across a few nanometers, producing titanic fields and
electrical forces (with field strengths of millions of volts/meter, near
breakdown-voltage where lightning bolts and glow-discharge may occur.)
Exotic things can happen down inside there. For further research, go
search on the 'Helmholtz double-layer.' That's the micro-thin film where
all the battery-chemistry and capacitor-physics is happening.
As you might guess, different metals have different attractions between
their internal atoms. Metals have their atoms bonded into
crystal lattices, and the different strengths of the crystal-bonds depend
type of metal. They resist water's aggressive attack by different amounts.
When dissolved by water, they'll rise to various different voltages before
the corrosion is halted. The exact voltage depends on the type of metal
Every half-cell voltage was created by chemical corrosion; by the water
aggressively attacking the metal surface and dissolving it away. The nasty
water molecules tear out the positive-charged metal atoms and transport
them away, out into the water. The water ends up positively charged, while
the metal becomes equally negative.
What if we could reduce that voltage between the water and the metal?
What if we could make it zero? In that case the water would again start
corroding the metal. The water would turn dark with dissolved metal, heat
quickly to boiling, and the metal surface might even melt or start
burning. The energy released by dissolving metal is very large,
about as large as when metals are burned in oxygen. And we can instantly
turn this energy-production on by lowering the voltage.
You can see where all this is going. A battery dissolves one of its metal
then it doesn't heat up. Instead, it uses this corrosion to produce a
voltage between metal and water. And, if some sort of electric device is
connected to the battery, then rather than heating up the water, the
energy flows to that external device, powering it. Connect a light bulb to
the battery, and the bulb lights up. The battery is still "burning"
internally: its metal is corroding away. But, rather than heating up the
water, instead the energy is all being used to produce a voltage and a
current: electric force and motion, where nearly all the energy goes into
the light bulb, and very little is wasted in heating up the water or the
battery-plates. (The incandescent bulb gets hot, but the
combustion-products in the battery stay cold! Pretty amazing, no?)
Heh, what if we could convert some flames directly into electrical energy?
Well, that's what batteries have always been doing, ever since they were
invented. But they do it slowly, and the internal fire stays cool, so that
we'd never notice that the battery plates are actually "burning." For
example, inside your old-style zinc D-cells, the zinc plate is 'oxidized'
into zinc chloride, exactly as if the zinc metal had been placed into a
tank full of chlorine gas, then lit on fire. The energy stored in metals
is quite enormous (similar to energy stored in un-burned wood or
gasoline.) But when zinc under water is silently turning into dissolved
zinc chloride, we have little intuitive gut-feel for the enormous energy
released. Not like flaming zinc powder exploding in a chlorine atmosphere.
OK, back to the original question: how do entire batteries
voltage? So far we’ve only explained how half-cells can do this. There's
always a certain zinc/water voltage, or a larger lithium/water voltage
Unfortunately there's no way to connect two metal voltmeter terminals to a
half-cell inside a battery.
Sure, we can connect one wire to the zinc plate. But how do we connect our
second wire to the water? There's a genuine voltage down between the water
and the zinc surface. But we have no simple way to get at it. The problem
is, if we touch any metal wires to the water, we create a second
half-cell, and its voltage is backwards. Take a zinc/water half-cell,
touch a zinc wire against the water. Now we have two zinc/water
half-cells. They're pointing in opposite directions (water positive, zinc
negative.) Significant voltage appears between the water and the zinc. But
in between the two zinc metal wires, the voltage is canceled out
to zero. The
voltmeter reads wrong. It cannot get at the genuine voltage which exists
between water and metal.
To solve this problem we just do as Luigi Galvani originally did (also
Alessandro Volta.) They used two different metals. Touch copper and iron
to the same dead wet salty frog, the dampness corrodes the metals, and the
two metals create two different voltages. Now touch the two metals
together. This greatly reduces the water/metal voltage on one metal
surface, and one surface starts corroding rapidly. Positive metal atoms
flood into the frog's conductive slime. This flow of positive charge is an
electric current. Now we have a genuine battery. But it's shorted out,
since the two metal plates are touching together. Inside the wet frog,
currents and voltages appear, which triggers the half-dead nerves and
makes the muscles twitch.
Here's something my texts never mentioned. Notice that, when two metals
are touching the same water, one metal is dissolving because it's voltage
was lowered. But the voltage at the other metal surface became higher,
and that half-cell is being "driven backwards." Yes, it's un-dissolving
or 'electroplating,' and consuming part of the energy being released by
the dissolving plate. In other words, both battery-plates aren't supplying
energy. Instead, the corroding plate always emits energy, and the other
plate always intercepts a significant amount! Fortunately the two voltages
across the half-cells aren't exactly equal. That means one plate puts out
energy, while the "backwards-chemistry-plate" is consuming a bit less.
Chemical energy gets moved from one battery plate to the other, while the
small remainder can be used by any devices outside the battery.
And finally, whenever we connect two different metals to some water, their
two voltages subtract. Their difference appears across the battery
And that's where the output-voltage of a battery comes from. We don't
measure the actual water/metal voltages produced by each battery plate.
Instead, we only see the
difference between the pair of water/metal voltages that had
by the two metal surfaces inside the battery.
Want more? Try this one, about metal electrodes, energy, and
misconceptions, a wbeaty QUORA.COM
What is single-electrode potential and its determination?
Also check out all my
518 and counting.