TOP  |


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 grandmother.)

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 at the 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 have a 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 it's possible to mix and match the half-cells. Normally we connect the two half-cells back-to-back in series, with the water-conductors touching.

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 halts.

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 dissolving halts.

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 the 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 microscopic layer, 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 the 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 the 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 on the 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 being dissolved.

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 plates. But 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 determine their 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 etc.

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 unwanted half-cell, and its voltage is backwards. Take a zinc/water half-cell, then 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 terminals. 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 been created by the two metal surfaces inside the battery.

Want more? Try this one, about metal electrodes, energy, and misconceptions, a wbeaty QUORA.COM answer, What is single-electrode potential and its determination?

Also check out all my articles, 518 and counting.


(Natural philoso-fiction?)
Created and maintained by Bill Beaty.
Mail me at: [my email address is my website addr preceded by billb atsign].
View My Stats