The process shown here uses a lemon, copper in the form of a penny, and zinc in the form of a drywall anchor. Although neither of these metal items is pure they have enough copper and zinc on their surfaces to work, and they are readily available items anyone can find.
Simply insert the penny and drywall anchor into the lemon as shown. These become your positive and negative terminals.When hooked to a volt meter, as shown, the cell measures 0.4V. What is happening is a chemical reaction at the copper along with a chemical reaction with the zinc in the lemon acid. So there are 2 reactions working together and each one is called a “half-reaction”. Each set of metals in electrolyte (lemon juice in this example) is a single cell. As long as the lemon is in one piece and the metals are not touching, we have 1 battery cell.
Although the lemon battery shows voltage the current is far too low to support even the simplest current demands. To find out how to increase the current to be able to power for example a light bulb, or what chemical reactions are happening inside the cell, keep reading. If however, all you want to do is make a lemon cell, you can stop reading.Here are the reactions in a lemon cell:
With the zinc: Zn → Zn2+ + 2 e-
At the copper: 2H++ 2e- → H2
What we want a battery to do is have electrons to flow through a wire from which we can utilize to power things. In each of the reactions in the lemon cell, the e-minus indicates electrons that are removed from the zinc to make a zinc ion and these electrons go over to the copper where they take hydrogen ions and make dihydrogen. All chemical cells work in a similar way. Other metals and electrolytes can be used, but one-half of the half-reactions will create an ion and the other half will reduce an ion. In our lemon cell, these ions will be flowing through the electrolyte (the lemon's acid) while the electrons are flowing through the wire on our voltmeter. It happens spontaneously if the circuit is complete but the reaction stops almost completely when ions cannot be made because no electrons are flowing through the wire.
Thus, any chemical reaction creating an ion that can be coupled via ion transfer with another reaction that reduces an ion can create a battery cell. The potential difference in the ion creation/ion reduction determines the amount of power the cell will have. Dissimilar metals are ideal for this, and there are also some other non-metal materials that can do the same thing. That’s why there are a number of different kinds of battery cells. Each cell has different reactions which have different properties; so we can apply the right properties as required to get the job done.
You don’t see too many copper-zinc-lemon acid batteries in the world actually powering anything. So let’s look at our lemon cell again. What properties does it have that we would want to use it in our test? It’s got common materials that are easy to get and put together. That’s it. Well, actually that and it makes a lovely twist in an after-test refreshment (remove the metal parts for best results). But a lemon cell doesn’t deliver a lot of current, and the voltage is rather low compared to other battery cells even when it is at its maximum possible voltage. In fact, if you look around the internet you’ll find a number of people that made lemon cells with voltages higher than 0.4V. Many of them got 0.8V or higher. Looking at the possible differences in these tests can give us more insight into the world of batteries. So what are they doing that we aren’t?
Maybe they use bigger lemons? No, that's not the reason, and I’ll explain why. We can determine just how high the voltage in a lemon can get. We can even determine the theoretical voltage of any cell by looking at the half-reactions and the differences in their potentials. Chemists have made tables with values for different metals and materials. Looking at the tables we can theoretically make a battery cell up to about 6V using different materials. And the highest theoretical voltage for a cell using water-based electrolyte is a little over 2V. The theoretical max is about 1.1V for a lemon cell. So that’s the highest possible voltage you’ll see in a lemon cell and factors that affect the voltage will be the purity of the materials, both metals and electrolyte, and placement (construction). Maximizing the reaction is the key to the highest possible voltage. Maximizing the reaction is what the other lemon battery experimenters were doing to get a higher voltage. What we could do to get better voltage is to wait for the materials to settle in and have the greatest surface area available for the reaction, use more pure metals, or perhaps break up the inside of the lemon to get the electrolyte to flow better (or maybe some lemons are better than others).
So doesn’t lemon size matter? If we had a lemon as big as a truck and sheets of pure copper and zinc the size of picture windows, wouldn’t the voltage be even a little higher? Nope; the best you’ll see is still 1.1V. But aren’t there batteries, like powertool batteries or car batteries that are higher than 6V? There are. That’s the difference between a battery and a cell. If cells are attached to each other in series, their voltages add together. Put those in-series cells together in one container and you have a battery. So if we get 100 lemons and 100 pennies and 100 drywall anchors we could put each cell together in a row, each with ones drywall anchor hooked to the next one’s penny until they are all hooked together, except the first one’s penny and the last one’s drywall anchor. If we check the voltage of this chain at that first penny and last anchor, we could theoretically have 100 x 1.1V, or 110V! Wow, that’s like house plug voltage! Couldn’t that be dangerous? No, not really. Hooking all those lemon cells together adds voltage, but not current. And the current delivered by a lemon battery is tiny, as will be shown in a moment.
There is another way to hook the cells together; in parallel. When we put our 100 cells in a row, but this time we hook all the pennies together and all the drywall anchors together the voltage would stay at 1.1V. So what good did all that slicing and poking do if all we get is the same voltage we started with? We got capacity. It’s just a little harder to see. If we ran our lemon cell down, using a device that ran on less than a volt, and measured how much energy we got out of it before it went dead, we could call it “1 lemon worth of energy”. Our 100-lemon battery hooked in parallel would also run that 1-volt device, but now we can run it 100 times as long (all things being equal) extracting all 100 lemons worth of energy. This is, in effect, making a bigger single cell. That truck size lemon may have had only 1V too, but it would run for a very, very long time. But there is something else putting together those lemons in parallel would do for us. Let’s look at an example to demonstrate the power of a parallel connection. Can our lemon battery light a 1V light bulb? A bulb for a Mag Solitaire flashlight (it runs on 1 AAA battery) will glow with 1V, and even a little less. However, even if we get the full 1.1V out of our lemon battery, it won’t do anything at all to Solitaire bulb. The reason is that the battery is pushing enough voltage through the bulb, but the bulb is trying to get more amperage from the battery than it can supply. Another way to say it is “the big hose is full of water, but the water just isn’t moving”. However, with enough lemons in parallel, or with that truck-sized lemon, we’ll have no trouble getting that flashlight bulb to light. It’s just a matter of getting enough reaction going to supply the electron needs of a lit bulb.
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