How to Turn a Potato Into a Battery
AbstractImagine telling your friends about your latest science project: using a battery to make a light turn on. You might get some blank stares...sounds a little boring and basic, right? Now tell them you will do it with a potato! Yes, you can actually use fruits and vegetables as part of an electric power source! Batteries power many things around you, including cell phones, wireless video game controllers, and smoke detectors. In this science project, you will learn about the basics of battery science and use potatoes to make a simple battery to power a small light and a buzzer.
Ben Finio, Ph.D., Science Buddies
Recommended Project Supplies
Measure how the voltage and current of potato batteries change when you combine them in series or parallel..
Batteries are containers that store energy, which can be used to make electricity. This method of storing energy allows us to make portable electronic devices (imagine what a pain it would be if everything had to be plugged into a wall outlet to work!). There are many different types of batteries, but they all depend on some sort of chemical reaction to generate electricity. The chemical reaction typically occurs between two pieces of metal, called electrodes, and a liquid or paste, called an electrolyte. It turns out that the moisture inside a potato works pretty well as an electrolyte, so you just need to add some metal electrodes to a potato, and you have a battery! While you do not need to understand the details of the chemical reaction to do this project, it is explained in technical note at the end of this introduction.
Next, you need to understand some basic concepts about electricity. The flow of electricity is called an electrical current, which is measured in a unit called amperes (A) (also called amps for short). Voltage, measured in volts (V) is what pushes electrical current through wires. Finally, electrical resistance, measured in ohms (Ω) (the capital Greek letter Omega) is a measure of how difficult it is for electricity to flow through a certain material. A common analogy for electricity is to imagine water flowing through a pipe. The amount of water flowing is like the current. The pressure pushing the water is like the voltage. The resistance is like the size of the pipe—it is harder to squeeze a lot of water quickly through a very tiny pipe than through a big pipe. Does that seem like a lot to remember? Table 1 summarizes voltage, current, and resistance.
|Current||Ampere (A)||The "flow" of electricity|
|Voltage||Volt (V)||The "pressure" that makes electricity flow|
|Resistance||Ohm (Ω)||How hard it is for electricity to flow through something|
An electrical circuit is a path through which the electricity can flow. Circuits can be very complex, with millions and millions of components (like the ones inside your computer), or very simple, with just two components, like a battery and a lightbulb. This project will focus on simple battery-powered circuits. In general, a battery supplies a certain voltage to a circuit. How much current is drawn out of the battery depends on the load, or what the battery is connected to.
Batteries have positive and negative terminals. In order for electricity to flow in a battery-powered circuit, there must be a complete path from the positive terminal, through the load, and back to the negative terminal. This is called a closed circuit. If the path is broken (for example, if one wire is disconnected), electricity cannot flow. This is called an open circuit. Finally, if there is a direct path from the positive to the negative terminal, this is called a short circuit. Short circuits are bad because they can cause batteries to drain very quickly and overheat (luckily, potato batteries can only supply a small amount of current, so short circuits in this experiment are not dangerous). Figure 1 shows diagrams of open, closed, and short circuits.
Figure 1. Closed, open, and short circuits. Electrical current is represented by the yellow arrows. In the open circuit, no current flows at all. In the closed circuit, current flows through the lightbulb, causing it to light up. In the short circuit, current flows directly between the battery terminals, bypassing the lightbulb, so it does not light up.
What about circuits that have more than just a single battery? You have probably used many devices that require two or more batteries, like toys or remote controls. Multiple batteries can be connected two different ways: in series or in parallel. When multiple batteries are connected in series, the positive terminal of one battery is connected to the negative terminal of the next battery (and this repeats if there are more than two batteries). When batteries are connected in parallel, all of the positive battery terminals are connected together, and all of the negative battery terminals are connected together. These two configurations are shown in Figure 2.
Figure 2. Batteries connected in series and parallel. Electrical current is represented by the yellow arrows.
So why would you choose one method over the other? The amount of voltage and current that can be supplied by multiple batteries changes depending on whether you connect them in series or in parallel, and certain electronic devices might require a certain amount of voltage or current. For example, have you ever noticed how a small device like a TV remote or computer mouse might only require two AAA batteries, but a larger toy or flashlight might require four or more AA batteries? This is because each device has different electrical requirements to operate properly.
You can measure how much voltage or current a certain number of batteries can provide by determining the batteries' open-circuit voltage and short-circuit current. A battery's open-circuit voltage is the voltage across a battery's terminals when it is not attached to anything. This is the highest voltage that a battery can supply (the voltage will drop slightly when the battery is attached to a load). The short-circuit current is the current when the battery's terminals are shorted together. This is the highest current the battery can supply (the current will also drop when the battery is attached to a load). How exactly do the voltage and current change when your batteries (potatoes) are configured in series or in parallel? In this project, you will use a multimeter, a device that can measure electrical circuits, to find out.
In a battery, chemical energy is converted into electrical energy. In general, electrical current consists of the flow of electrons, which are negatively charged particles. In a potato battery, the electrical energy is generated by two chemical reactions that happen at the electrodes (the copper and zinc metal strips). Because copper is more electronegative than zinc, it tends to attract electrons more easily than zinc. Therefore, electrons in a potato battery will flow from the zinc electrode, the anode, to the copper electrode, the cathode. But where do these electrons come from? The electrons are derived from the zinc metal. When zinc is in contact with the electrolyte, the following oxidation reaction takes place which results in the release of electrons:Equation 1 (zinc electrode):
While the the zinc joins the electrolyte as a positive ion (Zn++), the electrons flow through the wire connecting the electrodes (Figure 3 shows an LED connected between the electrodes, but as you will see in the procedure, this could also be a multimeter or a buzzer). Here, positively charged hydrogen atoms, or protons (H+), that originate from acids inside the potato (phosphoric acid and organic acids), accumulate as they are attracted to the negative charge created by the surplus electrons at the copper electrode. These protons take up the electrons from the copper electrode in a reduction reaction to become neutral hydrogen atoms and subsequently form hydrogen gas (H2) which you might see as bubbles evolving around the copper electrode:Equation 2 (copper electrode):
This reaction leaves behind a shortage of electrons at the copper electrode, which is the key reason why electrons keep flowing from the zinc to the copper electrode. The battery keeps running until it either runs out of protons if there is not a lot of acid present, or the zinc electrode is "eaten up" and completely dissolved into ions. The overall net reaction in a potato battery can be summarized as:Equation 3 (net reaction): (Hydrogen gas and electricity)
Note that the potato itself is not the sole energy source for the battery, but functions as an electrolyte to facilitate the transport of relevant ions such as zinc cations and protons that originate from organic acids and phosphoric acid that are present inside the potato.
Figure 3. Diagram of the chemical reaction that occurs in a potato battery, with an LED connected between the electrodes. Note that the arrows representing electron movement in this figure point in the opposite direction as those that represent electrical current in Figures 1–2. For more information about sign conventions for electric current, read this technical note.
Terms and Concepts
- Chemical reaction
- Closed circuit
- Open circuit
- Series circuit
- Parallel circuit
- Open-circuit voltage
- Short-circuit current
- What are the basic parts of a battery? How do batteries generate electrical current?
- What is electrical current? What is its unit of measurement?
- What is electrical voltage? What is its unit of measurement?
- What is electrical resistance? What is its unit of measurement?
- What are the differences between open, closed, and short circuits?
- How is the flow of electricity similar to the flow of water?
- If one vegetable or fruit battery is not enough to power a buzzer, what could you do to resolve the problem?
- Science Buddies. (n.d.). Electronics Primer: Introduction. Retrieved June 18th, 2013.
- Science Buddies. (n.d.). Multimeter Tutorial. Retrieved June 18th, 2013.
- The Physics Classroom. (n.d.). Current Electricity - Chapter Outline. Retrieved June 18th, 2013.
- Brain, M., Bryant, C., and Pumphrey, C. (n.d.). How Batteries Work. Retrieved June 25th, 2013.
For help creating graphs, try this website:
- National Center for Education Statistics, (n.d.). Create a Graph. Retrieved June 25, 2020.
Materials and Equipment
Recommended Project Supplies
- Veggie Power Battery Kit, available from our partner
Home Science Tools. Includes:
- Copper electrodes (3)
- Zinc electrodes (3)
- Alligator clip leads (6)
- Digital multimeter with test leads
- Piezoelectric buzzer
- Red light-emitting diode (LED) (3)
- You will also need to gather these items, not included in the kit:
- Potatoes (3), any large type like a russet. Make sure your potatoes are fresh. Old, dried out potatoes will not work well.
- Paper towels for cleanup as you prepare the potatoes
- Lab notebook
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- Insert one copper and one zinc electrode into each of the potatoes, as shown in Figure 4. Use a ruler to make sure you space the electrodes the same distance apart and insert them to the same depth in each potato (for example, 2 cm apart and 3 cm deep. The exact distances you pick may depend on the size of your potatoes).
Figure 4. Copper and zinc electrodes inserted into a potato. The copper electrodes are engraved with the letters "CO" and the zinc electrodes are engraved with "ZINC".
- Prepare a data table like Table 1 in your lab notebook.
|Series or Parallel||Number of Batteries||Open-circuit Voltage (V)||Short-circuit Current (mA)||Lights the LED |
|Powers the Buzzer
- Measure the open-circuit voltage of a single potato battery, as shown in Figure 5. Refer to the Science Buddies resource How to Use a Multimeter if you need help using a multimeter.
- Set your multimeter dial to measure in the 20 V range.
- Plug the red multimeter probe into the port labeled VΩmA.
- Plug the black multimeter probe into the port labeled COM.
- Use a green alligator clip to connect the black probe to the zinc electrode.
- Use a red alligator clip to connect the red probe to the copper electrode.
- Record the voltage in the first row of your data table.
- Refer to the Help section if you get stuck or have trouble taking a reading.
Figure 5. How to set up your multimeter to record the open-circuit voltage. Note: The multimeter screen has been blurred in the image on the left. We do not want to give away the data!
- Measure the short-circuit current, as shown in Figure 6.
- Leave the multimeter probes and alligator clips connected as they are.
- Change the multimeter dial to measure in the 20 mA range.
- Quickly record the current in your data table. The current will start to drop as the battery begins to drain.
- Important: Do not connect the multimeter to regular batteries (for example AA or 9 V) with these settings. Regular batteries can provide much more current than a potato battery, and can damage your multimeter. Refer to the How do I measure current? section of the multimeter resource to learn more about measuring current safely.
Figure 6. How to set up your multimeter to record the short-circuit current.
- Test if the potato battery can light up the LED, as shown in Figure 7.
- Disconnect the alligator clips from the multimeter probes (leave them connected to the copper and zinc electrodes).
- Connect the red alligator clip to the longer lead of the LED.
- Connect the green alligator clip to the shorter lead of the LED.
- Important: Current can only flow through LEDs in one direction. It is important to connect the copper electrode (positive electrode) to the longer lead of the LED, and the zinc electrode (negative electrode) to the shorter lead. Your LED will never light up if it is connected backwards.
- Record in your lab notebook whether or not the LED lights up.
Figure 7. How to connect the LED to your potato battery. Pay attention to how you connect the long and short leads of the LED.
- Test if the potato battery can power the buzzer, as shown in Figure 8.
- Disconnect the alligator clips from the LED.
- Connect the red alligator clip to the buzzer's positive (red) wire.
- Connect the black alligator clip to the buzzer's negative (black) wire.
- Important: The buzzer functions similarly to the LED. It has positive and negative pins, and it will not work at all if it is connected backwards.
- Record in your lab notebook whether or not you can hear the buzzer.
Figure 8. How to connect the buzzer to your potato battery. Pay attention to the positive and negative labels on the pins.
- Now connect two potato batteries in series, as shown in Figure 9, then repeat steps 3–6.
Figure 9. Two potato batteries connected in series. Use an extra alligator clip to connect the zinc electrode of one potato to the copper electrode of the next potato, and move the original green alligator clip to the second zinc electrode. This image shows the multimeter, but you can replace it with the LED and buzzer, as described in steps 5 and 6, respectively.
- Connect three potato batteries in series, as shown in Figure 10, then repeat steps 3–6.
Figure 10. Three potato batteries connected in series. Again, use alligator clips to connect the zinc electrode of one potato to the copper electrode of the next potato, and connect the black multimeter probe to the last zinc electrode using an alligator clip, forming a chain.
- Copy the data from the first row of your data table (Series - 1 potato) to the fourth row of your data table (Parallel - 1 potato). Remember that you need at least two potatoes to actually make a series or parallel circuit. This just makes it easier to graph your data later.
- Connect two potato batteries in parallel, as shown in Figure 11, then repeat steps 3–6.
Figure 11. Two potato batteries connected in parallel. Use one extra alligator clip to connect the copper electrodes of both potatoes, and another extra alligator clip to connect their zinc electrodes.
- Connect three potato batteries in parallel, as shown in Figure 12, then repeat steps 3–6.
Figure 12. Three potato batteries connected in parallel. Use two alligator clips to connect all of the copper electrodes, and two more alligator clips to connect all the zinc electrodes.
- Repeat the entire procedure (steps 1–11) two more times, for a total of three trials. Create a new data table for each trial. Remove and re-insert the electrodes into new locations on the potatoes each time. Remember to refer to the Help section if you have trouble at any point during your experiment.
- Analyze your data. Refer to the Create a Graph if you need help making graphs.
- Create a fourth data table for average values. For each configuration (for example, two batteries in series), calculate an average open-circuit voltage and short-circuit current across your three trials. These are the values you will use for your graphs.
- Make a line graph of open-circuit voltage vs. number of potatoes. Draw one line for series and one line for parallel. Make sure to include a legend on your graph so you know which is which.
- Make a similar graph for short-circuit current.
- How do voltage and current change in each graph? Are the lines different for series and parallel connections? Is this what you expected based on your background research?
- How much voltage and current does it take to power the LED? Is there a certain voltage or current below which the LED will not turn on?
- How much voltage and current does it take to power the buzzer? Is there a certain voltage or current below which the buzzer will not turn on?
- Cleanup: Dispose of the potatoes in the trash. Do not eat the potatoes after using them for this experiment.
Now that you are done with your project, you might be wondering if you can power something bigger than an LED or a buzzer. Can you use a potato battery to power a lightbulb or charge a phone? There are many videos online claiming that you can. Based on your results, do you think those videos are real? If you are not sure, watch this video for more information:
For troubleshooting tips, please read our FAQ: How to Turn a Potato Into a Battery.
Ask an Expert
- Repeat the experiment with different fruits and vegetables, such as apples, onions, oranges, or lemons. How do their open-circuit voltages and short-circuit currents compare to potatoes?
- Hook the battery up to a load (like a resistor, a buzzer, or an LED) and measure its voltage over a long period of time. How long does it take for the battery to drain? Is this time different for the resistor, the buzzer, or the LED?
- "Recharge" a dead potato battery by soaking it in water and repeat the experiment. Investigate how this works. Compare your results.
- Do a new experiment where you change the distance between the copper and zinc electrodes, and measure the effect of this distance on current and voltage.
- Do a new experiment where you change the amount of surface area of the electrodes that is embedded in the potatoes. How does this change the current and voltage? Hint: You can fit almost the entire electrode into a potato if you push it in lengthwise.
- Can you find a mathematical formula to predict the voltage and current delivered by combining potatoes in series or in parallel? If so, can you make different combinations, like two batteries in parallel combined with a third battery in series, and test your formulas on these more-complicated scenarios?
- If you were able to power an LED with your veggie battery, try putting two LEDs connected in series, or two LEDs connected in parallel, or a combination of an LED and a buzzer. Do certain combinations work and others not?
- What happens if you do the experiment with smaller (or larger) potatoes, or cut a potato in half? Does that change the amounts of current or voltage that are generated? What about the time it takes for the battery to drain?
Frequently Asked Questions (FAQ)
- For voltage, start with the 20 volt (V) scale. If the measured value turns out to be less than 2 V, you can move down to the next lowest scale for improved accuracy. On the DT830L multimeter that comes with the Science Buddies kit, these scales are labeled "20" and "2000m" on the upper left part of the dial. 2000m stands for 2,000 millivolts (mV), which is equal to 2 V.
- For current, start with the 20 milliamp (mA) scale. If the measured current is less than 2 mA, you can move down to the next lowest scale for improved accuracy. On the DT830L multimeter that comes with the Science Buddies kit, these scales are labeled "20m" and "2000μ" on the right side of the dial. 2000μ stands for 2,000 microamps (μA), which is equal to 2 mA.
If you need help using a multimeter or if this is your first time using one, you should ask an adult for help, or refer to the Science Buddies resource How to Use a Multimeter.
First, as you probably discovered during your science project, a couple of potatoes can at best produce only a few milliamps of current. Consumer electronic devices like cell phones and MP3 players typically require hundreds or thousands of milliamps to charge. So, it would take dozens, if not hundreds, of potatoes to produce enough current to charge something like an iPhone.
Second, as explained in the Introduction, in order for a chemical reaction to occur and produce electricity, the electrodes must be two different types of metal. In this experiment, those electrodes are copper and zinc. However, the pins on a USB plug are only one type of metal. So, even if a single potato could somehow produce hundreds of milliamps of current, there is no way for a chemical reaction to occur simply by inserting a USB plug. No chemical reaction means no current.
Finally, electronic devices typically require a current and voltage that are well regulated in order to charge properly. For example, USB-charging devices are designed to require 5 V, and the current may vary depending on the exact device. Cell phone chargers that plug into a wall are specifically designed to provide the right voltage and current. Not only can hooking up a device directly to an unregulated power supply prevent it from charging properly, but it can also damage or even destroy the battery. So, even if you could somehow get around the first two problems above, it would not be a good idea to charge an electronic device directly from a potato, without some external protective circuitry.
For a further debunking and explanation of this myth, see the HowStuffWorks article Can you power an iPod with an onion?
Your Veggie Power kit should contain a 1,000 ohm, or 1 kilo-ohm (kΩ) resistor; a small, tan-colored cylindrical piece with two metal wires sticking out of it. You can use this resistor to test your multimeter as described here and shown in Figure 13.
- Set your multimeter to measure current in the 20 mA range (the dial setting labeled "20m" on the right).
- Plug the multimeter's black probe into the port labeled COM.
- Plug the multimeter's red probe into the port labeled VΩmA.
- Use a red alligator clip lead to connect the multimeter's red probe to the positive (+) terminal of the 9 V battery.
- Use a green alligator clip lead to connect the multimeter's black probe to one of the resistor's leads.
- Use another green alligator clip lead to connect the resistor's other lead to the 9 V battery's negative (-) terminal.
- Your multimeter should read about 9 mA (maybe slightly less if you are not using a fresh battery).
- If this works, then you know the current measurement function on your multimeter is working. If you are still having trouble with your experiment, the problem is with something else in your setup. For example, you might have your electrodes or alligator clips connected incorrectly to your vegetables. See the other troubleshooting steps for more ideas.
- If this does not work, and you are confident that you set up the test correctly as shown in Figure 13, then your multimeter might be defective or have a blown fuse. Please contact firstname.lastname@example.org for assistance.
- When you are done, disconnect the alligator clips so you do not drain the 9 V battery, and remember to turn your multimeter off.
Figure 13. Setup for making sure the multimeter has a working fuse.
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