An iconic classic, the potato battery has been found in classrooms, in kitchens, and yes - even on the 80’s television series “MacGyver”. From a nutritional standpoint, the potato is sometimes hailed as a miracle food. Is there something special about potatoes that make them a miracle power source? No! The potato only serves as the “salt bridge” in a simple galvanic cell (battery). Replace it with any other suitable medium, including many fruits, and it works just as well.
To be an effective salt bridge, the fruit (or potato) must have an interior that provides an uninterrupted pathway of water and dissolved salts or acids. Since the interior of living cells is largely water and dissolved chemicals (like salts and acids), this requirement is easily met, though some fruits will perform better than others. (Some fruits will rot more quickly than others; some have higher moisture/salt contents, etc.) Experiment at will, but lemons (which contain a large amount of citric acid) are quite acidic and perform well. Apples and pears are similar in texture to potatoes, and would be good choices for additional experiments.
Beyond the salt bridge, the battery needs a pair of electrodes. The electrodes must be made of different metals. The traditional (and easily obtained) pairing is copper and zinc. You can use copper wire, a penny, or any other scrap of copper you may have as the copper electrode. Similarly, anything made from zinc or coated with zinc will work for the opposite electrode. The easiest source is anything made from galvanized steel (or galvanized aluminum). “Galvanized” indicates that the metal has been coated with zinc to protect it from rusting. Since only the zinc is exposed, it works as a zinc electrode. Galvanized nails are probably the easiest material to find in most houses.
Insert the two electrodes into the fruit, but leave enough sticking out so that you can attach a wire or an alligator clip to it, without the wire coming into contact with the fruit. (If you used copper wire for the copper electrode, you don’t have to worry about attaching more wire; just use a long piece of the wire for both connecting wire and electrode.) Make sure that the two electrodes do not come into contact either inside or outside the fruit. (If they do, you create a short, and get no battery power.) A smaller separation between the electrodes is better than a large one - start with about an inch (or a couple centimeters) between the two and then experiment to optimize battery performance.
Connect the two electrodes to your clock, LED bulb, or other electronic device using wires or alligator clips. Connect the copper electrode to the positive (+) terminal of whatever you are trying to power. Connect the zinc electrode to the negative (-) terminal. With a little luck, you’ll have enough electrical potential (or juice - if you care for puns) to run the device as is. Your fruit battery produces a very weak voltage, however, and may not be able to do it alone.
To strengthen your battery, you can string several pieces of fruit together in series. The voltages add together, essentially creating one larger battery. Each piece of fruit needs its own copper and zinc electrode pair. Connect (using wires or alligator clips) the copper from each piece of fruit to the zinc of the next piece to form a continuous chain. Finally connect the two ends to the device as before (copper to positive, zinc to negative). With enough fruit, you should be able to power most low-current applications. (For a large-scale potato-powered example, read about this vehicle’s sound system.) For a science fair project, you might try re-charging your cell phone with lemon-power.
Copper and zinc aren’t the only metals that can be used for electrodes. They’re normally used because both are easily obtained, rusting isn’t a problem, and they work well. The key to finding metals that work well is to look at their “standard reduction potentials”. There’s no need to go into the science behind reduction potentials here, just know that each metal has one, and that it is either a positive or a negative number. For any pair of metals, the difference between their standard reduction potentials is a measure of the relative voltage that they can produce. The greater the difference, the more powerful the battery you can create from a piece of fruit.
Here’s a listing of some metals and their standard reduction potentials, as listed on the website http://www.chemguide.co.uk/physical/redoxeqia/ecs.html (Oct. 2010). (Metals that react readily with moisture have been omitted here.) The values were measured in volts.
Aluminum (Al): -1.66
Zinc (Zn): -0.76
Iron (Fe): -0.44
Lead (Pb): -0.13
Hydrogen (H): 0.00
Copper (Cu): +0.34
Silver (Ag): +0.80
Gold (Au): +1.50
Copper and zinc are rather close in this list; the difference in their potentials is only 1.10 V. Sadly, the overall reaction isn’t really between zinc and copper. The reaction is between zinc and hydrogen (difference = 0.76 V), with the hydrogen supplied by the acid in the fruit. This means that the only way to get more power (based on this list) is to replace the zinc with aluminum (difference = 1.66 V). Replacing the copper with silver or gold won’t change the hydrogen reaction; it will just make your battery more expensive. For any metal pair you choose, the electrode with the more negative standard reduction potential should be connected to the negative terminal of the device you are powering. Similarly, if you have to connect multiple fruit cells in series, always connect the more positive electrode of one fruit to the more negative electrode of the next. If some of your fruit have zinc and others have aluminum, it will still work.