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Abstract

Objective

The goal of this project is to make batteries from fruits and vegetables using metal electrodes. You will use a digital voltmeter along with resistors and other loads to determine the voltage, current, and power that your batteries can produce.

Introduction

Batteries are like mini power plants that derive electrical energy from chemical reactions. You can make batteries with some pretty simple everyday materials. In general, all you need are:

• two different kinds of metal to act as electrodes (though not just any kind of metals will work),
• a liquid solution, called the electrolyte, which will react chemically with the metal electrodes, and
• a way to conduct the electricity from the metal electrodes to something that is using the energy that the battery provides.

Different kinds of batteries will have different characteristics. Some produce different voltages than others—like a flashlight battery at 1.5 volts and a car battery that is typically about 12 volts. Some can deliver a lot of current, and some will deliver less current. You'll learn more about voltage and current as you work on this project, but as you might already know, some things won't work at all unless the battery can provide a high enough voltage. Once this voltage is applied some things will draw more current from the battery than others. Current is a measure of how many electrons are flowing per second. The more electrons that flow per second (or the higher the current) the faster the battery will discharge. Also, if the item that your are trying to power with the battery tries to draw two much current then the voltage of the battery will drop and again the item might not work.

Many batteries are made up of more than one battery cell, also called a voltaic cell. When these voltaic cells are hooked up in series (see Figure 1, below), the voltage of the battery becomes the sum of the voltages provided by each cell. Car batteries typically have six cells, each producing about 2 volts, which added together provides a 12-volt battery. (This is why you see six little caps on most car batteries, allowing you to add water to each of the six cells.) The battery below is made up of 4 1.5 V cells in series, producing 6 V total.

Figure 1. Pictorial (top) and schematic (bottom) diagrams of batteries connected in series. Connecting battery cells in series increases the total voltage available. The total current available remains equal to the current of a single cell.

If a battery or a voltaic cell doesn't provide enough current, you can connect a number of batteries or cells together in parallel (see Figure 2 below). This keeps the total voltage the same, but now the total current that can be provided is the sum of the currents from each of the cells. Another reason to connect more cells or batteries together in parallel is so that they will power an item for a longer time before discharging. You'll learn more about this as you work on this project.

Figure 2. Pictorial (top) and schematic (bottom) diagrams of batteries connected in parallel. Connecting battery cells in parallel increases the total current available. The total voltage available remains equal to the voltage of a single cell.

Probably one of the most interesting things about batteries is the way that different materials and the way in which they are used can affect the characteristics of the battery. This means they can affect the output voltage and the amount of current that the cell can deliver. They can also affect something called the "internal resistance" of the battery. A battery cell made with a potato might provide a different amount of current than a battery cell made with a lemon or an onion. Battery cells made with different electrode materials, like copper, nickel, or zinc might produce different voltages. Batteries with different electrode shapes or surface areas might have different internal resistances. You will learn that the way the battery cells are made and connected with each other will determine if you can generate enough voltage and current to run a portable radio, a digital clock, or whatever small electronic device you choose to try.

So, go to the grocery store, buy some fruits and vegetables and then have some fun!

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• voltage, current, resistance, energy, and power;
• parallel and serial connections of power sources (batteries) and loads (resistors);
• Ohm's law;
• operation of a volt/current meter (also called a multi-meter);
• resistor values and color codes;
• batteries and the chemistry of batteries.

More advanced students should also study:

• voltage dividers,
• internal resistance of a battery,
• Kirchoff's laws, and
• ionic bonds.

Bibliography

• This site gives a good history of the invention and development of batteries:
  Buchmann, I., 2001. "Chapter 1: When Was the Battery Invented?" Batteries in a Portable World [accessed April 24, 2006] http://www.buchmann.ca/chap1-page1.asp.
• This is a great site with a lot of basics on electronics:
  Hewes, J., 2006a. "Welcome to the Electronics Club," The Electronics Club [accessed April 24, 2006] http://www.kpsec.freeuk.com/index.htm.
• Brain, M., 2006. "How Batteries Work," Howstuffworks.com [accessed April 24, 2006] http://science.howstuffworks.com/battery.htm.
• Hewes, J., 2006b. "Ohm's Law," The Electronics Club [accessed April 24, 2006] http://www.kpsec.freeuk.com/ohmslaw.htm.
• The colored stripes on resistors are a code that tells you the value and the tolerance of the resistor. This reference has more information:
  Phillips, W.D., 1999. "Resistors: Colour Code," Design Electronics [accessed April 24, 2006] http://www.doctronics.co.uk/resistor.htm#colour.
• Nave, C.R., 2006. "Voltage Divider," HyperPhysics, Department of Physics and Astronomy, Georgia State University [accessed April 24, 2006] http://hyperphysics.phy-astr.gsu.edu/hbase/electric/voldiv.html.
• Here are two webpsites with information on internal resistance. (Note: the first reference is a 3.3 MB pdf file and may take awhile to download):
  • DOE, 1992. "Internal Resistance," U.S. Department of Energy, Fundamentals Handbook, Electrical Science, Volume 2 of 4, p. 12 [accessed April 24, 2006] http://www.earth2.net/parts/doe/h1011v2.pdf.
  • Energizer Holdings, Inc., 2005. "Battery Internal Resistance," Energizer Technical Bulletin [accessed April 24, 2005] http://data.energizer.com/PDFs/BatteryIR.pdf.
• An online source for information on Kirchhoff's Laws is:
  Burley, I. et al., date unknown, "Kirchhoff's Laws," Introductory Physics Notes, University of Winnipeg [accessed March 14, 2006] http://theory.uwinnipeg.ca/physics/curr/node8.html.

Materials and Equipment

Here some basic materials that can be used. However, you should use your imagination and try some others.

1. Electrodes: The easiest materials to use with vegetable and fruit batteries are probably zinc-coated nails and copper wires. Nails are easy because you can just stick them into the fruit. It is also possible to push coins into some vegetables, like a potato.
   • about 20 large (~3 in long) zinc-coated ("galvanized") nails;
     Note: you can get zinc coated nails at almost any hardware store where they are usually referred to as galvanized nails.
   • galvanized metal squares (optional);
     Note: you can get small rectangular pieces of galvanized sheet metal at the hardware store. These can be cut with tin-snips into strips approximately 1/2 in × 3 in that will make even better electrodes than nails.
   • 3 feet of 12 gauge bare copper wire.
     Note: You will be cutting this wire into segments of about 3 in to use as copper electrodes. As with the galvanized metal, if you can find small sheets of copper and cut them into 1/2" in × 3 in strips, these will make even better electrodes.
2. Wires and connectors to hook up the various electrodes:
   • 10–20 jumper leads (with alligator clips), available at Radio Shack. Note: in many cases you can use the alligator clips to connect up to the battery terminals of an electronic device you'd like to try. If you need additional connector(s) for your electronic devices, Radio Shack is a good place to try.
3. Fruits and vegetables to provide the electrolyte liquid:
   • potatoes,
   • citrus fruits,
   • onions,
   • whatever!
   You will probably need more than one of each. In some cases you can cut them into multiple pieces to make more than one cell.
4. Various loads to hook up to the batteries:
   • various resistors (you can get a pack of assorted resistors at Radio Shack),
   • other loads.
     Note: These are things like battery-operated timers, calculators, low power buzzers, 3-volt radio, etc. For example, Radio Shack has a package of 3 calculators for $7; a piezo buzzer, 3–28 V, 5 mA for $4, and an ear-plug FM radio for $2.) It is best to look for items that run at 3 volts or less. Also, look for things that draw less than 0.5 mA (500 μA). These will be things that run on small watch-like batteries. It is best to use only things that you're willing to destroy in the process. Some things, like the buzzer mentioned above, will work "partially." The buzzer works; it just isn't very loud.
5. Tools:
   • voltmeter or multimeter,
     Note: you will need a volt meter or multi-meter. If you don't have one or cannot borrow one, they range in price from about $14 to several hundred dollars. Radio Shack has them, as do most hardware stores. You can get analog or digital meters. The digital meters are easier to read, but are a little more expensive. The lower-priced meters will work pretty well for this experiment. In general, look for a meter that will measure volts and resistance. If you can find a meter that measures current on a 200 μA scale this could be useful, but it is not really necessary. In general, the price starts to go up as you look for meters that can measure smaller amounts of current.
   • wire cutters,
   • tin snips (optional),
   • soldering iron and rosin-core solder (optional).

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Experimental Procedure

Safety note: do not eat the fruit or vegetables that have been used to make batteries!

To get you started, here are two simple experimental procedures to make one- and two-cell batteries. You'll see how to increase the available voltage by connecting individual cells in series. There are many more ideas in the Variations section (below) to get you thinking about experiments you can design for yourself.

Experiment 1

How much voltage can be generated using a zinc-copper potato cell? How does the voltage change as you hook up different loads (values of resistance) across the terminals of the potato battery?

1. Create a voltaic cell by poking one zinc (galvanized) nail and one copper wire (3 in) into each end of a potato. The buried ends of the electrodes can actually be pretty close together (maybe an inch apart), but they should not touch each other. The zinc nail will become the "−" or negative terminal of the battery (also called the anode) and the copper wire will become the "+" or positive terminal of the battery (also called the cathode).
2. Hook up the voltmeter across this voltaic cell by connecting the "+" terminal of the voltmeter to the copper wire and the "−" or "common" terminal to the zinc nail.
3. Measure and record the voltage. This voltage is referred to as the open-circuit voltage, because this is the voltage present when nothing is connected across the terminals of the battery—the circuit is "open". (For more information on using a multimeter, see the Science Buddies page How to Measure Voltage and Current.)
4. While leaving the voltmeter connected across the battery, use the wire jumpers to connect a 10 kohm resistor across the battery terminals. Record the voltage.
5. While leaving the first resistor in place, connect another 10 kohm resistor across the battery. Record the voltage.

You will note that the voltage dropped as you connected the loads (the resistors) across the battery. This is because current is being drawn through each of the resistors and the total current that is drawn also has to flow through the internal resistance of the battery. For this simple "veggie" battery cell the internal resistance is pretty high, so a noticeable portion of the battery cell's voltage is dropped across its internal resistance. This in turn reduces the amount of voltage that you measure at the terminals of the battery.

Experiment 2

How much voltage can be generated using two zinc-copper voltaic potato cells hooked up in series? Can this be used to power up something like a calculator or a low-voltage (ear-plug) transistor radio?

1. Create two separate cells like the one in Experiment 1, step 1, above. If you are using large potatoes, you can cut them into pieces and then use each piece as a separate cell.
2. Measure and record the open-circuit voltage of each of the cells.
3. Using jumper leads hook these two cells up in series to make a two-cell battery. (Note: Figure 1 in the Introduction shows how to connect cells in series.)
4. Measure and record the open-circuit voltage of the two-cell battery.

You should find that each individual cell has an open-circuit voltage of about 0.75 volts. When you connect the cells in series and measure the open-circuit voltage of the two-cell battery you should measure about 1.5 volts (maybe a little less). Now this is a battery that you can actually use to power any electronic device that is designed to operate at about 1.5 volts and that does not draw too much current. You will find that this means it will probably only power up things that will run on small watch-sized or calculator-sized batteries.

On Your Own

Working with veggie batteries is a lot more fun and interesting if they actually do something! Try powering up a calculator (see Figure 3, below) or maybe a small buzzer or low-voltage transistor radio.

Figure 3. Running an electronic calculator on veggie power! This is an illustration of a multi-cell veggie-power battery, described in Experiment 2, above.

There are even talking greeting cards that you might be able to find. You might need to get some help opening up the back of the calculator or other device and getting wires attached to the electrodes that normally hold the battery. (If you find that you need to solder a connector, the Science Buddies resource Electronics Primer: How to Solder Electronic Components has some helpful tips on using a soldering iron and making good, lasting connections between electrical components.) See the Variations section (below) for more ideas to get you thinking about experiments you can design for yourself.

Troubleshooting the Veggie Power Circuit

Sometimes, in hands-on projects like this one for making a veggie battery, something happens and you can't explain why. Or, you were expecting something to happen and it didn't. In both of these cases, you will have to troubleshoot what you have put together in order to figure out how to fix it. Here is a list of things to think about when troubleshooting this particular electrical circuit.

1. Copper forms an oxide layer that can prevent accurate electrical measurement. Try scrubbing the entire length of wire with a Scotch-Brite pad to remove the unwanted oxide layer.
2. Clean and dry the fruit or vegetable prior to use.
3. Make sure that the electrodes are completely inserted into the fruit or vegetable and that they are not touching each other.
4. Place the electrodes an inch apart.
5. If you are trying to power a device use a multimeter (for tips on using one, read Science Buddies' Electronics Primer: Using a Multimeter) to measure both the voltage and the current the device needs. Make sure your veggie battery circuit can supply enough voltage and current.
1. In order to increase the amount of voltage supplied by the veggie battery, connect the vegetable or fruit cells in series mode. See Figure 3 to understand how to connect the cells in a series correctly. Notice that the copper wire of the first cell is connected to the nail of the second cell. In this case, there is one path for current flow, and that is from one cell through the other.
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2. In order to increase the amount of current supplied by the veggie battery, connect the vegetable or fruit cells in parallel mode. See Figure 4 to understand how to connect the cells in parallel correctly. The copper electrodes are all connected, and the nails are all connected.
3. It is possible to create a circuit where the cells are in both in series and in parallel. You may want to look this up if you are having trouble getting enough current and voltage.

Figure 4. In this image, three lemon cells are connected in parallel. Avoid having the cells touch.

6. If you have several cells connected and you feel that something is not correct, take everything apart and test each cell separately with the voltmeter or multimeter.
7. If the battery cells are connected properly (supplying enough voltage and current), and the device that you have connected to the battery is still not working, try reversing the electrodes to the device. It could be that the negative/positive polarity is not correct.

Troubleshooting

For troubleshooting tips, please read our FAQ for Veggie Power!

Variations

There are many factors that can affect the performance of batteries and many variations can be done on the simple experiments suggested above. Here are some things you might think about to lead you to other variations that you can devise on your own. These are not listed in any particular order. They are just a number of different things you might think about or try.

1. How does varying the amount of load (resistance placed across the battery terminals) affect the voltage and current output from the battery?
2. By applying different loads on a battery can you determine its internal resistance? (See the references on internal resistance in the Bibliography, above.) What do you think you could do to lower the internal resistance of veggie cell battery? (For some ideas, see Variations 3, 4, and 5 below.)
3. With a particular set of electrodes and electrolytes, the battery voltage should be about the same (when driving high-resistance loads). Are there things that can be done with the configuration of a voltaic cell to change the amount of current that it can deliver before the voltage drops considerably? Does the distance between the electrodes matter? If so, why? Does the surface area of the electrodes in contact with the electrolyte matter? If so, why?
4. How do other materials affect the characteristics of the battery? For example:
   • What happens if you use other kinds of fruit or vegetables?
   • What happens if you use different metals for the electrodes?
   • Would the battery work with just vegetable juice? How about just vinegar or salt water?
5. Increase the number cells that you hook up in parallel, or in series, or both. How does this affect the voltage and current that the battery can deliver? Can you predict based on the characteristics of a one-cell battery how a given configuration of similar cells hooked up in series or parallel will behave? Could you build a multiple-cell battery that would power up a flashlight light bulb? How about a 9-volt transistor radio? How many cells in parallel and/or series would this take? (Remember for something like a transistor radio you have to provide both the right voltage and sufficient current.)
6. In general, given the specifications (voltage and current requirements) for a load (the thing that you are trying to power up with the battery), can you predict how may cells (of a given configuration) it will take to power the load?
7. How much power can a particular veggie cell or battery produce?
8. How long does it take for a veggie battery to run out of "juice" (electrical juice that is)? Why does it run out?
9. Using fruits and vegetables (or pieces of them), can you think of other physical configurations that will make good batteries? How about a little piece of potato sandwiched between a penny and 1-inch squares of galvanized (zinc) sheet metal? Can you stack up these penny-potato-zinc cells to create a higher voltage battery?

• For more science project ideas in this area of science, see Energy & Power Project Ideas.

Credits

Written by Craig Sander

Edited by Andrew Olson, Ph.D., Science Buddies and Michelle Maranowski, Ph.D., Science Buddies

Last edit date: 2012-01-13 12:00:00