Potato vs. Lemon Battery

[ssk]

Hi Guys, we need help. My 8 year old is doing a science fair project with a friend. We made veggie/fruit batteries using a copper penny as the positive pole and an aluminum pop top as the negative pole. We tested with on a multimeter. (easy I thought... acid = high reading) BUT we tested a lemon, apple, potato, tomato and an orange. The potato made the arrow go off the meter. We couldn't get the meter to move except on the OHM meter so that was what we looked at.
What have we done?

[Louise]

Hi Guys, we need help. My 8 year old is doing a science fair project with a friend. We made veggie/fruit batteries using a copper penny as the positive pole and an aluminum pop top as the negative pole. We tested with on a multimeter. (easy I thought... acid = high reading) BUT we tested a lemon, apple, potato, tomato and an orange. The potato made the arrow go off the meter. We couldn't get the meter to move except on the OHM meter so that was what we looked at.
What have we done?

Hi!

THis is actually a more complicated project than you might expect. (As I guess you found out!) I'd make the same hypothesis you did too, though. The sciencebuddies page on this experiement points out:

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.

So, with this in mind, I have a couple of questions for you:
1) Were the electrodes equally clean/ did you do the measurement at the same time interval after insertion of the electrodes? Basically, as the electrodes get junk on them (from the electrochemical reaction) the efficiency pluments?
2) Are all the pennies from the same year? THe composition of currency has changed a few times recently, so your electrodes may be of different materials.
3) I'm not sure what you mean about the OHM meter. Are you saying you measured the resistance. If so, does the potato going off the scale mean that it had very high resistance or very low resistance?
4) Usually this project uses zinc and copper electrodes. The penny is copper plated (assuming it is post-1982), but I don't know what the pop-top is made of. It coud be zinc, and not alumnium, but I don't know.

Here is the sciencebuddies page on this project:
http://www.sciencebuddies.com/mentoring/project_ideas/Elec_p029.shtml?from=Home

There have been quite a few threads on this topic in this forum. Here are one that talks about polishing electrodes:
http://www.sciencebuddies.com/mentoring/phpBB2/viewtopic.php?t=2513&highlight=lemon+battery

The entry on wikipedia might help too:
http://en.wikipedia.org/wiki/Galvanic_cell

Good luck!
Louise

[barretttomlinson]

Check this site out:
http://www.allaboutcircuits.com/vol_6/chpt_3/16.html

It suggests potato batteries have very high internal resistance, so voltage sags badly with even a slight load.

Also aluminum is famous for forming highly insulating oxide layers on its surfaces, and I do not think potatoes are very acidic. You might do better by using zinc rather than aluminum, judging from the websites I have seen.

Hope this helps. Sorry, I do not have a 100% certain answer for you< just a best guess.

[ssk]

Thanks so much for your responses and patience. We have since redone the experiment using a zinc coated nail and a penny. And have switched to the (sorry, I barely know what I'm looking at) the scale that reads DC mA on the 0.5 setting. The other scale, as you have mentioned, OHM X1K, I'm thinking is the resistance... and the potato reading off the scale meant it went off the scale to the zero side. There is one other scale that reads DC V with a lowest setting of 5 which does not move in our experiment. So, now, the results are potato 100, tomato 50, apple 50, lemon 150, and orange 25 (could be because the orange is not juicy anymore. We appreciate your input! So fun for us to find a science community!

[Louise]

Thanks so much for your responses and patience. We have since redone the experiment using a zinc coated nail and a penny. And have switched to the (sorry, I barely know what I'm looking at) the scale that reads DC mA on the 0.5 setting. The other scale, as you have mentioned, OHM X1K, I'm thinking is the resistance... and the potato reading off the scale meant it went off the scale to the zero side. There is one other scale that reads DC V with a lowest setting of 5 which does not move in our experiment. So, now, the results are potato 100, tomato 50, apple 50, lemon 150, and orange 25 (could be because the orange is not juicy anymore. We appreciate your input! So fun for us to find a science community!

Glad to hear this worked! You can always try rolling the orange around to squish it up and see if the reading on the meter goes up.

Ohm is the resistance, so a reading of zero means it is conductive.
mA is current (a= amps, m= milli) so these readings are the amount of current produced. (DC is direct current, like batteries.)

Glad things seem to be working now.

Louise

[Craig_Bridge]

I'm going to provide a little circuit analysis here to help you appreciate some measurement issues.

When you used the Ohms scale, you were doing something you should never do to a multi-meter. Measuring resistance in the presense of an external power source is potentially damaging to the meter. Fortunately, the amount of power available from a single veggie cell was probably small enough so that it didn't damage the meter, just caused erroneous results.

When you used the DC mA scale, you were effectively shorting out the battery and determining what is called the short circuit current capabilities of the battery.

Now we need to ask the question of how many ohms/volt is the internal resistance of your multi-meter. Many inexpensive analog meters are 1000 ohms/volt which means the 5v scale is effectively a 5000 ohm load to which you have to add the internal resistance of the battery to determine how much current you need to get a reading. Based on your short circuit current readings, the internal resistance of batteries are higher than your meter. Your battery won't supply enough current to provide an accurate voltage reading with your meter.

Most DVM (digital volt meters) have 200Kohms/volt or greater so they are better choices for this experiment.

[Louise]

I'm going to provide a little circuit analysis here to help you appreciate some measurement issues.

When you used the Ohms scale, you were doing something you should never do to a multi-meter. Measuring resistance in the presense of an external power source is potentially damaging to the meter. Fortunately, the amount of power available from a single veggie cell was probably small enough so that it didn't damage the meter, just caused erroneous results.

When you used the DC mA scale, you were effectively shorting out the battery and determining what is called the short circuit current capabilities of the battery.

Now we need to ask the question of how many ohms/volt is the internal resistance of your multi-meter. Many inexpensive analog meters are 1000 ohms/volt which means the 5v scale is effectively a 5000 ohm load to which you have to add the internal resistance of the battery to determine how much current you need to get a reading. Based on your short circuit current readings, the internal resistance of batteries are higher than your meter. Your battery won't supply enough current to provide an accurate voltage reading with your meter.

Most DVM (digital volt meters) have 200Kohms/volt or greater so they are better choices for this experiment.

Thanks for the info. I was confused why the Volts selection did not seem to work. When would you use the DC mA setting?


Louise

[Craig_Bridge]

When would you use the DC mA setting?

The DC mA setting is used to measure current. The internal resistance of meters on the current scales is very small so you usually need to supply some resistance or other load to prevent shorting out something like a battery. In the case of the veggie batteries, the internal resistance of the battery is high enough so that shorting it out is not a problem. Don't try this with normal batteries as they are designed to have a much lower internal resistance and will overheat, produce gas, and potentially explode.