Fruit Batteries

What would work better in making a battery out of a vegetable/fruit? A lemon, a potato, or an apple?
- Anonymous

[see correction below] Batteries work by moving and storing charges - a lot like electricity. In order for a fruit to be a battery, it needs to be able to conduct electricity. You may have learned about acids in school - if you haven't, acids make ions, or charged particles, when put in a solution like water. These charged particles from acids are the same type as the charged particles in electric current, so the fruit that will make the best battery is the one that is most acidic. Another general thing about acids - they tend to taste sour. The stronger the acid in the fruit, the more sour it will taste. Since a lemon is the most sour of the three, it will make the best battery. [or so we once thought]
-[name deleted]

(published on 10/22/2007)

That answer is not correct. A potato delivers a higher power (more Watts) than a lemon in both parallel and series circuits. I just did an in depth project in my Biological Engineering class at the University of Arkansas on biological batteries and I tested both lemons and potatoes. The potato always produced more power than the lemon. This means the potato is a better battery than the lemon.
- Wes Carr (age 19)

It sounds like your experiments show that the previous answer was not right.

The reasoning in the old answer was very sloppy. Several parts of the description of the role of electrical charges didn't even quite make sense. The factors which play a role in producing power include the acidity (which plays a role in the chemical reactions at the electrodes), the presence of various other ions (acidity just involves H⁺ ions), and the permeability of the cell walls, which can block currents. That’s not intended to be a complete list. As a result, it’s unlikely that some simple theory will predict well which complicated material (lemon, potato, ...) will produce the most power.

Whether your answer (more power from the potato) is correct in general or just for particular types of potatoes, etc. is another question.

Anyway, thanks for the correction. I've left the exchange in to inspire other critics.

Mike W.

Lee H
 

(published on 10/22/2007)

I have found that these fruit produce electricity in this order from greatest to least
apple,orange,lemon,grapefruit,---this appears opposite of what they should have in regards to Ph correlation ----------Can you explain?????
- dave
lincoln il

That’s an interesting result, consistent with other things we’ve heard. I can’t explain it offhand, but will update this post if somebody does figure it out.
Mike W.

(published on 10/22/2007)

While it is true that some batteries are acidic such as car batteries it is also true that some batteries are alkaline such as current commercial batteries (size AA, C, D etc.) for electronics. Could this be linked to the ph reversal of the fruits inthe above question? Is the alkaline referred to in these commercial batteries more than just marketing?
- Robert Escalante (age 41)
Houston, Texas, USA

Alkaline batteries really do have different chemistry than acidic ones. There’s a useful Wikipedia entry: .
I guess you're right that in an alkaline solution the simple zinc-copper battery could have the same reaction as a commercial alkaline battery:
Zn(solid)+2OH^-  ->Zn(OH)₂(solid)+ 2e^{-
rather than the reaction}
Zn(solid)  -> Zn⁺² (aqueous)+ 2e-.

However, you'd have to check the reaction potentials to see what's realistic in, for example, a potato.

Mike W.


 

(published on 10/22/2007)

Do take in mind that you can NEVER read something on Wikipedia and assume it's true w/o searching another site to back it up. Wikipedia can be changed by people who don't know what their typing about (or do know, but do it for fun. Once, someone had written that their school had blown up. They later got expelled and got their wiki account removed). Anyway, did you double check?
- JMtB03 (age 14)
Evansville, IN, USA

You make an excellent point. Wikipedia is reasonably reliable, but an answer that is good at the time we link to it can be changed to one that's incorrect. We hope that our readers understand this limitation. (I've made perhaps a couple of dozen Wikipedia modifications, and one of them was wrong, but quickly fixed by someone else.) If you know of any particular problem with that link, or any other one we've linked to, please let us know.

Mike W.

(published on 12/06/2011)

I`m makeing a frut powered rc car but I half to get 1.5v what can I use and how mach?
- noah (age 13)
OHIO

(published on 01/13/2012)

Thanks for being so open to discussion on the fruit battery idea. I want to chime in on the lemon battery/acidity/voltage theme. My son did his SF project this year on this topic, to specifically test the relationship between pH levels and voltage production. We tested the pH levels of six fruits: apples, lemons, oranges, strawberries, watermelon and bananas. We were quite careful, I think: We made purees of each fruit using equal amounts of fruit and water, tested pH levels using strips and also a pH meter, and repeated each test three times (ie made three batches of purees and tested pH levels on each batch). Then we tested voltage levels using a voltmeter. There was absolutely no correlation between acidity and voltage production. Apples were relatively low in acidity, yet created the highest voltage. Aside from why that is so (we theorized that it has something to do with saturation/concentration of metals in the fruits, and that the ash content in apples creates a particular strong charge), I find it problematic that so many educational sites suggest the lemon battery as a science fair project, and either state or imply that there is a meaningful relationship in this context between acidity and voltage. It is misleading and then requires unlearning.
- Hina (age 41)
Austin, TX

Many thanks for this info.

Mike W.

(published on 02/18/2012)

I never was a "science guy" and really don't understand the principals behind physics, chemistry, etc. However, my son had to prepare a SF project for third grade - he was interested in making a fruit battery and hypothesized that acid was a necessary ingredient to make a battery. That said, we experimented with various fruits and liquids including lemons, strawberries, apples, soda pop, tap water, filtered water (with and without baking soda added) and canola oil. In the end, every one of these items, with the exception of the canola oil, produced enough current to light the LCD watch. In the end would we be correct in concluding that acid is not the key ingredient but rather pH is what causes the zinc to release electrons and start the battery in motion? Would this explain why water works (because it contains chlorine) and why canola oil doesn't (even though it contains ALA omega-3 fatty acid)? Thanks for your help as I hope to inspire my son to like science a bit more than I did and I want to make sure I'm not starting him down the wrong path with incorrect answers! :)
- Scott (age 45)
Michigan

Let's start with the definite part of the answer. Canola oil is a mostly non-polar solvent, in which ions are almost insoluble. I don't believe even those "fatty acids" in it dissociate much into ions in this oily environment. The Zn, Cu etc. just don't go into solution, so there's no reaction to drive the current. With almost no mobile charges to carry current, it's also an insulator. So it can't form a battery, and can't even be included in the circuit.

All your other items were mainly water, with various extras. In them, the different propensities of Zn and Cu to dissolve as ions cause a voltage to build up, driving a current if the circuit is completed. The experience of everybody who has written in, as well as my own limited tests, indicates that pH has little effect on the voltage of these batteries. Since saying the solution is "acid" means pretty much the same thing as saying the pH is low, it looks as if the acid part doesn't play a big role. We were all surprised by that, but those are the facts. I guess the remarks in some of the answers above about alkaline reactions may have something to do with these results.

Here's how you can test if that chlorine matters. Use some distilled water (available at the grocery store) without chlorine. You could try then adding some baking soda or some clear vinegar  to change pH without adding chlorine.

Mike W.

(published on 01/27/2013)

I tried this science fair project, I used apples,pears and oranges, but I had a problem. when I connected each fruit to the LED (light emitting diode) the LED just wouldn't produce light which created a problem in the experiment,I tested the LED with a battery,it worked. I tested wire and the alligator pins, they worked.I tested the copper and zinc electrodes and they worked.so my only option to be a problem in the experiment was the fruit, I went to the store and bought fresh good fruit, I repeated the experiment but the LED just wouldn't produce light. So I bought a new LED, one that needed the least amount of energy possible.tested it.....it didn't work.I can't think of anything else that could be wrong.Could anything else be wrong with the experiment?
- michelle (age 11)
davie,florida,usa

Do you have access to a voltmeter? You could check that the voltage is high enough, and see how it drops when you connect the LED. Have you checked that the LED is connected with the + and - leads on the right electrodes?

My guess is that your battery just wasn't supplying enough current even for those little diodes. You might try using ten copper electrodes and ten zinc electrodes, wiring all the copper ones together and all the zinc ones together. Keep the copper ones spaced far apart from each other, and the same with the zinc, so that the ion currents inside the fruit  from different electrodes don't overlap too much. Don't put the copper and zinc very far from each other. You may need several pieces of fruit.

Mike W.

(published on 02/18/2013)

I would like to set up a fruit battery demonstration, and a similar effect where elementary school students put their (sweaty) hands on dissimilar metal plates (http://www.exploratorium.edu/snacks/hand_battery/index.html). Please could you advise me on what kind of ammeter to use. When I look at user manuals for typical multimeters, some say that they cannot be used to display current continuously but should be used to measure for 15 seconds then allowed to recover for 15 minutes. I would prefer a continuous display of current. Additionally, the “hand battery� produces about 100 microamps, which is low for most multimeters.
- Neil (age 55)
Boston, MA, USA

You should be able to get a good digital multimeter (DMM), that can measure down to 10μA or less for about $50. I believe the business about only doing short current measurements only applies for large currents. The small currents you're measuring should be fine for continuous measurements.

Mike W.

(published on 03/08/2013)

Hello, I have a question, for a chemistry project we tried to determine which fruit is the best for a battery,we measured the Ampère and Volt. As most people our hypotheses was that the lemon would produce to highest numbers because of it's Ph. However the apple won. Now we have to come up with reasons why the apple produced more and why this experiment doesn't rely on Ph level. Does anyone know the answer?
- Joep van Walsum (age 17)
Netherlands

Joep- Your experience is similar to those of our other readers. We're happy to post your question in the hope that someone will explain the result to all of us.

Mike W.

(published on 04/23/2013)

I think the ans. is too simple. Lemon/orange have chambers with tough impermiable wall(AS we squeeze lemon)-leads high RESISTANCE.But potato tissue is permiable(as in potato osmoscope) cause low resistance and less voltage-drop.is n't?
- X (age 22)
Kolkata,WB,India

That's quite possible. One of our earlier answers discused the permeability issue. One could do various tests, such as comparing lemon juice with a lemon.

Mike W.

(published on 07/26/2013)

Hi, I don't believe the issue of why an apple battery emits higher voltage is because of the permeability of the electrolyte within the fruit. I say this because my 9 yr old daughter ran each fruit through a juicer and extracted just the electrolyte from a lemon, and from an apple, she then taped the top of the clear glass container and punctured the tape with a copper wire measuring 2" and an inch apart from the copper she inserted a 2" galvanized nail. Her voltmeter reading was still higher with her apple battery than her lemon battery. but the issue of permeability was put to rest of us.
- Loretto E (age 40)
Riverside CA

That's a wonderful experiment! Thanks for settling that question.

We still don't know the reason, but you and your daughter eliminated one of our wrong ideas.

Mike W.

 

(published on 11/01/2015)

On #5 The diagram is a bit odd.I believe that the currant wouldn't finish because the curants are all facing the same direction.This next part isn't about my question:how do you paste a picture?
- James A (age 13)
Lake butler,Florida,U.S.A

It's ok because the currents from all the batteries go through the RC car as they loop around.

Unfortunately our question-asking function doesn't allow posting pictures. It's very limited because allowing more flexibility makes it easier for hackers to mess up the site.

Mike W.

(published on 09/15/2016)