Why doesn't this potato battery work?
May 18, 2010 6:23 PM   Subscribe

Why isn't my son's science fair project working? His classic potato battery isn't producing any electricity.

So my 10-year-old wants to make some batteries with different kinds of fruit and veggies for the 4th grade science fair. He sanded a penny to make it bright, inserted the penny and a galvanized nail into the potato, and -- nothing. Same with a lemon. We definitely are using galvanized nails, the penny and the nail are not touching, the multimeter is working fine, so why is this not working? Please help us Mr. Wizard!
posted by LarryC to Technology (24 answers total) 5 users marked this as a favorite
Is your penny really copper?
posted by Jimbob at 6:26 PM on May 18, 2010

What year is the penny? The composition of pennies has changed over time, it's plausible that that might have something to do with it.
posted by edwardvielmetti at 6:26 PM on May 18, 2010

This may be a completely insane answer but if the penny is dated after 1983, it's mostly zinc. Can you make sure you have a real copper penny (1982 or earlier) and try again and see if it matters?
posted by jessamyn at 6:27 PM on May 18, 2010

Also a good opportunity to check around the house for other things that might be made out of copper. Could be a good "side note" to the science presentation! A sort of "Did You Know...?" thing off to the side!
posted by carlh at 6:31 PM on May 18, 2010

Just tried some older pennies and no luck.
posted by LarryC at 6:35 PM on May 18, 2010

Do you have any scraps of copper pipe lying around that you could try?

Otherwise you may need to recharge your potato.
posted by backseatpilot at 6:40 PM on May 18, 2010 [3 favorites]

How moist is your potato? You need the potato to make good contact and conduct electricity. That's your electrolyte, so it has to be pretty juicy.
posted by misha at 6:47 PM on May 18, 2010 [1 favorite]

Test your setup on a lemon: if it works the potato is the problem....
posted by Rumple at 6:52 PM on May 18, 2010

Gotta be a fresh potato. Try different potatoes, and different types of potatoes.
posted by Cool Papa Bell at 6:57 PM on May 18, 2010

Are you sure the multimeter is setup correctly? There are normally three potential places to plug in the two leads: one for measuring voltage, one for measuring current, and one common. Similarly the knob can be turned to measure voltage, current, or resistance. You want both of them setup for voltage.
posted by Rhomboid at 7:09 PM on May 18, 2010

I don't know how much current a potato battery usually provides, but if you are using an old-fashioned VOM (analog unpowered multimeter) it may be loading the potato too much to register.

If you are using a VOM, try a VTVM or FETVOM (analog meters with internal amplifying devices made to reduce load on the measured circuit) or a good quality digital VOM. I say this because I find it odd that you are getting nothing at all. I'd guess you'd get some voltage no matter how bad your materials are.
posted by fritley at 7:14 PM on May 18, 2010

Tried a lemon and several juicy potatoes, no luck.

I leaning towards the multimeter being the problem. It is this Sperry SP-10A Pocket-Sized Analog Multimeter and I have it set at 1.5 volts DC. I tested it on a AA battery and the needle kicked right up--so I thought that could not be the problem. But maybe it is.
posted by LarryC at 7:17 PM on May 18, 2010

Since it's powered that model should have sufficient input resistance to not load the potato too much. Have you tried both combinations of polarity?
posted by Rhomboid at 7:22 PM on May 18, 2010

Yes, tried both models of polarity.

In one of the videos that my son and I watched a second grader made a potato battery. We should be able to do this.

I just called the neighbor and he has a digital multimeter but won't be home for a few hours.
posted by LarryC at 7:27 PM on May 18, 2010

Rhomboid is mistaken: the single AA battery in your VOM is used only for measuring resistances. The voltage measurement is unpowered/unamplified. I am really pretty sure the meter is the problem.
posted by fritley at 7:28 PM on May 18, 2010

Thanks everyone. Wonderboy and I are going to keep working on the project as if the batteries were working and when the neighbor gets home we will borrow his digital multimeter. I'll let you know what happens.
posted by LarryC at 7:42 PM on May 18, 2010

It is the meter! We hooked up a tiny LED and it definitely lights up. Success!

My son has a word:

Seriously the difference between a digital and needle omg lol so funny (and kinda annoying :( )
posted by LarryC at 8:16 PM on May 18, 2010 [4 favorites]

posted by jessamyn at 9:34 PM on May 18, 2010

If your battery is capable of delivering enough current to light up a LED, it should have no problem at all persuading a VOM - even a crap one like that little Sperry - to show a nominally open-circuit voltage.

Use the 10 DCV setting on the range switch. If you use the 1.5V B.TEST setting, the meter will probably be switching in a dummy load that's more than your potato can drive.
posted by flabdablet at 9:44 PM on May 18, 2010

Heh. Just looked up the specs for that meter (manual here). 2kΩ/V? And they're charging $100 for it?!

As a guide, any decent multimeter should be at least 20kΩ/V. Outside of certain specialty uses, or $10 junk from the local auto / junk stores, I haven't seen an analogue multimeter below 20kΩ/V for 30 years or so. By comparison, I can buy this here piece of crap with 1MΩ/V impedance (that's 1000kΩ/V, or 500x that of the cheap Sperry meter) for under AU$15…

(Actually, calling that Digitech meter crap is a little unfair. For the price, they're not a bad little meter.)

On second look: the 1.5v & 9v ranges on your Sperry meter are battery test ranges, which means they're designed to test batteries with a low-resistance load. Try it on the 10v range & see how it goes…

(Or, on preview: what flabdablet said!)
posted by Pinback at 10:02 PM on May 18, 2010

Hey, on 10v I get a reading! Thanks again, friends. My son is so excited about his first science fair, it is so cute and wonderful to watch him.
posted by LarryC at 10:52 PM on May 18, 2010

What is that reading, just out of interest? And how many potato cells do you need to wire in series to get your LED to glow?
posted by flabdablet at 3:42 AM on May 19, 2010

The reading is about half a volt for one potato or lime. If we shut the lights off we can see a tiny glow in the 2.5v bulb with one potato.
posted by LarryC at 3:43 PM on May 20, 2010

At the end of the science fair he wired about 8 or ten fruit batteries together and got 8 volts!
posted by LarryC at 5:03 PM on May 21, 2010 [1 favorite]

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