Sunday, January 10, 2010

Chapter 1 - Exercise 5

Experiment 5 is a fun one... using lemons to create your own battery to try and light up an LED. Definitely brings back memories of old kitchen table science experiments when I was younger...

I bought 2 lemons, cut them up into halves, and then proceeded to wire up three halves at first as shown on page 32. I took a voltage reading - about 1.4v. I then wired up my LED into the mix and... no luck. I tried about 3 other LEDs from the little bag I purchased, but none of them would give even a flicker. (It is very possible, as the author mentions, that these might not be low-current LEDs... even though I'm hitting the voltage, the current may not be enough to light 'em up.)

So, I tried to wire in the remaining half - two full lemons. I took another voltage reading... the meter kept moving between 1.5 and 1.6. The extra lemon was giving a slight boost in voltage, but would it light up an LED?

Again... no luck. But that's okay. I got the main point of the exercise which was to get a little better understanding of how batteries work. It's kinda cool to wire up four lemon halves with pennies and zinc-plated nails and get a voltage reading... any voltage reading. But 1.4 to 1.6 volts for 2 lemons? That's like a single 1.5v alkaline battery- just not as efficient apparently.

My other readings matched up with the author - almost 2mA of current and the resistance of each lemon half was around 25-35K when penny/nail were inserted very closely (same segment of lemon).

After letting the setup sit for about 15 minutes, I took some more readings... a slight decrease, so I imagine the lemon battery isn't going to last too long... of course, it is exposed to air (unlike real batteries) and I'm not using the cleanest nails and pennies for the experiment, so there's a lot of factors that are controlling voltage and current levels. Still, a fun experiment.

Just a few notes:

1. When inserting the nails and pennies, try to get them as close together as possible without touching... when I first inserted them on opposite ends of each lemon half, the voltage reading was down around .2 to .3... moving them to the same lemon segment AND about 1/4" apart really bumped up the voltage.

2. I may try this experiment again at a later date if I can locate some low-current LEDs... if anything, just to prove to myself that I wired it up correctly.

If any of my readers try this experiment and can get the LED lit up, please take a picture and send it to me - I'll try to post one or two of the first successful lightings.


  1. What about wiring lemon halves in serial and parallel, say 3 in serial, with 2 parallel paths, for similar voltage and more current?

  2. David,

    I didn't even think of wiring them in parallel to increase the current...argh. If someone wants to try this and report back, that'd be great. I already threw out my lemons...

  3. I ran them in series and got it to light up a bit. If I had enough clips to do one more half I think I could have got it nice and bright.

    I seem to be a little behind the times, but I'm just starting the book.

    Thanks for the great blog and feel free to use any of my images.


  4. The boy and I wired up the lemon battery yesterday. (Placing a lemon half on a shot glass prevents the lemon from rocking and made it easier to insert the electrodes.)

    We measured .7V on one half and .8V on the other, and when we put them in series, we measured almost 1.5V -- looks like the voltages are additive, kinda like I'd expected. While I was having the boy write down our results (he writes slowly), I noted the voltage reading was trending steadily downward, indicating these batteries lose voltage pretty fast.

    We tried to light the LED at this point and failed. (One thing that was a little worrisome was how to orient the LED -- the long lead is supposed to go to the positive side of the circuit, but I don't know how to determine this, though I'd assume there's a way -- anyone?)

    So, we added two more lemon halves to the circuit. At this point, we measured about 2.5V in the whole circuit, and were able to faintly light the LED. Was kinda cool to sit back and see the LED driven solely by lemon power! (Sorry, we didn't read your blog until after the experiment, so didn't take a photo.)

    We didn't measure current or lemon resistance or experiment with straight lemon juice, as suggested by the book, though we did leave the rig hooked up to the multimeter and I assigned the boy to take readings every so often while the wife and I went out on a date.

    He observed a rolloff to about 1.7V after 20 minutes or so, where it stayed roughly the same for the next two hours. Now, this is weird -- the next morning, roughly 15 hours after we created the battery, the voltage had actually INCREASED back up to 2.5! However, it now fails to light the LED. We're currently reading 0.05mA - substantially less than your readings (and the author's prediction) -- looks like the current may be the factor that depletes over time. I can't do a comparison against yesterday, because we didn't measure current during our initial setup.

  5. Two days later, we were still at 2.5V, but back down to 0.03mA. Went ahead and tore down our lemon batteries at this point and moved onto the next experiment.

    Thanks -- Thrall (aka 'Scott')

  6. If the resistance of a lemon is high, why is a lemon used to make a battery?