YouTube clip - Onion Battery
do they really work or simply end in tears?

Note: these articles have been published in InfoChem, the supliment to Education in Chemistry produced by The Royal Society of Chemistry.
Many are based on the two BBC OU TV series - Hollywood Science

A recent YouTube clip claims that an ipod can be charged-up using just it's charging cable, a few 100ml of sports energy drink and an onion! The clip has apparently attracted over 5 million viewers [1]. But is it true, can it be done?

The clip shows an onion being drilled in a couple of places and then left to soak for 30 min in some 'high energy' sports drink. An i-pod charging lead is then connected up to the i-pod and the other end (which usually goes into the AC adapter / charger) is simply pushed into the onion. They claim that this can charge up the i-pod.

Now pushing two metals into an electrolyte will produce a potential difference across them providing the metals are different [2]. So it's possible that pushing the plug into the soaked onion will produce a voltage if the plug and its pins are of different metals. But the potential won't be very much probably less than 1V and the current will be small, a few mA or so depending on the surface area. (Note: there may be more than one set of pins on the usb plug but unfortunately the voltages can't add like a crude series circuit because the conducting electrolyte shorts them all in parallel).

Now an ipod will require several 100's mA charge current at a few volts (ca. 5V). So unless something very unusual is happening within the onion layers it looks like there won't be nearly enough electricity to charge an i-pod. So assuming the whole thing is not a deliberate joke / swindle why might they think its working?

An ipod is basically a specialised computer designed to play MP3 tracks etc. It has many programs held on its hard disc that it runs to be able to function. One program will be the 'charge battery and show details on ipod screen' program. When the plug is pushed into the onion the electrolyte (a good conductor) probably completes a crude electrical circuit that sends a signal to the ipod. This will either wake it up from its sleep mode or if it's already on will trigger this charging program (it's the charge lead that is plugged in to the ipod and so its pins are wired up for this purpose).

I think what we see on the clip is the residual power in the ipod battery running the first steps of this charge program on the i-pod screen but probably not actually getting so far as charging the battery at all.

It's easy to get fooled as leaving the onion in place for an hour might well produce a higher reading on the battery level meter when you turn it back on again. But then so will leaving the i-pod unused for a while. This is because when you stop draining a battery, and give it a rest, it can recover a little so when you start to use it again its voltage will also have improved a bit.

If you are still not convinced then think about it this way. AA batteries are improving all the time but its taken 60 years or so of research to get to today's high capacity alkaline batteries we take for granted. Even four of these batteries would be quite hard pushed to completely charge a power hungry i-pod so really what chance have you got just shoving a plug into an onion !?

Overall then, I think the clip is not showing what it claims to be showing (others agree [3]). You can make electricity in this simple way but not in the quantities required to charge an i-pod. I would be delighted if you can prove me wrong though. One thing is for certain it's a great way of ruining your i-pod charging lead!

[1] YouTube clip: How to charge an i-pod using electrolytes and an onion
[2] Experiments with Homemade Batteries, M. Bullivant, J. Hare, Education in Chemistry, RSC, Vol 43, 1st January 2006, p12-14
[3]YouTube clip: Charge-an-ipod-with-an-onion

How teachers can use these articles in a lesson

Why Hollywood Science

Open University Hollywood Science web site

Call for clips - do you have a film clip that needs investigating?

Jonathan would like to thank Robert Llewellyn, Gill Watson and Harry Kroto (Vega Trust), all the BBC teams, The Royal Society of Chemistry and all at the Open University.


Dr Jonathan Hare, The University of Sussex
Brighton, East Sussex. BN1 9QJ.

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