It is interesting that you should take this video at face value, chemisttree.
I am far more skeptical of its validity, especially when considering the other videos that same YouTube user puts out which are even more difficult to believe (for example,
http://www.youtube.com/watch?v=0rA-zhTJuFU&feature=related").
Although it is not entirely implausible to think that some type of voltaic cell is being constructed using the onion and USB charger, I think a more likely explanation is that the video is faked.
I do not know what types of metals they use to make USB plugs (perhaps Copper, Gold, Nickel, or steel), but whatever they are they are probably not
that far apart in their reduction potentials. The voltage from such a cell would probably not be very high, and in order for it to recharge the battery it would need to be at least 3.7 volts (the voltage of the Li-ion battery the iPod uses).
Not to mention the very small reaction area over which the redox reaction could occur, the so the current would be very limited and the voltaic cell would supply a very small power.
How do we know that the iPod in the video is actually operating on the ‘power’ supplied by the onion-cell? For all we know it does have internal battery power and that is what it is using and off-camera someone turned it on when we (the viewers) could not see.
Perhaps another possibility is that the iPod turns itself on when it detects a closed-circuit in its battery charger (I don’t have an iPod, so I cannot check this).
