" But, you can actually use chemical energy stored in a lemon and two metals to make a current and light up a small LED. It’s true and we’ll show you exactly how it’s done."
Steve Spangler Science
Et tu, Steve?
There are many ways to get to the free energy available in a fruit.
The simplest is to eat it. You have a few billions years worth of living behind you, and animals are pretty good at using the energy found in the chemical bonds of foodstuff to recombine phosphate and adenosine diphoshate, giving your cells an abundant supply of their basic energy currency, ATP.
You could dry it out until it's kindling then light it on fire.
You could mix it with yeast and let it ferment. The yeast will reward you with a wine that will warm you up. If that's not enough you could distill the wine, concentrating the ethanol enough to make a flammable liquid that can be used for heating your home or powering your car.
You cannot, however, get free energy from a fruit to power a battery. This is bad science. The fruit simply acts as a bridge for the electrons of one metal to travel to the other, different metal.
There's a reason you do not find most metals in a pure state in nature. If you want iron, you dig up iron ore, then smelt it. That, of course, requires free energy. When you connect copper and zinc together with some sort of liquid bridge. (Acids work, so does salt water--the salty vinegar dripping from your chips would work great.)
Fruits have tremendous stores of free energy, and animals, fungi, and bacteria take advantage of this--you want to eat the fruit before it rots, but rotting is just another organism beating you to the rich deliciousness of fruit.
If you want its energy, eat it, don't waste it jamming metals into it. If you want to show that fruit can serve as connection bridge in a battery, because it's cool and gets kids interested in science, go for it!
Just don't tell them that the fruit is powering the battery.
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