Our plan is to build a battery.

A potato battery.

A lemon battery / potato battery is a device used in experiments proposed in many science textbooks around the world. It is made by inserting two different metallic objects, for example a galvanized nail and a copper coin, into a lemon. The copper coin serves as the positive electrode or cathode and the galvanized nail as the electron-producing negative electrode or anode. These two objects work as electrodes, causing an electrochemical reaction which generates a small potential difference.

The aim of this experiment is to show students how batteries work. After the battery is assembled, a multimeter can be used to check the generated voltage. In order for a more visible effect to be produced, a few lemon cells connected in series can be used to power a standard LED. Flashlight bulbs are generally not used because the lemon battery cannot produce the amount of current required to light such bulbs. Digital clocks can work well, and some toymakers offer small kits with a clock that can be powered by two potatoes or lemons.

The energy for the battery does not come from the lemon or potato, but rather the energy comes from the chemical change in the zinc. The zinc is oxidized inside the lemon, exchanging some of its electrons in order to reach a lower energy state, and the energy released provides the power. The lemon or potato merely provides an environment where this can happen, but they are not used up in the process.

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In a lemon battery, both oxidation and reduction occur. Consider the case of a zinc-copper battery; this battery is similar to the original "simple voltaic cells" invented by Alessandro Volta. At the anode, metallic zinc is oxidised, and enters the acidic solution as Zn2+ ions:

Zn → Zn2+ + 2 e-.

At the copper cathode, hydrogen ions (solvated protons from the acidic solution) are reduced to form molecular hydrogen:

2H++ 2e- → H2.

Potatoes, apples, or any other fruit or vegetable containing acid or other electrolyte can be used, but lemons are preferred because of their higher acidity. Other non rusty metal combinations (such as magnesium-copper) are more effective: for example, using a magnesium strip instead of zinc increases the voltage from 1.1 V with zinc to 1.6 V with magnesium. (The exact voltage varies depending on the lemons.) However, zinc and copper are usually preferred because they are reasonably safe and easy to obtain.