The construction (as opposed to chemistry) of alkaline cells differs significantly from ordinary carbon-zinc cells, however. Alkaline cells are effectively turned inside-out. The shell of the alkaline battery is nothing more than that–a protective shell–and it does not play a part in the overall chemical reaction. The anode of the cell is a gelled mixture of powered zinc combined with the electrolyte (itself a mixture of potassium hydroxide–a strong alkaline-and water), and the combination is linked to the negative terminal of the cell by a brass spike running up the middle of the cell. The cathode, a mixture of carbon and manganese dioxide, surrounds the anode and electrolyte, separated by a layer of non-woven fabric such as polyester. The figure below illustrates the construction of a Duracell alkaline battery.

It consists of 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 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.

The energy used to power the circuit ultimately comes from the energy used in the electrode factories to reduce metallic ions forming the solid metallic electrodes. The lemon simply provides a salt bridge for the reaction to occur. In a lemon battery, both oxidation and reduction occur. Consider the case of a zinc-copper battery. At the anode, zinc is oxidised:

Zn → Zn²⁺ + 2 e^-

At the cathode, hydrogen is reduced:

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 metal combinations (such as magnesium-copper) are more efficient: 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.