A lemon battery uses the juice inside the lemon as an electrolyte to facilitate electrons moving between a pair of electrodes inserted into the fruit. These need to be two dissimilar metals, such as a copper penny and zinc-coated nail. Completing the circuit creates a weak electrical current.
Inside the lemon, the electrolyte solution reacts with the zinc electrode, freeing electrons that pass through the solution to the copper electrode. There, the free electrons combine with hydrogen atoms, producing bubbles of hydrogen gas. This process can produce a weak voltage if the two electrodes are connected in a circuit. Over time, the chemical reaction degrades the zinc electrode and depletes the electrolyte, rendering the battery useless.
The amount of electricity generated by a lemon battery may vary depending on the individual characteristics of the fruit. In addition, the makeup of the nail and penny can alter the characteristics of the battery, as many of these items are made with alloys instead of pure metal. In most cases, a single lemon cell provide slightly less than a volt of power. However, lemons may be connected in series to provide a higher voltage, suitable for illuminating LEDs or driving simple low-powered demonstration devices.