A battery is a device that uses a chemical reaction to push electric charge through a path. It has two different ends, called terminals. When both terminals are connected by a conducting material such as a wire, electrons can travel through the circuit and transfer energy to a device. A flashlight battery is made of several materials sealed in a case, but a homemade battery can use everyday items such as a lemon, metal strips, and wires. A battery does not create energy from nothing: it changes stored chemical energy into electrical energy. The electrical current stops if the circuit is open, if the reacting materials are used up, or if the connection is poor.

Simple batteries need two unlike metals, an electrolyte, and a way to connect the parts. In a lemon battery, a zinc-coated nail and a copper strip act as electrodes, while the acidic lemon juice is the electrolyte. The metals should not touch inside the lemon because that creates a shortcut, called a short circuit, that sends charges directly between the metals instead of through a wire or device. Work with clean, undamaged materials and wash hands after handling coins, nails, or acidic foods. Never cut open commercial batteries, heat them, charge nonrechargeable cells, or connect their terminals with loose metal objects. Ask an adult for help when using sharp tools or testing equipment.

To make a lemon battery, gently roll a lemon on a table to loosen the juice inside without breaking the peel. Push a clean copper strip or copper coin into one side and a zinc-coated galvanized nail into another side, leaving space between them. Attach one alligator-clip wire to each metal. The copper connection is usually the positive terminal, and the zinc connection is usually the negative terminal. Connect the free wire ends to a voltmeter to check for voltage, or connect several lemon cells in a chain to try powering a very small LED or calculator. For a series connection, join the copper electrode of one lemon to the zinc electrode of the next. Record which changes improve the reading.

A coin-cell battery can be built by stacking common materials to form repeating chemical cells. Place a small square of paper towel soaked with vinegar or salt water between a copper coin and a zinc washer or galvanized washer. The wet paper is the electrolyte, and it must be damp rather than dripping. Make more layers in the same order, with copper facing the wet paper on one side and zinc facing it on the other. Press the stack firmly and hold it with tape or rubber bands while keeping the top and bottom metals exposed for clips. Measure the voltage across the outside layers. If the reading is low, check that each layer has both metals, the paper is moist, and no clips touch each other.

Inside a simple battery, chemical reactions move electrons from one metal to another. Zinc atoms tend to lose electrons more easily than copper atoms. At the zinc electrode, atoms can become zinc ions and leave electrons behind in the metal. Those electrons travel through the external wire toward the copper electrode, where another reaction can use them. Meanwhile, ions move through the lemon juice or wet paper to balance charge inside the cell. This teamwork creates electric current only when there is a complete circuit. The amount of voltage depends on the materials used, while the available current also depends on electrode size, moisture, temperature, and how easily charges can move. A battery eventually weakens as its reactants are changed or used up.