How do batteries actually work?

A battery contains two electrodes made of different materials sitting in an electrolyte. At the negative electrode, called the anode, a chemical reaction releases electrons. At the positive electrode, the cathode, a reaction consumes them. The electrolyte allows ions to move internally to balance the charge but blocks electrons from taking that shortcut, so the only path available to them runs out through the external circuit, which is your phone, flashlight, or car starter. The chemical energy difference between the two electrode materials determines the voltage, which is why different chemistries produce characteristic voltages regardless of the battery's physical size.

Size determines capacity rather than voltage. A AA and a D cell of the same chemistry both produce about 1.5 volts, but the larger one contains more reactant and therefore stores more energy, measured in milliamp-hours. Non-rechargeable batteries use reactions that are difficult to reverse and consume their electrode material permanently. Rechargeable batteries use reactions that reverse when current is forced through them backward, restoring the original chemical arrangement, though never quite perfectly, which is why capacity fades over hundreds of cycles as unwanted side reactions accumulate.

Lithium-ion, which powers essentially every modern portable device, works by shuttling lithium ions between a graphite anode and a metal oxide cathode. Its dominance comes from high energy density, meaning more stored energy per unit of weight, and reasonable cycle life. Its drawbacks explain most battery advice: it degrades faster when kept at full charge or run to empty, dislikes heat, and can fail catastrophically through thermal runaway if damaged or manufactured poorly, which is why airlines restrict them in checked baggage. Practical implications worth knowing: do not mix old and new batteries in the same device, since the depleted one can be driven into reverse and leak, and never dispose of lithium batteries in household trash or recycling bins, since crushed cells have caused numerous fires in collection trucks and processing facilities.