Electricity from a chemical reaction
A battery contains materials whose chemical reactions can release or take up energy. In a discharging battery, a spontaneous redox reaction separates the movement of electrons from the movement of ions. The battery’s structure makes electrons travel through an external circuit, where they can do useful work.
Two routes, one circuit
Consider a typical rechargeable lithium-ion cell. It has two electrodes, an electrolyte that conducts ions, and a separator that keeps the electrodes from making direct electrical contact while allowing ions to pass.
During discharge, lithium ions move through the electrolyte from the anode toward the cathode, while electrons travel through the external circuit. The electrons cannot normally cross the electrolyte and separator directly, so the external path is what lets a current power a device.
What charging changes
When a charger applies a voltage in the appropriate direction, it drives the cell’s chemistry backward. Lithium ions move back toward the anode, and electrons are supplied through the charging circuit. The energy is stored as chemical potential rather than sitting inside the battery as a reservoir of free electrons.
Why batteries run down
As discharge proceeds, the reactants move toward a lower-energy state and the cell voltage falls. A battery is considered discharged when it reaches its specified cutoff—not necessarily when every possible chemical reaction has stopped. Real cells also lose capacity over time because some reactions are not perfectly reversible.
Remember: electrons move through the outside circuit; ions move through the inside of the cell. The coupled movement keeps charge balanced and allows the chemistry to continue.