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Why can rechargeable batteries be charged again after they are discharged?

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Cross-section diagram of an alkaline-manganese dioxide battery showing labeled internal components, including the steel-plated covers, cathode, anode gel, current collector, separator, steel can, and brass rivet. — battery Cutaway view of an alkaline–manganese dioxide power cell.

Rechargeable batteries, also known as secondary or storage batteries, can be charged again because they are specifically designed to allow their internal chemical reactions to be reversed.12

During discharge, a battery converts chemical energy into electrical energy as electrons flow from the anode to the cathode through an external circuit. When you connect a rechargeable battery to an external direct current (DC) power source, you apply a voltage that is higher than the battery's own voltage in the opposite direction of the discharge.31

This process, a form of electrolysis, forces electrons back through the charging circuit and into the battery's electrodes and chemicals. This action reverses the chemical changes that occurred during discharge, effectively restoring the battery to or near its original energy level and power capability. While this process is highly efficient, commercial batteries may experience a very slight loss in capacity over many recharging cycles.3241