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What chemical changes occur when a storage battery is charged?

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Crystal growth — electrochemical reaction Different mechanisms of crystal growth.

When a storage battery is charged, it undergoes a process of electrolysis, where an external electric current is used to drive chemical reactions that do not occur spontaneously. Essentially, the electrical energy provided by the charger is converted directly into chemical energy within the battery.12

During this process, the chemical reactions that occurred during the battery's discharge are reversed. For example, in a lead-acid battery, the discharge process consumes lead dioxide, lead, and sulfuric acid to produce lead sulfate and water. When the battery is recharged, the electric current forces these products to react and revert back into their original forms—lead dioxide at the positive electrode and lead at the negative electrode—thereby restoring the battery's capacity to produce electricity.1342

Because the electrolyte's composition changes during these cycles, the state of charge can often be determined by measuring the specific gravity of the electrolyte.4