In a simple battery, a chemical reaction between two different metals and a mixture of acid and water causes a potential energy, or voltage. When the battery is connected to an external load, current will flow. The current will continue flowing until the two metals become similar and the mixture becomes mostly water.
When current is sent into the battery by an alternator or a battery charger, however, the reaction is reversed. This is a chemical reaction caused by current flow. The current causes an electrochemical reaction that restores the metals and the acid-water mixture.
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