How do Batteries get charged?

Inside each cell, chemical reactions take place involving the two electrical terminals (or electrodes) and a chemical known as the electrolyte that separate them. The good news is that if you're using a rechargeable battery, you can make the chemical reactions run in reverse using a battery charger.
Generally all smartphones we use contains lithium ion batteries.
A lithium-ion battery or Li-ion battery is a type of rechargeable battery in which lithium ions move from the negative electrode to the positive electrode during discharge and back when charging.

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