There are a variety of chemical and mechanical devices that are called batteries, although they operate on different physical principles. A battery for the purposes of this explanation will be a device that can store energy in a chemical form and convert that stored chemical energy into electrical energy when needed.
Keeping this in consideration, how much energy is required to charge a battery?
The answer: 1 kWh. This is the amount of electricity you'd need to power ten 100-watt incandescent lightbulbs for an hour. Far from anything worth being sheepish over, 1 kwh costs about 12 cents. To be specific, your iPhone battery holds a charge of 1,440 mAh, or about 5.45 watt hours.
How does a rechargeable battery charge?
In a rechargeable battery, however, the reaction is reversible. When electrical energy from an outside source is applied to a secondary cell, the negative-to-positive electron flow that occurs during discharge is reversed, and the cell's charge is restored.