The Mophie Juice Pack charges when connected to a power source by the USB cable included with the device. You can also plug the juice pack into a computer or into a wall using an iPhone wall plug.
The Mophie Juice Pack charges on its own or with an iPod or iPhone inserted. You can also use the juice pack in stand-by mode, which gives more battery life to the iPod or iPhone. In order to know that the juice pack has fully charged, look at the indicator lights on the case. The case has a full charged when all four LED lights have solidly lit.
To get the most out of a Mophie Juice Pack, you should not switch the pack to the On position until the iPhone or iPod battery is low, or when it is standing at about 10 to 20 percent battery life. Keep the juice pack on until the device has charged up to 80 percent. At this point, you can leave the device alone until the battery has gone back down to the 10 to 20 percent battery life, or until you can top off the battery any time it falls below 80 percent.