Replace the battery in the iPhone easily with this ‘how to’ Letsfixit guide. Sometimes the battery may have been exposed to water, or it just malfunctions from no fault of your own. Apple will replace your battery if you have purchased their battery warranty and the cause of the battery malfunction is covered under that warranty. If your battery is not covered under Apple’s warranty, you can easily replace it yourself.
Turn your iPhone off and unplug it from any power source.
Insert the flat-head screwdriver into the crevice on top of the phone between the LCD and back cover, and pry it open. As you pull it open, notice the three cables connecting the LCD to the motherboard. The cables are numbered 1, 2, and 3. Remove them in numerical order, and disconnect the first two with your fingers. The third one will require you to slip the flat-head screwdriver underneath it on the motherboard to pry it up.
Lift the motherboard away from the bottom assembly using the flat-head screwdriver to pry it up at varying points. Be gentle--the motherboard is still connected to the camera by a cable.