• Let your device completely discharge until it automatically shuts down (0% battery left).
• Leave it off for at least 5 hours (best overnight) without trying to turn it on.
• Charge your device to 100% while leaving it idle. Only use official Apple cables!
• Leave your device charging at least 2 more hours after it reaches 100%, to make sure that the battery is charged at its maximum capacity.
• Your battery should now be reset. If your problem persists, try restoring your device with iTunes before going to an Apple Store!
NOTICE: if you have either a beta version of iOS or a jailbroken device, it’s entirely possible that your battery is suffering from it. Try switching to a stable iOS version without jailbreaking it.