How to calibrate iPhone, iPad and iPod battery:
Step 1: First and foremost, you will need to run your device’s battery down to zero. And by that, we mean that your device runs completely out of juice and powers down on its own.
Step 2: Next, you’ll need to charge your device’s battery all the way up to the 100% mark.
Step 3: Once you’ve reached the magic 100, leave your device on charge for another hour before unplugging it.
Step 4: Use your device as normal, letting it drain down to zero. Ensure you do not charge at any time.
Step 5: As with steps 2 and 3, you’ll need to charge up to 100% again, and leave it charging for a further hour once it’s full.
Step 6: That’s it – your device’s battery is now calibrated.