FireWire Compatibility

• If you're using a charging cable that uses FireWire instead of USB, the cable may be the cause of the charging issue, depending on which version of the iPhone you own. FireWire cables cannot be used to charge the iPhone 3, iPhone 3GS or the iPhone 4. If you own one of those iPhone versions, check the box or documents that came with the charger to see if it uses FireWire instead of USB. If you've got an original iPhone, then FireWire should charge the phone.
  
  

Computer Power-Saving Modes

• If you're attempting to charge your iPhone using a USB cable on your computer, check that your computer is not in a power-saving mode. Computer states, like Sleep or Hibernate, do not supply enough power to the iPhone to charge it. Keep the computer turned on and in an alert state to ensure that power is flowing through the USB charger to the iPhone.
  
  

Remove Other USB Devices

• If you've got your iPhone plugged into a USB outlet on your computer, check that you don't have too many other USB devices plugged in. If you've overloaded the computer with USB devices, your iPhone may not be receiving a strong charge. Disconnect anything you don't need so that you can determine if the iPhone is getting a charge.
  
  

Troubleshoot the Charger

• If you've got access to another charger, either USB or a power adapter, try using it to determine if the problem lies within your phone or within the charger. Always plug the charger in securely and check for loose connections to ensure that the charger is fully connected with the power source and the phone. Using an alternate type of charger, such as a USB instead of a power adapter, can also help you to determine if the charger is the problem.