Ampere is a free app for Android (4.0.3 and higher) that will help you find out why your phone or tablet is not charging as fast as it should be.
Ampere monitors battery charge and drainage, as well as temperature and health of your battery, and tells you which of your chargers or cables is no longer up to the task of charging your device. When you first launch the app, it will provide you with a reading containing the milliamps your device is using. Don’t connect your device to a charger during this reading. Jot down this number and then plug your device in to charge.
Find your charger’s listed amperage (each AC adapter has a set output) – it should be on the AC adapter or, if it’s a Qi charger, on the back of the receiver. Compare this number to the number you get from your second reading (the one you get while your phone is charging). If they don’t match up (give or take ~100 mA; this fluctuation is expected, because of your phone’s discharge rate), you have a faulty cable/adapter on your hands.
You can download and learn more about Ampere right here.