If you tend to carry your iPhone 5, 5c, or 5s in your pocket there is a good chance that pocket lint will accumulate in the Lightning connector. This will tend to cause a poor connection with the Lightning cable and make for erratic charging. The best way to fix this is to take a plastic or wood toothpick and clean out the connector. Never use the end of a paper clip because a conductive material like metal could cause electronic damage. This was the remedy for my poor iPhone 5 charging. Sure enough there was lint in the connector. Now that it's clean I am getting more reliable charges.