iPhone 5s will not charge

My mom got her iPhone 5s and has had charging issues every day since. After testing, I found her lightning cable that came with the phone was bad. I tested it on my iPhone 5 , then found my lightning cable worked on her iPhone 5s. So I figure the problem is solved, bad cable. Went yo the apple store and they exchanged the cord, no problem. The charging issues, happened again after she went home and charged her phone over night. Dead battery when she woke up! Crazy...so she took her phone back to the apple store and they exchanged her iPhone 5s , for another one. Guess what, the same problems continue with the current phone! It will not charge. She hasn't been able to use the phone more than a few hours since her purchase of the first iPhone 5s. She wants to return it. I feel bad, and bummed Apple is putting out such defective devices. QA, must be slacking. I've never had this issue with any iPhone since the first one. I know there is a recall of some iPhone 5s , but is it possible a lot of store stock is still bad. Is anyone having issues like this? I really hope there is a better solution than my mom has to return her new phone and go back to using my old iPhone 4.

John