My phone won't charge anymore

My phone has stopped charging, it can't go past 56%. I have been trying to get answers for a couple days now, my battery sometimes will jump to full charge, or then instantly back down to 10%... Like it can't figure out how much it has left. I have taken it into the Apple store and they turned it on for me after it died and i couldn't. They suggested a DFU restore if problems continued... As they did i finally did a restore today and it did not fix it. Is there a way to fix it? I have not had this phone for more then a couple months... I doubt the battery is already destroyed. Could it be a defect? Will Apple replace that or my Sprint carrier? I simply would like to know all the neccesary information to get my phone back to normal. 🙂 Thanks!

-Chris