Why can't i charge my iphone 5 when its on?

Hi first off i'm on sprint and yes i contacted them and did what i was suppose to but let me just tell you the story first of when i got my iPhone 5 3 moths ago it worked fine and it still does its just that when i charge it it will only charge when its turned off for 3-5 minutes then it turns on and then its at like 5-7 percent and it won't charge it just has a red and half empty symbol and it turns off for a while saying its charging then turns on and then turns off when its n like 5 percent so anyways i just got a new charger 4 days ago because my old charger that came with phone wire broke or torn but as soon as i got the charger it worked for 3 days then on the third night i put it on charge so in the morning i an use it and it was not charged i tried to charge it again it turned on then it showed me the red sign of it not charging and i was so confused i just need help i also contacted sprint today right away and we went through some steps like resetting my WHOLE phone and also pressing and holding the home and power button various times but it WON't WORK i can't charge on my computer because it can't find my device i just need help write away please don't give me that contact apple crap or nothing like that because its already been done i'm tired and i need my phone 100% so i can use it for many of things including emergencies…

It sometimes happen to me can be the charger change it, it work for me

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