Why does my iPad 4 charge only when the screen is black?

So I have my own personal iPad 3 and a customer just brought in an iPad 4 both with the same problems. They both won't charge unless the screen is off. When you are using the iPad with it plugged in, it says "not charging". But if I watch the percentage with the screen off, I notice it goes up while plugged in. What do I do to fix this? Is it a new battery, or is it a chip?