iPhone dies when removing charging adaptor!

Hi guys,

I have an iPhone 5s. It is working properly when is connected to the laptop or wall port, but once I disconnect and lock it, it dies (please note that when it is disconnected but screen is on, it is working properly too, but it dies when it is locked). I make to force restart it by pushing home and power buttons to turn it on. This way works sometimes but mostly is not effective. Then I have to wait one day and recharge it as it power drains to zero even though it is off!!! I am wondering is the problem from battery or it is something else?

I would be grateful if somebody help to sort out this problem.

Thanks,

Dariush.