My iphone 4 always shuts down even it is not yet drained. And if I plug the charger, charging works but the Drained Battery Sign still showed and if I remove the charger, it's still battery drained. Sometimes, my iphone turns on but I need to force turning on, and few minutes it will shut down again even it's still 100% charged. How to fix this? Please? Thank you so much. I'm just worrying.