Why the iPhone turn it selfs off?

I've just replace the display (LCD + touch) to this iPhone, it turns on and works perfectly, but... don't know why it turns off randomly! When i've reassembled the iPhone, i've lost the metal covering of the internal speaker, and used some other screws, so on the LCD - touch - camera metal cover i've used the remaining screws. Someone can help me ? wish i haven't broken the MB...

The battery % doesn't change, but it's strange because after the reassembling it charged from 26 to 49%, and then started this issue.