Thanks for all the answers.
Actually, I have not use any flash shoot yet, but I used my LCD to check shots and do the setting. I am going for the butterfly photography event, so I do not want to miss half of the event for a dead battery. I myself not sure if I charged full 4 hours before use it,since I am beginner and I got the camera,recently.
I guess, I take Sportz Guy advice, and buy a second battery to be in a safe side.