I ALREADY KNOW WHAT IT IS, I LEFT THIS VIDEO UP TO HELP OTHER PEOPLE.

is it a secret? i really dont know ._., the answer has been found. I'll leave this video up for those who dont know what it is. If your to lazy to scroll down and read the comments, I'll put the answer here. Ok the compartment is for the clock on the wii. The wii needs a battery to operate the clock because what happeneds when you turn the wii off and theres no power on at all to operate the clock? (not in sleep mode) something needs to help keep the clock working.