Circuit Help Answered

hi, i saw a video by the king of Random    
https://www.youtube.com/watch?v=hCxmn1ERbXc

He hand cranks up a drill to charge a phone..
but what i wanna do is similar but different, instead of charging a phone i want to charge a battery and what i need to know is,
what component do i need to stop the current from the battery to go back into the drill once i stop cranking so it doesn't depletes..