Battery is locked when turned clockwise. Unlocked when turned counterclockwise.
Tip for removing battery: Starting from locked position, turn key 90 degrees to go horizontal. Pull on key to remove battery.
My preferred method is to unlock battery completely (turn counter clockwise until stop) and use pliers to pull battery out.
Make a note that the battery can be pushed in too far and the lock mechanism actually retains it into proper position. It has to be flush with the green line seen in the picture below.