The bottom part of the silver nerf bar mouldings extend over the edge of the battery box, which might be stopping the battery coming out. You could try removing one or both the mouldings by undoing the two screws that fasten them to the tub (they are screwed in from the inside of the battery box - use a long screwdriver). Also take off the driver moulding which will reveal some big holes in the battery box so you can push the battery out from the top.