The zip ties are basically just there to secure the battery during installation. The battery mounts inside of an angle iron frame, with a sheet of 1/8" thick ABS plastic underneath it. The battery is secured two ways. There is an ABS cover that was designed to act as a spacer between the top of the battery and the bottom of the van floor to keep the battery from rising out of the angle iron frame once installed. There is also a small gap (about 1/4") between the angle iron frame and battery case sidewalls. This gap is filled with Sikaflex sealant which secures the battery to the frame. The Sikaflex sealant also prevents small pebbles from becoming lodged between the angle iron frame and battery case, which will wear a hole in the battery case.
If you don't have the ABS cover or Sikaflex sealant, you'll need something more than the zip ties to secure the battery (and to prevent small pebbles from causing major problems).
While I haven't seen a battery ever come out of the angle iron frame, I have seen the damage a couple little pebbles can cause!