This is a picture of what happens to empty battery pods when subjected to about 6 bar of pressure. I typically test underwater housings to their rated depth before using (usually empty), however these need to have the additional internal support of the batteries to avoid collapsing. So don't do what I did. Also, don't remove the batteries and electronics and toss you new ROV over the side of the boat on a line either, which is probably what I would have done if the battery pods hadn't fit in my water filter pressure pod. Colin sent me a nice explanation of the problem:
"Its unfortunate that your tubes collapsed under pressure testing; The tubes can withstand pressures much greater than 100 meters, but only if they are supported by batteries in the tube.
Since the tubes are extruded PETG, the concentricity of the tubes can vary from batch to batch. Assuming that the tubes are perfectly concentric, the PETG on its own can theoretically withstand the pressures. But in reality no tube is ever perfectly circular, so any slight deviance under pressure will result in a slight hoop stress concentration that will grow with increased pressure and eventually cause the tube to strain+deflect more and eventually buckle, resulting in the failure you observed. The batteries support the battery tube under pressure; they act as support rings that line the entire length of the tube that ensure it doesn't deflect too much to keep the tube from buckling and failing, as well as keeping the endcaps and o-ring seals concentric with the tube. "
Thanks for sharing this Bob! Definitely a mistake I was about to make. You didn't happen to capture the implosion on camera did you?