@inetdog
If we assume that it has to be able to tolerate more than the pressure differential between pressurization altitude and maximum flight altitude, you can get a lower limit on the overpressure required at low altitude to rupture the disk.
Maybe the lower limit on the overpressure required to rupture the disk has to be higher than you are suggesting.
Initially the air pressure in the sealed system will be whatever the atmospheric pressure was at the place and time that the system was closed. If at a later time the system and its battery is hotter then the pressure in the system will be higher. So the lower limit on the overpressure required to rupture the vent disk maybe ought to be the difference between this higher-than-atmospheric-pressure and the pressure at maximum flight altitude (plus a good margin). This is a higher limit than you suggested.
I was interested to see that a presentation at one of the NTSB hearings (by a manufacturer who seems not to be involved with the 787) mentioned an example of a burst disk operating pressure of a cell (not a battery and not a sealed enclosure) of 200psi.
http://www.ntsb.gov/news/events/2013...ry%20Forum.pdf
What I would like to know is whether there is also some sort of very restricted air bleed to allow for a slow equalization of internal pressure over the normal time between maintenance openings of the box.
I hope not. A “a very restricted air bleed” is how moisture can accumulate inside a system, something Boeing has already found in testing.
http://www.pprune.org/tech-log/505695-787-batteries-chargers-part-1-a-53.html#post7759373