Willoz
So correct me if I am wrong please, but they still dont know what the cause is (and probably never will), so the "fix" from Boeing is to "contain" a runaway battery, ie contain and released fluids and thermal damage inside a stronger box, not stop it from happening in the first place, am I correct?
I don't think you are (entirely) correct. From what I can gather in reading about this 'fix' and speaking to a 787 engineer who has been in contact with Boeing engineers during the investigation, it is more comprehensive.
Details seem to be under wraps at the moment (why?) but I believe the 'fix' incorporates the following: a software change to the charging system, re-designed cells with better quality-control, re-configured battery internals (cell arrangement), insulation between cells, more comprehensive monitoring of cell condition and monitoring of insulation condition.
All of that was Boeing's solution, and I believe the FAA then insisted on an improvement in the containment device, just in case..