The box frame idea is reinforcement, like ring stiffeners for a cylinder, 
  only moreso.
  
  Hi Vance and Adam.
   
  Could either of you tell me what is the purpose 
  of isolating the batteries from each other into seperate compartments? 
  
  I was thinking about what the reasons could be 
  and I could only come up with two possibilities.  Is it 
  because, if there
  was a leak in one specific spot on the main box, 
  that only one battery would be contaminated by sea water leakage
  and only form enough chlorine gas as one battery could make? Is another reason to contain 
  any hydrogen gas formation
  in seperate compartments for some reason? That 
  wouldn't seem to make sense though because if one compartment 
  exploded and the others were seperate but 
  contaminated with hydrogen, they probably would rupture from the explosion and 
  blow up
  also. Or is it some kind of structural 
  reinforcement thing? You are talking about a battery box inside the sub's 
  interior not outside it
  correct?
  Thanks,
  Bill Akins