Unlike the majority here who seem to think they know better than the qualified guys and girls at Boeing, Thales and Yuasa, I believe that the batteries and associated circuitry are fundamentally well designed and built (and no, I'm not a Boeing fan!)
Well-built batteries should only fail if they are mistreated electrically, and since we know that the entire electrical system was never tested as a whole before the planes started flying, that is where we should look for a root cause.
If this is the case, then the additional margins on charge and discharge adopted by Boeing are a correct measure to enable the battery to absorb abnormal electrical conditions. I would hope that these limits have been worked out on the basis of measurements on actual aircraft, and not plucked out of thin air by the bean-counters.
All this fuss over the battery box is purely political: the FAA mandated that there should be no fire, Boeing believed the batteries wouldn't fail catsatrophically (after all, they ARE an essential system) and perhaps took a rather cavalier attitude towards containment. But they did fail and catch fire, hence the need to show compliance even if all goes well and no other battery will ever catch fire again.
Another reason for the fuss over the box may be to divert attention from the design oversight in not fully testing the complete electrical system before flight, and the consequent self-regulation can of worms.
What would be interesting, and not publicised as far as I know, is how much the battery capacity has been reduced and how this impacts on emergency operations when the battery is doing what it is supposed to do (length of flight without other sources of power, number or duration of brake applications, number of APU start attempts...)