Just as Boeing pursues an interim solution to the 787 battery problem, battery maker Yuasa publicly disagrees with the issue behind the battery failures.
Boeing believes their interim solution solves a battery failure problem. That is, they believe by containing the battery, they solve the safety of flight issue. This does suggest a belief that the battery design may be at fault.
Yuasa, on the other hand, believes that the batteries it has examined show clear signs of an over-charge which pushes fault outside the battery itself and onto the control system(s). Boeing says that their quadruple redudant systems would prevent such a thing from happening.
My thoughts: If this were a fundamental battery design flaw, I think we would have seen more battery problems much earlier in this program. The charging system and other power systems are complex and reportedly do cause some pain to the operators.
Why would a charging system fail now as opposed to during test flights? Conditions. Operating aircraft will be subjected to greater loads, real world power consumption profiles, etc.
The greater issue is this: The battery maker and airframe maker are disagreeing in public. Goodbye interim solution, in my opinion.