Really agree with this statement by Elon Musk as linked in Airman1900's post.
"They [Boeing] believe they have this under control, although I think there is a fundamental safety issue with the architecture of a pack with large cells," writes Musk in an email. "It is much harder to maintain an even temperature in a large cell, as the distance from the center of the cell to the edge is much greater, which increases the risk of thermal runaway."
The shorted cell in the NTSB briefing appears to show exactly this effect, short along the centerline of the battery plates. That is where the temperature sensing devices belong in each battery cell.
Last edited by Machinbird; 30th Jan 2013 at 01:17.