areobat
I think you are on the right page. The CT scan shows bulging battery cases, with intact interior folds. My sense is that the cases are bulging due to thermal expansion of the contents, perhaps including some trapped gaseous electrolyte.
The prismatic shape of the batteries does of course bear on the internal stresses. To the extent that the interior stresses of folding work against a dissipation of "wear", in my opinion, the separator material is subject to abrasive forces. The application of the paste to the poly film creates a stress simply by virtue of the relative physical properties of the two materials.
Any one who has opened a water bottle at cabin altitude after launch knows the pressure differential has an effect on the clear plastic bottle, the bottle will snap and crack, audibly.
Simplistic? Most problems start just that way.
The poly separator in the batteries is twenty five microns in thickness. The standard is a minimum of ten, to a maximum of forty (microns) dependent on application, (from the industry website I posted.
These batteries have a patent failure profile, consistent with the FAA considerations, the research data, and the evidence of failures since 2006.
Including the dramatic failures which caused the Japanese safety board and FAA to ground the fleet.
bill
I don't think the problem is mysterious. The actual temperature range for suitable performance of the poly may be much more narrow than that allowed.
I've had experience with plastics in normal temps. When it gets cold, it disintegrates, when it gets hot, it loses its resilience, and expands/contracts sufficiently to destroy its "shape". Since the perforations in this application are critical, my guess is that the poly deforms, the perforations change dimension, and allow short circuiting, which greatly accelerates the deterioration, and causes the need to "replace" the unit. Feverishly replaced, as Boeing has shown.
Last edited by Lyman; 9th Feb 2013 at 14:35.