Packs not built for the physical punishment they are to be subject to. This is the same issue the 787 had with its batteries, the packs were not built with th

Packs not built for the physical punishment they are to be subject to. This is the same issue the 787 had with its batteries, the packs were not built with the idea that they will be rattling around for years and years, so there was not enough layered abrasion resistant insulative material and protections to keep the pack from arcing. This also includes making pockets for the wiring and running it through those pockets so that wires do not get pinched, and protective electronics to prevent dangerous electrical conditions. In cell phones most of these things are dealt with by placing extra tape in a number of places, battery packs in cell phones have an outer insulative layer and are held in place with double sided tape.