Originally Posted by
Grunff
The sad part is that Li-Ion + metal box weights more than NiCd it was meant to replace.
The problem was that NiCd didn't have the necessary energy density - in short a NiCd of the needed capacity wouldn't fit in the available space. So they redesigned the Li battery with enough isolation between the cells that if a cell shorted and ran away, it couldn't propagate to additional cells. Unfortunately, the damage to the failed batteries was such that they couldn't definitely determine the cause of the failure, so they couldn't prove they'd fixed the root cause - necessitating the steel box in a sort of belt and suspenders solution.