since most devices use multiple batteries in series : why don't they just integrate this damn chip ?
make a chip for 1 cell , 2 cell , 3 cell , whatever. and sell those.
the device makers can take this energy harvesting chip , slap it inside the device and done.
no more fidgeting with sleeves, no stuff that ends up in landfills.
spoiler : linear tech and maxim (and probably many others) already have such chips. they can really suck a stack of batteries dry ...