I'm thinking about the simple scheme of charging a portable device's rechargeable internal battery from an external battery (which should have a little higher voltage, obviously). The portable device should have an internal charge controller to protect its battery and manage the charging.
Find a suitable battery or stack of cells, and plug it in. Simple.
Using AA cells in an product like Belkin's iPod charger is an example.
I'm looking for specific instances of products, or of people doing this themselves and making the suggestion to others. One caveat, these instances have to be during the 1990s or before. Sorry! :-) That rules out Belkin and their AA-cell holder, since Apple introduced the iPod in 2002 and they brought out their product sometime later.