And such a gadget is one that no one would buy, and consequently no one would produce. Electronics designed to run off AA batteries already extract as much ener

And such a gadget is one that no one would buy, and consequently no one would produce. Electronics designed to run off AA batteries already extract as much energy as is practical from them. Indeed. If your gadget can make use of NiMH rechargables, then you're already having to deal with about 1.2V per cell, and that's when they're fully charged! So if you need 3.0V or 3.3V nominal for your main supply, you are probably already using a buck/boost switching regulator. As long as the battery can supply enough current, your gadget will keep going, and drain pretty much all the useful energy from it.