Battery Charger
Not a battery recharger: a battery charger. You plug it into the wall, you pop in a standard-sized non-rechargeable alkaline battery into one of the slots provided (from AAA to E), and within half a minute it charges up the battery for you from wall current. It always works. It doesn’t seem to hurt the battery. It doesn’t care what brand the battery is, or how old it is. You can buy a Battery Charger from a sidewalk vendor in most major cities in the Americas for about twenty bucks.
Just don’t show one to a chemist, unless of course you like seeing people have nervous breakdowns. The Battery Charger cannot actually work, because non-rechargeable alkaline batteries rely on a particular kind of chemical reaction that can’t be reversed simply by shooting enough electricity through it. Unless you have a Battery Charger, apparently. The Battery Charger does things to the internal composition of the battery it’s charging that the charger, well, simply shouldn’t be able to do. And neither the chemists nor the physicists who will end up looking at one of these devices will be able to make heads or tails of it. But the Chargers work! Somehow.