Most charging cables are made of copper wire, which is just one of many valuable metals in old electronics. | Jorg Greuel/Getty Images In an old cabinet, stashed away at the back of my closet, there are not one, not two, but three drawers full of old cables and devices. Every generation of USB is represented as is every major brand of gadget. I know that I won’t use these cables again, but I also know they don’t belong in the trash. I have no e…