I dug the Wii U out of a box again for the kids to play with, and discovered that the charging dock's mechanism had rusted apart (3/5), picture taken after I already disassembled and cleaned it.
Rather than fix the "pop-up" mechanism I replaced it with exposed charging contacts made from solder wick, (4-5/5). This allows the contacts to touch when the controller is put in the dock (1/5).
Not pictured is putting a bit of solder on the GamePad's recessed contacts, and then folding solder wick and soldering it on.
It's important that the wick is soldered just enough to be securely attached, but not so much that it isn't flexible, 5/5 shows that it's permiated with solder on the inside (so it stays in place), but the finished 1/5 shows it's still flexible where the contact is made.
It does have the "drawback" that live voltage is exposed if the contacts are touched, but as this is only around 5v that's safe, I don't know why Nintendo opted for this overly complex mechanism.
And for anyone's stressing about the nominal 4.75v nominal voltage v.s. 5v (comes up when people are looking into e.g. retrofitting USB-C), my OEM charger outputs 5.3v, and the GamePad is still working after all these years.
Posted by avar