Do you have to be a Mr. Wizard to replace the batteries? I'm a retard with a sodering iron. I'd love to have Shadowrun on the Genesis again, but the one my friend borrowed to me didn't save.
+1 for 3DS Flash Memory
+1 for passwords
Being a retard will do just fine.
All you need at the very least is nothing but a soldering iron, that's it. Preferably something to pry the battery away when the 2 solder points are heated and some fresh solder as well, but you could just reapply a new battery to the existing. Just tip to solder, goes liquid, pull up one side, then the other. Hold tabbed battery over space where old was, re-heat existing solder and the tab will attach to it, same on the other side. About the best and worst way if you don't apply more solder to make sure it sticks.
I'm no expert and I can handle them on any handheld gaming cart, 8/16bit carts, etc. Even done large ROM chip transplants from board 1 to board 2 for Genesis before.