Concerning the use of batteries you would need to tap individual plates to lower volts and amps, but that process cuts both amps and volts equally. That would mean tearing into the batteries. Not a safe proposition I would Think! I doubt one could find many usable low volt/amp combinations. In a shtf situation the old school methods were they used rivets ,screws, and bolts. For "old school" you are overthinking the problem. Forge brazing and soldering aint hard to do.