The “bus bars” on the battery holders are 14 AWG copper wire:
Slightly stretching the wire straightens and work-hardens it, which I’d been doing by clamping one end in the bench vise, grabbing the other in a Vise-Grip, and whacking the Vise-Grip with a hammer. The results tended to be, mmm, hit-or-miss, with the wires often acquiring a slight bend due to an errant whack.
I finally fished out the slide hammer Mary made when we took a BOCES adult-ed machine shop class many many years ago:
The snout captured the head of a sheet metal screw you’d previously driven into a dented automobile fender. For my simple purposes, jamming the wire into the snout and tightening it firmly provides a Good Enough™ grip:
Clamp the other end of the wire into the bench vise, pull gently on the hammer to take the slack out of the wire, and slap the weight until one end of the wire breaks.
With a bit of attention to detail, the wires come out perfectly straight and ready to become Art:
The wires start out at 1.60 mm diameter (14 AWG should be 1.628, but you know how this stuff goes) and break around 1.55 mm. In principle, when the diameter drops 3%, the area will decrease by 6% and the length should increase by 6%, but in reality the 150 mm length stretches by only 1 mm = 1%, not 3 mm. My measurement-fu seems weak.
Highly recommended, particularly when your Favorite Wife made the tool.
The Harbor Freight version comes with a bunch of snouts suitable for car repair and is utterly unromantic.