You CAN use many things for
battery wire, but there are reasons the wires are made and used differently. The closest thing to
battery wire is IIRC "number 3 machine wire" used for flexible machine
power cords. Welding wire uses finer strands than either of these, because the finer strands provide more surface area and less cross-section in each wire, and welding is more efficient when there is more surface area as the AC
power just flows on the skin of the conductors, not in their whole cross-section the way that DC battery power does. So even if the resistance of the
cables is "the same" at 12 volts, it is very different at AC frequencies.
Because the strands in the welding wire are finer, and it is not tinned, you will be losing more cable to surface oxidation more quickly. And even if you tin the ends, as the cable thermal cycles from daytime and nighttime changes, the entire cable sucks in ambient air and humidity, which rots out the entire length of cable, despite tinned or sealed ends, if there's even a pinprick in the
insulation. Yes, careful tinning and sealing can minimize that--but you're still using thinner strands, so you'd need a heavier gauge to get back the cross-section area.
You can hitch a donkey up to an ox-cart yes, and it will still
work. As long as you are aware that there are real differences, and you're willing to
work with them.