Type of cables to use for connecting 6v batteries?

2 gauge cable 9" in length can successfully carry 1,000 amperes and not even blink. I used to rip out the insane 2/0 battery post snapping interconnects on big rigs and replace them with 2 gauge welding cable. The trucks went on to spin 855 Cummins engines at 12-volts in -20F winter mornings.

The only issue with using steel is everyone is not fastidious about keeping cables and connections perfectly clean. I have seen oxidized steel interconnects melt lead posts down to molten globs.