Not sure the size of your batteries... but I made my interconnects from 1/4" thick, inch or inch-and-a-half wide solid copper busbar I bought from a switchboard manufacturer as scrap.
Here is where the run turns around and heads back - the end one is a little wider and thicker, just to be sure!
As for crimped cables, I did mine with 35 sq mm welding "superflex"
Stripped end
Lug on
Crimped properly
Heatshrink and gland on
Finshed set
I always thought soldering was a better system too, but evidence is that a PROPER crimp acts more like a spot-weld, has a lower connection resistance and is actually superior to soldered connections.
The other problem with soldering multistranded cable is that where the solder ends tends to become a fracture/failure point and cables tinned and soldered thus, often fail prematurely (especially if there is vibration or movement over time).
Good crimps, especially if you seal them when completed, seem to last better and outperform soldered connections.