Blue crimps are typically used on 2.5mm2 cable. Rating depends on spec, but the best stuff is tri-rated which can handle 30A. Personally, I would use 4mm2 or 6mm2 for the 'link' between batteries (rated 41 and 53A respectively for tri-rated type), both of which you use yellow crimps on. I'm lucky in that I do industrial electrics for a living and have a pretty much endless supply of cable

Still not sure if OP has solved charging problem.
HTH, Bri