I've been through this with my 2000 V70-- the positive battery cable was getting hot to the touch and I measured a significant voltage drop between the alternator output and the voltage across the terminals, indicating that that cable needed to be replaced.

I read elsewhere that the aftermarket OEM replacement cables weren't very good quality, and the pricing on the genuine Volvo ones was outrageous: about 3x the aftermarket cost. I was planning on fabricating one (the only thing that is special about it is the post with the 10mm nut to attach the B+ cable), but I ended up spotting a new genuine Volvo one in a pick-and-pull and used that instead.

Others have suggested having a cable fabricated at a place that does marine electric work.

Good luck!