How to solder large gauge wire to terminal
Soldering 8 ga multistrand is not a good idea. The problem is that, assuming you can get it hot enough to take the solder, solder will wick up along the individual strands, causing a fairly long region where the cable is essentially solid. The end of this region is susceptible to long-term degradation since it acts to concentrate any flexing stress and work-harden the copper.