Me, I'd use Anderson PP45 connectors and gang up as many as necessary to carry the current. That way they'll break away non-damagingly in the event of a cable pull. SB50 is a good connector, but it can take a serious amount of tension to unplug. 14ga zip cord to each pair, not more than 35A per pair.
Also I wouldn't carry a battery on my back if I could carry it on the bike.