Bill, instead of putting them in parallel... add the battery, put the accessories onto it's lugs and buy a automatic battery isolator for the charging circuit. Find that charging feed wire from the big motor, pull it off the original battery and put it on the feed of the isolator and then feed the two batteries off the dual outputs of the isolator. No more damaging voltage drops to accessories when you start the big motor. Bonus is if you accidentally kill either battery you simply jump positive to positive to get the big motor going.