What spruce said above mate, but understand that a split charge relay is useless, unless you are doing serious mileage to put the charge/amps back into it! I killed lots of battery's in the early days, without knowing this. I do have a S.C.R but i do very little miles for it to be my main charger. I use 2 leisure batts, and bench charge them daily, and rotate. They will give you a better life span, as you wont be (?) taking them under their 50% drain value, (based on that 20hr rule). If u want to keep costs low ish, i would recommend the following Lead acid ncc class b, 200 charge cycles batts @ 50% drain. The Yuasa 100ah efb (i use) or Varta ldf 90ah / bosch L5 90ah (same batts) both have powerframe tech, which though research, i understand will take a charge close to 13v. (Ive found tayna batteries are cheapest, £90-£110)
Good luck