2 batteries in series (negative bat1 -> positive bat2)
sets in parallel (negative bat_pair1 -> negative bat_pair2, positive 1 -> positive ->2)
be sure to put the charger
across the battery bank as well for the most even charge. (charger negative on bat 1 of battery pair 1, charger
positive on positive of battery pair 2)
definitely use 6-8 gauge copper wire with screw on connections (never alligator clips or the like)
Also, in my work in solar
power we learned that one should never mix and match batteries to make a bank... they should all be new, and of the exact same type else you will significantly degrade the life of the batteries.