2 batteries in series (negative bat1 -> positive bat2)
2
battery 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.