How to wire 6-6V batteries-Oh yes, I'm still here

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As to the double wires going to the battery bank, my guess is that the reds and blacks are connected somewhere down the line. Something like boths blacks going to a single ground lug and both reds going to the inverter. If so, then connect 2 six volt batteries in series (one positive to one negative), then 1 red to the other positive and a black to the other negative each set of 2.