IF you have 6-volt batteries (3 fill caps on each battery), then the 2 batteries are connected in series - that is, the positive of one battery connects to the negative of the other battery, and the remaining positive goes to the converter with the remaining negative going to ground.
On the other hand, if you have 12-volt batteries (6 fill caps on each battery), then they are connected in parallel - that is, positive connects to positive, negative to negative and the positive lead connects to the converter with the negative lead connecting to ground.
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