Here is a simplified pictorial on how to connect two 6V Batteries in series to get 12VDC... after connected in this manner then just conside these two 6VDC batteries connected in serial as a 12VDC battery...
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When connected in series note that your DC VOLTAGE will double but your AH rating will stay the same as one 6V battery rating...
If you want to increase your AH rating install a second series connected group of 6V batteries in parallel to the first group. Using the pictorial above of 220AH batteries you can get 12VDC at 440AH capacity by doing this...
Here is a schematic of how this can be accomplished...
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Roy Ken
