These are simplified drawings for both the 12VDC and 6VDC battery wiring. The idea is to get them hooked up for 12VDC and then you can parallel as many of those 12VDC groups you want. Just consider the two 6VDC batteries in SERIES as one large 12VDC battery...
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photo from google images
Something like this for 6EA 6VDC batteries used for 12VDC output
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I have always mounted my batteries in a separate vented compartment. They will gas fumes from time to time so you want to keep them away from electrical spark things...
Roy Ken