The more I think about this the more I think using car batteries for this
is a tad dangerous, sealed, old-school with caps, no spill batteries, etc
no matter the type/style. Chemically created energy in a thin walled box is
not to be played with, particularly when it sits ideal a lot. When in a car
they are 'exercised' regularly. Sitting in the situation described here
they will be doing nothing. Much like dry-cells, I have seen car batteries
that experience a chemical breakdown when they just sit.