If you hook two batteries car to negative, then from that battery's positive
to the negative of the other battery and positive of the second battery to
the car you have hooked them in series. This will double the voltage but
not change the amperage.
If you hook positive to positive and negative to negative you will double
the available amperage, but will not change the voltage. Hooking the
batteries this way is known as parallel.
In either case it does not matter if we are discussing positive or negative
ground, both systems would work the same way.
Seeing as how you can buy some insanely large (measured in CCA) batteries
nowadays, I can't see the advantage of a second battery in parallel.
if you have a slow crank issue, you might have a voltage drop problem in the
battery cables, or a starter issue.
hope this helps,
Rick
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