Will asked about using two 12V batteries in parallel instead of two 6V
batteries in series. You do indeed add the current in the parallel
configuration, while in the series config the voltage adds. It may seem
like therefore the 12V parallel config would increase your cranking
ability. HOWEVER - in reality if the batteries in question are about the
same physical size, the 12V batteries will individually have much less
current capability than the 6V. All lead-acid batteries are made up of
individual cells of 2V each, three cells per 6V battery and six cells per
12V battery. If the batteries are the same physical size, the cells in
the 12V battery must be only half the size as the ones in the 6V battery,
and the current they can supply is proportional to the total area of the
plates in the cell.
Automotive batteries generally have a 'Cold cranking amps'
rating. If the 12V batteries each have more than half the cold cranking
amps as the 6V batteries, then the 12V parallel system wins. If not,
stick with the 6V.
Len Bugel '50 TD