To charge an 18V battery at 10A means your charger will have to draw at least 15A from the battery (18V/12V x 10A), since they are basically switching boost converters, and power in = power out + overhead power + inefficiency losses.

Again, you can calculate how many Amp-hours you need out of a marine battery, and purchase the appropriate size. If you use your car battery, your alternator will certainly handle a 15 -20A drain on the battery.

Like I said, if you use your car battery, monitor the voltage and be sure you start it up before it gets too low. The guys with the larger LiPos leave their engines running quite a bit, if not all the time.

540 cranking amps is NOT a capacity measurement. It is a measure of max current output for a short period of time.

Marine batteries can have 80-100 amp-hr capacities. With say 16 amp-hrs to charge one of his packs, that means he could charge 5 to 6 of his batt packs off of a deep cycle marine battery.