I've jump started two cars in the last week off three of my 6v batteries in my electric car, 18v makes the starter spin kind of fast! you can also get 8v deep cycle batteries, but 8v and 16v chargers are harder to find.
DC/DC Converter Regulator 24V Step Down to 12V 20A 240W | eBay
Is one of the cheap DC to DC converters, mine is a Sure Power Industries converter that is designed for golf cart light kits.