Replacing a PC's AC-DC converter with a DC-DC converter for mobile power supply
Yes, DC-DC converters do "actively" convert one voltage into another, but with some loss, with efficiency 80-90%.

So, if your computer consumes maximum 3.4 A at 19 A (assuming absolute max), a normal DC-DC converter will take 3.4 * 19 /12 /0.8 = 6.7 A from a 12-V battery.

There are plenty of automotive adapters for laptop power. For example, DELL-compatible 65W mobile supply, or TARGUS brand.