Powering a GPS laptop without an invertor

[silverbullet]

I know this is strictly a Tech query, but since there's a few of you who are pretty clever with this sort of thing I'll ask anyway!
Aside from the technicalities of mounting the laptop in/on the dash, can I use a step-up DC-DC convertor (something like fleabay item 150877299187) to provide a supply to power and/or charge it with a suitable charger? I don't want to lay out for a Sterling or an invertor, total overkill for my needs and I'd like to fit a supply to more than one vehicle.
Requirements are 18v and 70 Watt but I'm not sure there's enough headroom with this 80 watt unit?

[BOXY]

If you're using the laptop for mapping software why not hack a Navman 510 to run memory map?

[discipleofsketch]

I use one of these to power an old apple macbook (16v, with home-made magsafe adapter), and dell netbook (19V):-

http://www.amazon.co.uk/Trust-Compact-F ... B001D3344M" onclick="window.open(this.href);return false;

Works well and comes with loads of different tips - will drain your leisure/ main battery surprisingly fast though if the engine isn't running - starts whistling (I don't think that's an intentional feature!), when the power source gets too weak..

[silverbullet]

Thanks guys. Sat-nav screens are a bit tiddly when trying to identify things off in the distance, although my old one is on the way out anyway! I can't think why I didn't think of a universal in-car adaptor, I just looked at the label over the power port and thought "how can I get 18v?"