That sounds about right. Have you measured the actual amp draw of the laptop while running?
The division is straight forward - 17 amp hr battery pack will last 3.4 hours in theory. I say in theory, because all batteries are designed to run at a certain amp draw, above that and they will not last as long. Your laptop draw is probably greater than the rate draw of the SLA inside the 'Powertank', so you'd be losing some time because of that.
Better quality batteries can handle high current draw rates, I believe. Typically 'powertank' type off-the-shelf things have cheap batteries in them.
You should however find that your laptop doesn't draw 5 amp all the time, that's quite high. My Toshiba draws about 2.5 amps while doing autoguiding through my webcam with the screen off, about 3 with the screen on.
See this thread for more info that might be helpful:
http://www.iceinspace.com.au/forum/s...ad.php?t=14245