Back when I was living in the boonies with an extended power failure every 2 weeks, I just bought a car battery and charger. Thing weighed almost 50 pounds, but it was "portable" in that I could lug it to my car and take it with me. Wouldn't want to take it in a bag though.
When the power was out for more than a few minutes, I'd clip the 12V leads to the cigarette lighter plug of a car power inverter (rated at enough watts to power the laptop). Then plug the laptop's AC adapter into the inverter's 110 Volt outlet.
It held about 1000 Watt-hours (12V * 85 Amp-hours), which is about 20x what the built-in battery on that laptop holds (49 Wh). So in theory it has enough juice to last 20x longer. Maybe 2/3rds that after figuring conversion losses for converting to AC and back to DC. If you used this a lot you'd be better off with a DC-DC converter to step up the 12V to the 19V your laptop takes, though you'd probably need to add some voltage regulation since the 12V battery's voltage varies quite a bit with charge state.
Edit: All of this effort turned out for naught. While I could power my gaming laptop most of the day with this setup, it turned out the cable company's battery backup at their distribution hub nearest my house only lasted about 1h 45m. After which I would lose Internet (my router and cable modem were on a UPS) and no more online gaming for me.
/www.ncix.com/detail/msi-ge60-apache-pro-i7-dd-96309-1139.htm
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