I searched for this and found nothing. Apologies if I'm search-challenged ...
I ran across a back up battery solution that's cross-platform for many laptops.
http://www.valence.com/ncharge.aspI'm running a Dell Latitude D800 and even with two internal batteries I'm limited to 6+ hours (okay for a one-off club act but sketchy for two days at Langerado camping). The N Charge powers the laptop and/or recharges its batteries depending on how its connected. Pretty nice. It's also very slim (about half an inch) and has a smaller footprint than the laptop itself so it fits right in the same carrying bag.