An elegant solution is a DC-to-AC inverter. I found a $40 inverter (Wagan Tech) that's compact, quiet, delivers 400 watts (or 150 from a cigarette lighter socket), and includes two AC receptacles that let you to plug-in whatever you want. This kept the laptop running easily, while charging a second small device as needed (Nintendo DS, cell phone, and so forth). I don't know why I didn't think of this sooner.