May I offer a suggestion. I didn't look to see what laptop you are using, but if it has a PCMCIA port, you could get an inexpensive Firewire PCMCIA card which usually contains a number of 6 pin firwire ports which carry power. Of course how much power your back needs to power it up is a figure I don't know...but if the PCMCIA firewire card doesn't supply enough power to the back, then most of these PCMCIA cards have a tiny jack on them, that allows you to plug in a AC adapter into the wall to increase ouput power. Sometimes the AC adapter is even included with the firewire PCMCIA card.
Dave (D&A)