OK, here’s an idea. How about a USB to USB gagdet that a person whose laptop battery is dying can use to suck power from someone nearby who is plugged in? Obviously you could not power a whole laptop solely from the USB draw, but the idea would be merely to replenish while there is still life left in the battery, forestalling laptop shutdown. Patent!
An inverter is likely to fry your buddy’s usb hub. Charging a surrogate battery betwixt the two laptops would be more feasible, but then the draw on the host USB hub could not exceed 5v at 500mA at a maximum, and hopefully your host doesn’t have anything of his own plugged into USB.
As well, USB contains no overload protection and could feasibly give up more draw than it could handle, commiting unfortunate damage to the host.
Ever hear of someone with a couple cheap passive USB hubs plugging eight devices into two? That’s a bad idea.
Further, all those silly plugin devices, such as reading lights and fans, do not comply with the USB standard which requires negotiation between the device and the computer before the device is supposed to draw more than 100mA. Most of these devices start drawing whatever they want. Using even one of these devices is dangerous. Using more than one is an invitation to the sparkle and smoke show.
Your best bet would be to have an extra battery pack with 16v 3A out(typical laptop draw) that plugs directly into the DC in on your laptop. Such a pack could offer multiple lines out.
But you might as well carry around a Sears Die-Hard car battery with a 12v DC-AC inverter in a backpack. Although that’s tempting as well. Maybe you could employ power backpackers and have these people walk around airports selling regulated juice along with free wifi and coffee to the discontented and disconnected. Perhaps we could hang batteries from a rolling IV pole…
It’s tough to disconnect. We are become cyborgs.