Possible Duplicate:
Charge a laptop via USB power (500mA)?
I will fly to China in the next month which will take a mammoth 18 hour flight. The plane has USB power source on it.
I know USB only has 5v and 0.5 amps output and laptop is 20v and 3.25amps. So 4 USB's would be short on the amps but supply enough voltage. Is there a transformer out there that could make this set up work?
First, if you got a transformer, you'd need more than 4 USB ports, as the power draw will be constant at 65W. As each USB provides 2.5W, you'd need at least 26 USB ports, and that's assuming that you have perfect efficiency in your transformer (which you won't). I'm pretty sure the airline won't let you have 26 USB ports. Even if they did, it's unlikely that an airline would let you plug in what would have to be a handmade transformer. And, even if they did, it is almost certain that airport security wouldn't let you through with such a transformer in your bag.
This is pretty much guaranteed to fail. There is a reason we don't power laptops by USB. My suggestion? Get an extra battery or two, and just swap them in and out as needed.
Firstly, the power draw will NOT be constant at 65 watts. The maximum rated output of the normal laptop adapter is 65 watts.My laptop adapter is also rated at 65 watts. When not charging the battery, my laptop typically draws between 10 and 25 watts.
If you really have to draw from the USB, I'd do it by charging an intermediary battery and cycling power to the laptop in waves supply power for at least a minute. Depending on your battery chemistry, perhaps start applying power to the laptop at 80% charge and remove power at 40%. Size your intermediary battery accordingly.
You won't be able to reasonably power your laptop fully, but you will be able to power it enough to appreciably increase your run time with the use of four USB ports. (10 watts nominal). Voltage conversion is an exercise for others.
It is likely that those adapters may be "charging" type adapters that put out a full amp at 5 volts. That 5 watts from one port would still be useful and could give you a freshly charged laptop battery if you fell asleep on a long-haul flight.
This is going to sound a little silly but it is completely allowed by FAA regulations.
You are allowed to bring in sealed Lead-Acid batteries.
Attach a 12v cigarette adapter to that then hook that up to a small power inverter (you can get laptop ones for fairly cheep). You can then use that to charge your battery in flight.
Here is a DIY video of how to make one.
Are you sure it's just USB because many planes now have a power connector for laptops - for example:
http://www.engadget.com/2007/07/24/laptop-power-in-flight-internet-coming-to-qantas/
Quite often the airline's Web site will cover their fleet's features so it's worth taking a look - and this site (below) also has lots of useful info:
There's absolutely no way you can charge a laptop via USB or a USB docking station. The only way is through the AC adapter or via expansion port/dock.
