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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?

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marked as duplicate by Ƭᴇcʜιᴇ007, Journeyman Geek, Renan, Simon Sheehan, slhck Jul 15 '12 at 20:11

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

20V x 3.25A = 65W required. 5V x 0.5A = 2.5W available. 65 / 2.5 = 26 USB outlets needed. – RedGrittyBrick May 17 '11 at 14:06
argh! I'd have to ask 25 other passengers if I can use their USB ports lol – Jefro May 17 '11 at 14:55
New device found: Airbus 380. Do you wish to configure the device now? – Simon Richter May 17 '11 at 15:00
@Simon Richter: – starblue May 17 '11 at 19:57
Since RedGrittyBrick pointed out so quickly that power requirements make this impossible (though you don't really need that much power), this question is probably over-and-done, but I want to clarify: Finding a transformer is not a good question for Electronics. We'd welcome questions about the design of a DC-DC converter, but we're just as adamant (if not more adamant) about prohibiting bad shopping questions as Superuser. – Kevin Vermeer May 17 '11 at 21:51

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.

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haha... I can see the faces of security now as they open my bag to find a mass of 26 usb cables all wired into a handmade box... – Jefro May 17 '11 at 14:44
Or, with 7 parallel blocks of 4 supplies in series you get 20V, 3.5A. This would require 4 * 7 = 28 5V supplies, but does not need the transformer. Not that that is any more feasible... – sblair May 17 '11 at 15:20
Note that USB ports for power may supply more than 0.5 amps, as ipods/smarthphones may draw more than that if available. My 'droid says "connect to charger, usb port doesn't supply enough power" when I run tethering sometimes. – Macke May 17 '11 at 18:20
It make me sad to up-vote this... – Trufa May 18 '11 at 0:29
since those ports are probably "dumb" power supplies, you can most likely draw a lot more power from them. – Martin May 20 '11 at 14:48

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.

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Personally, I wouldn't risk it, but you're quite right, and I like your solution. +1 for you. – Lukasa May 17 '11 at 16:50

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.
enter image description here 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.

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As long as the SLA doesn't contain more than 100mL of acid... runs – Ignacio Vazquez-Abrams May 17 '11 at 18:00
+1 this cracked me up – Mehrdad May 17 '11 at 18:14
good answer <.> – Wildling May 18 '11 at 6:09

Are you sure it's just USB because many planes now have a power connector for laptops - for example:

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:

share|improve this answer Economy class seats are equipped with USB and Ipod chargers only :( – Jefro May 17 '11 at 14:53
Well, that settles that then :-( – Linker3000 May 17 '11 at 16:15
@Jefro, better upgrade to First Class :P – Kevin Peno May 17 '11 at 16:48

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.

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That sounds like a challenge... :P – Richard Marskell - Drackir May 17 '11 at 14:07
@Drackir - If I had written this I would have qualified it with 'reasonable' haha – Shinrai May 17 '11 at 14:15

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