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Charge laptops through the USBs

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I expect there's a good reason why this isn't done but don't know what it is, so i'd like someone to tell me.

Many things can be charged through their USBs. Netbooks in particular don't seem to demand much power. Therefore, why is it that laptops can't be charged in that way?

nineteenthly, Aug 06 2010

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       I've used USB for the ground connection when replacing a laptop power connector that lacked a good solder point for the ground wire. That still leaves the positive connection though; to my knowledge you can't reasonably supply the ~50 needed watts through regular USB, it would burn the traces.
Spacecoyote, Aug 06 2010
  

       I suppose the power supply could "intelligently" negotiate whether/how much power to supply, like Power over Ethernet (PoE) does. PoE provides up to 25 watts of power (without interfering with legacy/unpowered devices, I think), which is more than USB could ever do.
Spacecoyote, Aug 06 2010
  

       Well maybe that's the way to go - via Ethernet.   

       Clearly this isn't standard for a USB but could the connector not allow this? If the traces are the problem, could that be addressed by making them thicker or more conductive?
nineteenthly, Aug 06 2010
  

       My laptop has a magnetic connection, which means that if you snag the cable, it doesn't pull the computer unto the floor. This would not be possible with a usb, but I appreciate that having a usb charging input would be a good option.
xenzag, Aug 06 2010
  

       It might be doable with PoE in the near future, some EEEpcs for instance requires 30 watts, future models may require less. PoE equipment is expensive (because it has by necessity fancy protection circuitry and such) however.   

       Maybe USB 3.0 could be adapted, it has more wire pairs to put the power over than regular USB. It is uncommon on laptops however.
Spacecoyote, Aug 06 2010
  

       USB runs at 5V; most laptop batteries range between 15V and 24V. So an internal switcher would be needed to boost the input to charging levels.   

       This is not in itself a problem.   

       Most laptop PSUs can deliver at least 30W. At 5V, that's 6 Amps - enough to warm up a USB connector.   

       Although it could probably be made to work in the short term, the conectors would degrade too rapidly to make it practical; once the USB socket on the motherboard is damaged, it's a big job to replace it.
8th of 7, Aug 06 2010
  

       [+] very much. I'll take the wall-wartside bit now please, to recharge my camera when my computer isn't on, and ship me a netbook with the other bit when you've got it in production. Nice one.
FlyingToaster, Aug 06 2010
  

       Also, phone lines can deliver a reasonable voltage (and a modest current). It would be handy if the modem socket could also be used to charge a laptop.
MaxwellBuchanan, Aug 06 2010
  

       //I thought USB was just for power output?// USB type-A connectors are just for power output ("upstream port"), and type-B connectors are just for power input ("downstream port"); the asymmetry is deliberate to prevent current loops. Or so Wikipedia tells me. So this idea would require adding a B-type port to the laptop.   

       Might be more feasible with IEEE 1394, if only that were more widespread.
mouseposture, Aug 06 2010
  

       Just thinking: how feasible would it be to charge using several USBs at once, say four? Would that be enough to solve the heating problem?   

       As it happens we do have Firewire.
nineteenthly, Aug 06 2010
  

       [xenzag], there's nothing in the spec that would prohibit the addition of magnets in the USB connector, in addition to the mechanical connection that is in place.   

       The problem with the main idea is that pushing that much current through a connector designed for low current operation is electrically dicey. Arcs and shorts do not make a computer or power supply happy.
Cedar Park, Aug 06 2010
  

       Maybe I've missed something here. Is it *essential* that the USB power input delivers a full 30W? If the aim is to have another option for charging the laptop, presumably even a small current is better than no current.
MaxwellBuchanan, Aug 07 2010
  

       But what powers the USB? Would you need a special USB cable that has a wall-wart at the other end?
AntiQuark, Aug 08 2010
  

       My main source of irritation with laptop chargers is their apparent fragility, the need to buy an entirely new one when they break and the fact they get hot and waste electricity if they're left plugged in. The USB can already be used to charge other devices and you'd just have to plug in widely available and cheap cables to do this. The other end is effectively something like a desktop PC PSU, or perhaps a hub.
nineteenthly, Aug 08 2010
  

       Here's why not: The battery pack isn't a 5v battery and charging it from a 5v source would require an additional internal transformer which would in effect be redundant with the conventional charger AND the transformer that actually would powers USB which would need to be switched off.
WcW, Aug 08 2010
  

       // apparent fragility, the need to buy an entirely new one when they break and the fact they get hot and waste electricity //   

       I can't see how a different type of cable would cure those problems. I suspect those issues are inherent with power supplies in general.
AntiQuark, Aug 08 2010
  

       I've been able to solve the first two problems by replacing the connectors with 2 pin molex connectors. UL probably wouldn't approve, though.
Spacecoyote, Aug 09 2010
  

       A laptop charger has a heavy lump in the middle which can pull the wires if it's lifted stupidly.
nineteenthly, Aug 09 2010
  

       I can use ny bathroom faucet to till my water glass. So why can't I use that faucet to refill the reservoir up the mountain?
DavideAndrea, Aug 10 2010
  

       That makes me think about the idea of other sources of power, like the rain filling the reservoir indirectly, gradually recharging the laptop, but i know that's been done. In fact, rain itself could be used if the laptop was waterproof enough.
nineteenthly, Aug 10 2010
  
      
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