Can I charge a computer through its USB port?

I have an EEE PC with an additional charger which I can use inside my car. It plugs into the cigarette lighter thingie that seems to be standard in every car. When on the move, it does allow me to charge my laptop that way. Very practical.

But charging through an USB port is only possible at one side of the USB connection, not on both sides. Basically, the USB ports on a laptop are used to send a signal out, thus they can be used to power up something. Many rechargable USB devices are designed to get a signal in, thus they are waiting until they receive power through the port. Technically, this is all just a one-direction street.

Still, they could make a laptop that can be recharged through the USB port, if they add a receiving end to it. Basically, that would mean that you can't connect a device to your laptop, but you would be connecting your laptop, as if it's just another device, to another PC. Would this be practical? Maybe if you want to do this to set up a special network or whatever but no one has seen some true value in such a setup. Basically, you'd be downgrading your laptop to a portable hard disk.

Still, as mentioned by the others, an average laptop will require a bit more power to fully charge than other devices. Especially the big screen and the disks are big energy consumers, although the CPU itself also puts some strain on the system. The power a device receives through the USB port is just a fraction of the power that a device would receive through a direct power connection.

The closest thing to a computer that can be recharged through an USB port would be a Smartphone with Windows Mobile. Or perhaps an UMPC system, but I don't know any good UMPC that is recharged this way. I assume you also have a few wishes concerning operating system, diskspace and memory.

Applying power to a USB port while the computer is powered off sounds like a method of damaging something.

To charge the computer, you need a direct connection to the battery.

A USB port won't give you that. The 5V which comes out of the USB is (more or less) carefully regulated by electronics. Electronics doesn't work backwards.

I'm aware of no computer that can be charged using the USB port. The capacity of a USB connection is only up to 500 mA (milli-Amperes) at 5 volts, or 2.5 Watts per powered port. This is not sufficient to actually run a typical laptop these days. Its worthy to note that USB1.1 ports only 100mA.

(Source: our host Jeff Atwood's blog.)

In principle you might be able to get a separate external charger for laptop batteries that was USB-powered, but it would be very slow, and I've never heard of one.