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Here's how to make a 4 x AA USB battery pack for an external USB hard drive, Sitnalta writes - "Not long ago, I upgraded the hard drive in my laptop from an 80GB 4200RPM to a 120GB 5200RPM. Not wanting to let my old 2.5" drive go to waste, I bought a rather nice USB enclosure for it. Ah, but there was a catch. USB ports can only supply a maximum of 500 milliamps. The hard drive will need 550mA, meaning that the use of an AC adapter (or special USB cable) would be necessary under high load conditions...The way this HD enclosure "solves" the power problem is to include a special USB Y-cable that can draw power from two ports. So one port handles power+data while the other just supplies power. This means that sometimes I'd have to give up two ports for one HD (or use an after-market AC adapter.) Not cool...But I'm not here to complain. I'm here to awesome this problem out of existence. And we do that with some bits from Radio Shack, and, of course, a tin of Altoids." - Link.
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The 500mA rule is a bit of a misnomer. 500mA is the minimum specification. Most desktops can supply more than enough power for any 2.5" hard drive. Laptops, however, are very stingy about USB current.
A voltage regulator with a low voltage drop out would be a welcome addition to this project. You're likely to damage components built for USB power that don't contain regulators or low voltage drop outs.
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The 500mA rule is a bit of a misnomer. 500mA is the minimum specification.
Absolutely, completely WRONG. According to the USB 2.0 spec (read it here), the maximum current a bus-powered device can draw from the hub is 500 mA.
Of course there are many devices that need more than 500 mA, so these devices have some external power source (like perhaps this battery pack) and the device tells the host that it is Self Powered.
Most desktops can supply more than enough power for any 2.5" hard drive. Laptops, however, are very stingy about USB current.
Nope. The max port current is still 500 mA, regardless of whether the machine is desktop, laptop, or nuclear-powered server. One non-obvious caveat: if you are using a bus-powered hub with four downstream ports, then the current available to each downstream port is only 100 mA: you can't supply 500 mA to each port when your input supply is capable of sourcing 500 mA! This is why most hubs come with a wall wart.
Now, there IS an issue with some laptops, which is that a laptop might consider its USB ports to be part of a bus-powered hub, in which case its ports have a 100 mA current limit. Apparently, this is the the case for certain machines (I've never run into one) when using battery power. When on mains AC, the ports can source 500 mA.
A voltage regulator with a low voltage drop out would be a welcome addition to this project. You're likely to damage components built for USB power that don't contain regulators or low voltage drop outs.
The REAL problem with this idea is that you've got the batteries in parallel with the USB 5V supply. I wonder how long it'll work until the batteries overheat and explode.
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