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Josef Prusa has published a short article on a recent hack that he has done to a Netbook battery.  He added a USB socket to the battery to allow him to charge devices such as i-phones independently of the PC.  His Acer Wind in fact is surplus to requirements these days so this is an interesting way to recycle the old parts.

Be warned however, you can’t just wire up the USB socket to the batteries, notebook batteries output 12v or more. 

Josef used a lm7805 voltage regulator for the task of reducing the voltage down.  As he mentions in his article temperatures get pretty hot, 70c with a heatsink.

The choice of regulator is not the best, this is an older design which although a good IC, is going to leak a hell of a lot of power as heat during charging.  The comments section of his blog has already gathered a few better suggestions for the power conversion 

USB iPhone battery pack from MSI Wind battery « PRUSADJS.CZ