Whenever I'm away from home for an extended period of time, I would charge my phone and mp3 player from my laptop. This way, I get about 3 or 4 full charges out of the laptop battery and I could leave the phone and mp3 player chargers at home. But carrying around a laptop just to charge a phone and mp3 player seemed a little excessive.
Later on I discovered the now hugely popular Minty Boost , a small battery-powered device for charging USB devices. However, after making my own I found that the two AA batteries used in the Minty Boost just didn't have enough juice for what I needed (a couple of 2000mAh AA cells gave me about half a charge on my phone before giving out).
So I decided to combine the capacity of a laptop battery with the portability of the Minty Boost: A heavy duty portable charger.
The device is based around a DC/DC boost circuit, a microcontroller (I used a PIC), and a handful of 18650 lithium-ion cells. Laptop batteries usually contain 8 of these cells (although I notice my netbook only uses 3, which explains the dismal battery life). I harvested the batteries for this device from an old Dell laptop battery, but you can buy 18650 cells cheaply on ebay, (you can see one in the top right of the picture below),.
Note: for this instructable, you will require experience with circuit building, programming and using microcontrollers. I have included my source code for the PIC12F683, but the same circuit applies to Atmel or other microprocessors.
Note2: While I have designed the simple circuit from scratch, the general principles behind such circuits are well established, I am sure many people who have built such devices will have arrived at very similar circuits. Infringement is not intended.
Pictured is the final device charging my phone and running a USB fan at the same time, this one contains 4 18650 cells, has two USB sockets and is built into an 8cm CD wallet, which I found was a perfect size.