The goal of this project was to build an efficient Altoids tin iPod (firewire) charger that runs on 3 (rechargeable) 'AA' batteries. Firewire delivers 30 volts unregulated. An iPod can use 8-30 volts DC. To get this from 3 AA batteries we need a voltage booster. In this instructable a switch mode power supply based on a microcontroller is used.
Standard disclaimers apply. High voltage....deadly...etc. Think about how much your iPod is worth to you before connecting it to this little stun gun in a tin can.
For all the math and dirty details of SMPS, read the nixie tube boost converter instructable:http://www.instructables.com/ex/i/B59D3AD4E2CE10288F99001143E7E506/?ALLSTEPS
Read on to see how the nixie tube SMPS design was adapted to be an iPod charger....
A ton of previous work inspired this project. One of the first DIY chargers used a combination of 9 volt and AA batteries to charge an iPod through the firewire port (works for all iPods, mandatory for 3G iPods):http://www.chrisdiclerico.com/2004/10/24/ipod-altoids-battery-pack-v2
This design has the problem of uneven discharge among the batteries. An updated version used only 9 volt batteries:http://www.chrisdiclerico.com/2005/01/18/altoids-ipod-battery-pack-v3
The design below appeared on Make and Hackaday while this instructable was written. It is a simple design for a 5 volt USB charger (this type will not charge earlier iPods, such as the 3G). It uses a 9 volt battery with a 7805 5 volt regulator. A stable 5 volts is provided, but the extra 4 volts from the battery is burned off as heat in the regulator. http://www.instructables.com/ex/i/9A2B899A157310299AD7001143E7E506/?ALLSTEPS
All of these designs have one item in common: 9 volt batteries. I think 9 volters are wimpy and expensive. While researching for this instructable I noted that an 'Energizer' NiMH 9 volt is only rated 150 mAh. 'Duracell' doesn't make rechargeable 9 volters.
A 'Duracell' or 'Energizer' NiMH 'AA' has a healthy 2300 mAh of power, or more (up to 2700 mAh ratings on newer rechargeables). In a pinch, disposable alkaline AA batteries are available everywhere at a reasonable price. Using 3 'AA' batteries nets us 2700mAh at ~ 4 volts, compared to 150mAh at 9 or 18 (2x9 volts) volts. With this much power we can live with switching losses and extra energy eaten up by the SMPS microcontroller.