I just made one of these for my girlfriend (in a altoids gum tin no less), and when she tried to recharge her iPod it wouldn't work. Her DS can charge using it, so I think it's the iPod. I haven't had a chance to get it back from her yet, so I can't test it. Has anyone ever had this problem? Anyone know what's wrong? Any advice is appreciated. Thanks.
The 9v battery doesn't have enough current, it's probably only about 150mAH, the ipod is expecting at least 500Mah from the usb port. Other designs i've seen use 2 9v batteries in parrallel to raise the available current. I made one of those, but it worked more like a battery extender than a portable charger ( I used a 270mAH NIMH 9V cell). I now use 4 2500 mAH NIMH AAs, they put out 4.8v but at a high enough mAH rating to charge my 4th Gen Photo a couple of times, it'll even charge my Dell Axim X51v whilst I'm using it and that is one power hungry device.
I actually found that you have to short one of the data lines to ground and then put the iPod in sleep mode and then it'll start charging. Pretty weird, but it works. I didn't have enough time to see if it worked as a battery charger or just an extender, but when my girlfriend gets back I'll ask her how well it worked. Also, her friends were impressed with it, and apperently I've been comissioned to make several more of them. I'm also considering making one with 4 alkaline AAs, but it'd be pretty pointless to make a regulator to drop 6 volts to 5.
NiCd (or whatever) rechargeable cells are typically 1.2V IIRC, so 4 will give you 4.8V which should be fine without regulation, unlike standard 1.5V alkalines or whatever.
My setup will charge the ipod whilst it's being used, like i said in my original post, it's not the voltage, it's the current that makes the difference in this case. 4 NiMh AAs will outperform the 9v battery due to the fact that the 9v battery is designed for low drain devices. 4 Alkaline cells are not suitable, not only are they 1.5v each, they just don't have the capacity of the NiMh cells. 6 cells may work if you use 2 sets of 3 wired parrallel to double the capacity at 4.5v (3x1.5V). Look for NiMh batteries with high mAH ratings, at least 2500mah, they're not too much more expensive to buy but will pay for themselves in a couple of charge cycles.
Alkaline AA's typically have about 2900mAh. Also, when my girlfriend got back from her trip she said the battery performed well. When she started her flight the ipod was about to die and the battery lasted the whole trip (about 3 hours). Not sure if the ipod charged in that time, I'll have to ask.
5g ipods accept 5-30V, look on the back. Also, you don't want to short the usb data line to ground, you want to pull both data lines low with a certain value of resistor. (Look on the internet, try ipodlinux.com)