http://www.headphoneamp.co.kr/ftp/sijosae/Gallery/ - view Sijosae's Designs
Originally I was assmebling Radio Shack pre-made battery packs (2xAA or 4xAA) in Altoid tins with USB (5V) connectors. However these AA cells were large & required both frequent battery changes, low voltages, required cables and had poor charge times for a dead iPod...problems.
See Ted's excellent reviews: http://home.speedfactory.net/tcashin/ipodbattery.htm
Firewire has better potential, smaller packages and requires no cables. Newer iPods only...
Using rechargeable NiMh 9 volt batteries provide higher voltages for use with firewire than AA's, although most have only 700mAh while the AA batteries can be obtained up to 1200-2700mAh. However AA Packs of 6 or 8 can be awkward, heavy and hard to manage easily. Alkaline 9v's can be had online or at discounts as low as 55 to 99 cents ea.!
So we use 9v in series or parallel with firewire!
comments on AA vs. 9v batteries? Rechargeables vs. Alkaline dry cells
Step 1: Concept & Specifications
Working on small Cmoy amps I found many small designs online I liked. The Duracell was perfect for a single 9V solution. An Altoid tin will easily house Dual 9v (serial or parallel) and provide power similiar to the Apple AC adapter and the IEE1394 firewire output from a desktop or laptop.
Having used USB powered at 5 volts this promised to provide better performance for both old and "new" iPod's.
Firewire specs: http://www.1394ta.org/Technology/Specifications/