cool, I AM here to help, but as above, I just wanted you to think this through first or you will waste time and money in a potentially fruitless direction. doing is defintely a good way to learn, I do it all the time, but I always set clear goals for a project first, then research the hell out of it before taking the first practical steps.
LiFePO4 are great batteries, I use A123 myself in my transportable dac/amp, but they arent small and dont make the mistake of thinking you can use them directly for 3.3v, well depends on the tolerance of the parts used, but at fully charged you are looking at around 3.65v and I cut off at 3v, but they DO spend most of their time around .45->3.2v. also there are plenty of dubious fake or crappy LiFePO4 on ebay so watch it on that, a less than stellar LiFePO4 battery isnt much better than a quality low self discharge NIMH cell, which are a lot easier to charge
I dont use an internal charger, the type you can fit in your box will take too long to be very useful IMO, I use an FMA balance charger designed for radio control racing and it'll charge and balance the cells (4 x 2300mah 3v3) in less than 1hr. I also use active battery voltage sensing and I can second the recommendation for Linear tech stuff. I use LT3032, LT1963A and LT1763 regs, LTC2935 battery monitor and push-button controller to sense low battery, give warning, then at 3v it pulls the shutdown pins on my regs low and the unit goes to sleep till I can turn it off and charge. i'm using 4 batteries in series for a ~+/-6v6 supply.
hey jpaul, good call on the 3092 CCS on your other projects, little noisy for a hifi CCS, but handy as hell
