iPhone 4 USB allowable voltage range for charging

I am the very happy and satisfied owner on an iPhone 4, but am seeking mobile backup power for on the go use (and to limit unnecessary charging cycles in preparation for being away from an outlet). To that end I just purchased a Griffin TuneJuice charger for use with my ample collection of AA NiMH batteries. The TuneJuice expressly claims compatibility with NiMH and NiCd batteries as well as Alkaline and Lithium ones. Regrettably, even with fully charged brand new NiMH AAs the TuneJuice showed me no love with rechargeable batteries on my iPhone 4 and is being returned to from whence it came tomorrow. (It appeared to work fine with the included Alkaline batteries, but I have no interest in using it in that capacity.) That said, Id like a solution that does work well with rechargeables.
Unless someone has any great alternative product suggestions, I expect I may be building a homebrew alternative. My initial diagnosis is as follows:
+ NiMH batteries are rated to deliver about 20% less voltage than their alkaline brothers.
+ the TuneJuices 3 AAs in series was already under-volting the USB interface with the 4.5 volts 3 AA alkelines supply
+ even an overcharged NiMH battery delivers about 7% less than what a half-used up alkaline is rated at (a fully charged NiMH will typically deliver just under 1.4v when newly charged, but will quickly fall to between 1.3 and 1.2 volts.)
+ as charging with alkaline batteries did seem to work, I expect the voltage difference was the culprit.
So my main question is:
1. What are the allowable voltage ranges for charging an iPhone 4 (or iPods/iPhones in general) over USB. What are the minimum and maximum current an iPod/iPhone can draw while charging?
I realize that USB is specd to 5 volts, and as particular as I am about taking good care of it, Id like to give it 5v 100% of the time, but the truth is that batteries do fade before they are mostly dead, and I imagine that interrupting a partial charge would do more harm to the iPhones battery then let it finish charging at 4.9volts.
As NiMH batteries are rated at 1.2volts, but often deliver as much as 1.4 initially; I think 5 AAs in series (delivering, in the theoretically maximum case of alkelines, 8 volts; and more typically 5-5.5 volts with a less than full charge) can be voltage-regulated down to a steady 5 volts. If I had 2 sets of 5 AAs in parallel, I could remove one set to charge while still using the other without impacting to target voltage. (sets of 4 AAs would be much more convenient, but would mean that my delivered voltage could drop as low as 4.6 volts as the batteries begin to die. Is this within allowable parameters? Am I really just going overboard on this? Please provide any suggestions. Id honestly rather buy than build.)
So having done a small amount of cursory research on homebrew USB to charge iPods and iPhones I have 2 related follow-on questions:
2. At one time, low-current reference voltages of 2.5v and 1.8v were required on the data pins to initiate charging. Are these reference voltages still required? Is there any official documentation specifying those?
3. I have been more than happy with the service Ive received as part of my AppleCare warranty and wish to do nothing to invalidate it. Assuming that I am careful to check my work to ensure that Im generating the voltages I should be, would anything Ive described above invalidate my AppleCare warranty? How would it be different than charging an iPod/iPhone on any 3rd party vendors USB port. (I concead that as a hobbist and solo artist Im more likely to make mistakes than a company using 6 sigma processes to produce a run of several million units, but thats not what Im asking.) In short, will this void my warranty?
iPhone 4 iOS 4