If you want to build something to charge a device via USB along the way, even if it is no Apple product, it pays to read the documentation of the Adafruit „Minty Boost“: https://learn.adafruit.com/minty-boost/
The first conclusion is: if you do have an Apple product, get the voltage dividers right (if not: just short the data lines).
A good start on those annoying proprietary stuff is cutting an old cable and building an adapter to have a standard USB „wall wart“ charge an old iPod:
Then there is the possibility to use four NiMH cells in series to charge a USB device directly without any regulation.The no load voltage is 1.32 V per cell, so four in series is 5.28 V (not for long though).
A description (in german) can be found here: http://www.mikrocontroller.net/articles/IPod_Ladeger%C3%A4t#Notladung
My Motorola Milestone got along with this quite well.
…but I had seen that Minty Boost and it’s spec. I don’t care if THT parts are easier to solder and I had a little Tim Taylor sitting on one shoulder and a Jeremy Clarkson on the other – both yelling „More power!“
Once this worked I coud refine the schematic and start routing.
Everything went well, but using three AA cells was demanding on them…
If you want to do this properly: Max. voltage is 5.00±0.25 V and many chargers just start by delivering 5.2something volts. Once the device charged demands too much current they drop that voltage (on purpose …or not) and the IC handling the e.g. LiPoly battery scales back. Chances they find their equilibrium are quite good.
Too bad I went for the fixed voltage …032 IC and stress on the AAs was still high at 5.0 V. But The circuit could be easily taped onto a pair of paralleled „16650“ cells salvaged from dead laptop batteries. …which lead to more power!
Obviously, with a remaining capacity of 2.9 Ah (in sum), no mobile phone would draw enough current to let their voltage drop – especially as they share the current.
Nowadays „Powerbanks“ are everywhere. I have seen smaller ones as giveaways. But I gave mine away and kept this.
Need even more power and/or you devices constantly recharged for days from let’s say LR20 / D-cells? Six in series? Easy: Use a car charger. They step down – and most work a fair way lower than 9 or 12 V.
(Summarized from http://www.metalforum-owl.de/viewtopic.php?f=21&t=138&start=100#p40287, August 4th 2011 and http://www.metalforum-owl.de/viewtopic.php?f=21&t=138&start=160#p46882, May 13th 2012)