Don't put non-rechargable batteries in the holder though, because you will wind up with 6 volts instead of five. The USB socket is supposed to supply five volts.
One nice thing about this setup is that the batteries stick together as a set of four. They get drained together, and they get charged together. Also there are no complicated circuits.
Step 1: Buy a battery holder, batteries, and a charger
You can usually find a battery charger at a thrift store or in the free box at a college dorm building. Often times the charger is missing its power cord, and you can just match it up and be on your way. Buying a charger would be silly with all the free ones out there, ready to go in the trash because nobody wants them.
Most chargers charge as two sets of two (one LED for two batteries) but some charge all four cells as individuals (one LED for each battery). Those are better. Everyone deserves to be treated as an individual.
You can get rechargable batteries in a set of four. Cheap ones are 1500mAh, expensive new ones are almost 3000mAh. Get whatever is handy, and upgrade if you need to. Don't mix batteries that weren't born together, they will end up beating each other up. I am serious.
Step 2: Find a USB socket, or cable
The first picture is a pair of USB sockets from a desktop computer. These are a good choice because they can be found free in any computer-junk pile, and the red and black wires are already figured out for you. Don't cut off the other wires; you will need to put resistors on them later.
The second picture is a socket-end of a USB extension cable. This is the easy too, because when you cut the cable you will see the red and black wires again.
If you choose another type of USB socket, use the diagram to make sure you are getting the Plus and Minus correct. Red goes to Plus, Black goes to Minus. Make sure you don't have it wrong or you will destroy your fancy IPod. Then you can write an instructable about how to make an altoids tin out of your dead ipod (just remove dead ipod guts and add hinges).
Another option, if you have an extra IPod USB cable, is to cut the USB plug off of the cable and wire the red and black wires to the battery holder directly. If you do this with your only IPod cable, however, you will never be abe to change your music again!
Step 3: Wire battery holder to USB connector
Now you have to join the black and red wires from the battery compartment to the USB connector. Strip the wires, solder them, and tape them. Red to Red, Black to Black.
The result will be that when you put four RECHARGABLE AA batteries in the holder, you get 5 volts at the USB socket, in the correct polarity.
Step 4: Add resistors to the data lines
Get a pair of 100K ohm resistors (brown black yellow something, or brown black black orange something) and twist them together at one end. Connect that end to the red wires. The other end of each of the resistors goes each of the remaining wires from the USB socket.
You are basically connecting one 100K resistor between VBUS +5VDC and Data+,
and another resistor between VBUS +5VDC and Data-.
The second image is from the Mintyboost version 1.2, and is for reference purposes only. Ignore the right half of the screen. Ignore the fact that the resistors do not exactly connect to +5VDC in that schematic. If you want to make a mintyboost though, use that schematic.
For more information about the Mintyboost, see the Mintyboost 1.2 FAQ:
Step 5: Use it!
A hot-glue gun is the correct way to stick the USB socket to the battery holder once you're sure everything is done right. Make sure you don't get hotglue into the holes of the battery box where they will block the batteries from contacting their terminals.
Now would be a good time to put an easy-to-read warning sticker on your battery holder that says, "DO NOT PUT NON-RECHARGABLE BATTERIES IN HERE OR DAMAGE MAY RESULT!"
This is because non-rechargable batteries are 1.5 volts, which would total SIX volts - more than USB is supposed to supply. Probably the IPod would be okay with this, but no guarantees that it won't turn your IPod into a shiny coffin for burnt circuitry.
That's it! Hopefully the Ipod will continue to charge until the battery pack totals 4.0-4.4v and then stop. At that point it is time to charge the batteries the next time you get the chance. Unplug the Ipod from this device when you're not using it, so that it doesn't keep trying to charge the Ipod after it's full.