My iPod Touch was dying and I didn't want to turn my laptop on just to charge it. But then I thought of an old laptop charger I had that was still good, so I got to work. And so I have hacked together an USB charger.
Step 1: List of Things You Need
three alligator test leads
USB car charger
Old laptop charger: 20V max, 1.5A is reasonable
DMM ( optional)
Step 2: Prepare the Laptop Charger
First, cut the DC out cord in two. It doesn't matter where. Mine was shorted in a few places so I cut it before the first split. Then you need to cut the outer insulator about an inch. After that, remove 1/3 of the shield. This shield will be your ground or your positive rail, depending on the charger. After you remove the shield, you will have 2/3 white insulation in the center. Remove 1/2 of this revealing the center wire. You should now have 1/3 shield 1/3 insulation then 1/3 wire. The charger is now ready. Use the DMM to verify that the charger is correct in voltage and that the center wire is indeed positive.
Step 3: Connecting the USB Charger and Switch
Now for the easiest part of the instructable. Grab one the test leads and connect it to the outer terminal of the USB charger and to the ground of the charger. Grab the next test lead and connect it to the positive rail of the charger and to the switch's common terminal. The last lead goes from the switch's any other terminal to the center connecter of the USB charger.
Step 4: Verify It Works
Plug the laptop charger in and flip the switch. If the USB charger has an indicator LED it should light up. Else you have to plug something up to it to see if it charges. No big deal as that was the whole point here.
Step 5: Variations
One thing you can do differently is replace the laptop charger with a 12 V 5-7 Ah battery and take it with you. I calculated that on a 7 Ah battery, you can go at least 30 hours before an iPod Touch even starts running on its own internal battery. That was using an average estimate I obtained by using a multimeter (which gave me an average of 0.225 A while charging). But your iPod probably won't be charging for the whole 30 hours, so it should actually go even longer.