Motivation:
Have a load of general purpose 12V lithium-ion batteries and want to be able to charge USB based devices on the go.

Solution: Modify a 12V car adaptor to connect to 12V battery.

Parts:
12V 6800mAh Li-ion Rechargeable Battery Pack €14 inc. delivery
USB Dual Car Charger €4 inc. delivery
2.1mm DC socket €0.76
2 core wire ( I used 6 core alarm wire that was at hand)

I have over 20 of these batteries, who's main purpose is for an LED costume (hopefully I'll put an instructable about it soon). When not using the costume, these batteries are free for general use. These batteries can comfortably provide 1A at 12V, and I've drawn 2A out of them in the past, so they should have no trouble charging 2 devices each drawing 1A at 5V.

The first version I made was the black lead. The batteries are 12V and USB is 5V, so I used a 5V regulator. I snipped a spare micro-usb cable. Attached Red to +5V, Black and shield to Ground, and shorted out Green D+ and White D- so that any USB devices will charge in AC mode, not just USB mode (more here). This worked well, but will only do micro-usb phones. What about my GoPro or friends iPhone? I decided that a USB A female connector would be most versatile.

Step 1: USB Car Adaptor

I ordered a few USB Dual Car Chargers. Didn't get the very cheapest ones, instead got ones that promised to charge iPhones, since they are the most particular due to proprietary configuration of D+ and D- pins (watch this).

I broke it open and snipped off the springs. I also confirmed that this model uses a switched regulator rather than a linear regulator, so that more than half my power isn't lost in heat. Unfortunately didn't photograph the back side before sealing up, but I could see a chip with the code "ACT4060A". A quick read of the data sheet confirmed that it was switched, and could take upto 2A.

Step 2: Putting It All Together

I had 6 core alarm cable at hand, so used that. Because there could be up to 1A travelling through this low gauge wire, I paired them up. Soldered onto 2.1mm socket and covered with hot glue and heat shrink. Then soldered at the other end the +5V to where the spring was attached and the Gnd to the wire attaching the side contacts.

Then sealed it back into the case with hot glue.

Step 3: Testing

Important to test before attaching a €500 phone to this!
Used a severed lead to test each of the pins. Google USB pinout for more details.
Confirmed that RED was +5V.

Then for iPhone fast charging on USB, we need aprox: 2.8 V on D- and 2.0 V on D+ (more here and here)
I tested this and got an interesting result, on one USB port, this was correct, but on the other it was reversed.
This isn't an issue for my Samsung GS3, will have to test on someones iPhone later.

To check on android that you are using fast AC charging and not USB, type on the dialler: *#*#info#*#* or *#*#4636#*#*

Finally, to tighten up the connection and stop it accidentally falling out, I stick a flat screwdriver into the socket and spread the split pin a little for a tighter fit.

Hope this helps someone.