I am no expert in electronics but a little reading and a little logic seems to have worked here.
You will need:
A wire stripper
A Phillips screwdriver
a 1 ohms resistor
A USB cable with type A head. (see below)
Step 1: Where to Connect
Step 2: Where to Connect
It's possible to solder directly to these points. I just thought it was easier to solder to the battery terminals as you'll see.
Step 3: Where to Connect
Cut it at the tip and peel the cable back to reveal the 4 wires inside.
We only need the black and red wires which are what carries the power down the wire. I chopped the other 2 off.
Use a wire stripper to reveal the copper under the coloured wire.
I used a wood/metal file to make a small hole in the side of the pop as you can see in the picture to feed the USB wire through to the battery terminals.
I also held the wire in place with a cable tie.
Step 4: The Soldering Part.
I used a resistor to lower the power being carried down the usb by 0.5 volts using a 1ohms resistor.
I am really not into electronics at all. I used an online calculator to help me pick up the correct resistor. http://led.linear1.org/1led.wiz
As you can see in the image, I soldered the negative from the usb (black) to the negative battery terminal. I soldered the resistor between the positive usb wire and the positive cable terminal.
Step 5: Complete
I now have happy kids not moaning that their batteries have run out and a portal of power that connects to the PS3 USB socket. :-)