Well, you unplug it first.
With credit to csealpcr for posting it and whoever figured it out first!
NOTE: I TAKE NO RESPONSIBILITY FOR DAMAGE DONE TO ANY PHONE, BATTERY, ETC. FOLLOW THESE STEPS WITH CAUTION TO AVOID RISK OF DAMAGE.
1. Take your cable and cut off one of the ends, leaving a standard USB plug at the opposite end to plug into your computer.
2. Now you need to strip about and inch of the wire off until you get to the core where you should see 4 colored wires (red, black, white, and green). The colors don't really matter and they may vary, just as long as you can see a red wire and a black wire.
3. Strip off about a quarter inch of the rubber sleeving on the red and black cables, exposing the actual metal wire.
4. Twist the metal strands of each of the wires a couple of times so that all of the wires from the black cable are together and all of the wires from the red cable are together. This is just to make sure that you don't create a short circuit.
5. Here's where it gets a little tricky. You are going to use this cable to make the phone think the battery is fully charged, when in fact you will just be powering the phone using the power from the USB port. Pull out the battery on your phone and find the 4 exposed metal slots (the battery terminals).
6. Each of the far terminals should be labeled with a plus (+) or a minus (-). What you need to do is put exposed leads on the red wire up to the positive terminal and the leads of the black wire up to the negative terminal.