So basically charging a device with a microUSB like your nexus works like this. There are 4 pins in the charger port, 1 through 4. 1 is positive power, 2 is positive data, 3 is negative data, and 4 is ground. While you could hack it and just use pins 1 and 4, pins 2 and 3 are important because if the android operating system doesn't see a specific constant voltage being applied across these two pins, the tablet will only charge in "USB" mode, drawing a max of 500 mA. What you need is an android friendly charger that will trick the tablet into thinking its plugged into a wall allowing it to enter "AC" charging mode and draw as much amperage as is safe for the device. I use a USB plug in charger that delivers up to 2100 mA, which is what I would suggest you do, no cutting, no mess, no fuss.
Keep in mind your screen on, Bluetooth running, and multiple apps are capable of drawing at least 1500 mA, so you'll constantly lose charge unless you do this correctly.
Edit:
I asked around on the android forums and was pointed to this:
http://www.amazon.com/dp/B0088U4YAG/...0_M3T1_SC_dp_1
The nexus guys have had great luck with it, and it delivers 3100 mA!