I don't know if you read the other forum.
I'll put here the conclusion.
Before trying to change AREF to external, try to put a capacitor between AREF pin and GND, with Arduino GND and 3.7V LiPo GND connected. Check if values are good enough.
If not, and if you want to try changing the AREF, you must:
1) open ...\arduino-0010-win\arduino-0010\hardware\cores\arduino\wiring.c
search this lines:
// set a2d reference to AVCC (5 volts)
change to this:
// set a2d reference to AREF
Upload your sketch.
2) connect Arduino GND to 3.7V LiPo GND;
3) connect 3.7v. to AREF pin.
Don't connect nothing to AREF pin until change the wiring.c, open IDE, and upload sketch.