I had simmilar issue using VIN. I can't believe Pycom doesn't provide any schematic for these expansion boards, or atleast proper descriptions on how they can be used. I was also under the impression that VIN can be used as an input, instead of using the USB. There are NO WARNINGS anywhere that you should not be using VIN as an input, while using a battery as a backup. Luckily my house didn't burn down as I soon enough figured out that VIN and the Battery + seem to be linked together... I'm new to Pycom's hardware - and thus far really disappointed in the lack of usefull (and up to date) documentation

My suggestion? Don't bother with ADC measurement. Get a "gas gauge" breakout, like the one from SparkFun: https://www.sparkfun.com/products/10617 I've created a thread with feature suggestions for the various expansion boards. One thing I'd like to see is the gas gauge IC integrated to all boards; preferably the MAX17048 (which is a newer, more power-efficient version of the IC on the previously-mentioned breakout). Forum thread here: https://forum.pycom.io/topic/2476/expansion-board-suggestions