Work on Pi power control by others

LiPoPi - Daniel Bull

This uses an Adafruit PowerBoost 500C to power up a Raspberry Pi from a switch and shuts it down when the Low Battery pin on the PowerBoost goes low.

Controlled shutdown duration test of Pi model A with 2 cell lipo - Alex Eames, RasPi.TV

This describes how to set up an Analog Digital Converter with a Raspberry Pi to monitor the voltage of a LiPoly battery. This uses a MCP3002 ADC - in my experiments I'm using a MCP3008.

Analog Inputs for Raspberry Pi Using the MCP3008

Adafruit tutorial on using the MCP3008 ADC with the Pi.