crispy:
I'm using a Lithium Polymer battery from an RC aircraft to power my Arduino project. This is a "dumb" battery - it has no internal protection circuit. To avoid over-drawing and damaging the battery, I need a way to shut down the Arduino when the voltage drops below a certain threshold.Reading the battery voltage can be done on one of the analog input pins, with a resistor to drop the 11.1 volts down below the max 5V input (calibration will be needed to correlate sensed voltage in with actual voltage).
My biggest question is, can the Arduino power itself off in software, or will I need to rig up a relay that mechanically disconnects the battery?
Well an arduino board cannot actually cut all power off, the AVR chip does have several power-saving modes. However the Arduino IDE does not provide any library files to utilize those power saving modes directly but there should be much examples posted in this forum if you search them out. Keep in mind that even if the AVR chip is put to sleep the board still consumes some current via the USB serial converter chip, +5vdc on board voltage regulator if using Vin to power the board, the power LED, and auto-voltage selector components. When I need to use a manual power on, auto software power down I use a 5vdc single coil latching relay circuit:
http://img25.imageshack.us/img25/563/08miq7.jpg
Lefty